Assessment of autonomic function in patients with neurally mediated syncope: augmented cardiopulmonary baroreceptor responses to graded orthostatic stress

Vasovagal syncope with mild versus moderate autonomic dysfunction: a 13-year single-center experience

BACKGROUND

An adequate large-scale pediatric cohort based on nationwide administrative data is lacking in Korea.

PURPOSE

This study aimed to differentiate patients with VVS by autonomic dysfunction severity using the composite autonomic severity score (CASS) and compare the clinical manifestations and prognosis between patient subgroups.

METHODS

We retrospectively reviewed the medical records of 66 VVS patients divided into 3 groups by CASS. To compare the differences between these groups, we analyzed VVS type, triggers, prodromal symptoms, management of syncope, and prognosis between patients with mild versus moderate autonomic dysfunction.

RESULTS

Of our 66 patients with VVS, 41 had mild autonomic dysfunction (62.1%) and 25 had moderate autonomic dysfunction (37.9%). We found no significant intergroup differences in age, sex, inducible factor (P=0.172), prodromal symptoms, laboratory findings, head-up tilt test, type of syncope, or prognosis (P=0.154).

CONCLUSION

We found no evidence that autonomic dysfunction degree is affected by VVS characteristics, test findings, parameters, or prognosis; therefore, no further evaluations are needed to classify autonomic dysfunction severity.

The Role of the Baroreflex in Tilt Table Testing: Outcome and Type of Response

OBJECTIVES

The purpose of this study was to better understand the role of the baroreflex in tilt-induced vasovagal syncope (VVS).

BACKGROUND

The role of the baroreflex in tilt-induced VVS remains controversial. The authors hypothesized that: 1) patients with positive tilt table test (TTT) results have greater baroreflex gain (BRG) compared with patients with negative TTT results; and 2) patients with tilt-induced asystole have greater BRG compared with patients without asystole.

METHODS

Using the sequence method, BRG measurements were obtained in 438 consecutive patients undergoing TTT. Two hundred sixty-eight patients (61%) had positive TTT results (mean age 50 ± 21 years; 34% men), and 170 patients (39%) had negative TTT results (mean age 48 ± 21 years; 35% men).

RESULTS

Mean BRG was significantly higher in patients with positive TTT results compared with those with negative TTT results (12.9 ± 6.0 ms/mm Hg vs. 11.5 ± 6.0 ms/mm Hg; p = 0.01). Among the 268 patients with positive TTT results, 23 (9%) had more than 3 s of asystole (mean age 37 ± 17; 30% men), and 245 patients had a mixed vasodepressor or cardioinhibitory response without asystole (mean age 51 ± 17 years; 34% men). Mean BRG was greater in patients with tilt-induced asystole (>3 s) compared with patients without asystole (15.3 ± 5.9 ms/mm Hg vs. 12.7 ± 5.9 ms/mm Hg; p = 0.03).

CONCLUSIONS

The results of this study demonstrate that baseline BRG was higher in patients with positive TTT results compared with those with negative TTT results, with greater values noted in patients with tilt-induced asystole (>3 s) compared with those without asystole.

Slower Lower Limb Blood Pooling Increases Orthostatic Tolerance in Women with Vasovagal Syncope

Background and Aim: Slower lower limb blood pooling and associated blunted sympathetic activation has been detected in healthy women prone to orthostatic syncope. Whether these findings are true also for patients with vasovagal syncope (VVS) is unknown. The aim was to investigate initial blood pooling time (pooling_time, time to 50% of total blood pooling) together with hemodynamic responses and orthostatic tolerance during lower body negative pressure (LBNP) in VVS and healthy controls.

Methods and Results: Fourteen VVS women (25.7 ± 1.3 years) and 15 healthy women (22.8 ± 0.8 years) were subjected to single-step and graded LBNP to pre-syncope. Lower limb blood pooling (ml · 100 ml⁻¹), pooling_time (s), hemodynamic responses and LBNP-tolerance were evaluated. LBNP induced comparable lower limb blood pooling in both groups (controls, 3.1 ± 0.3; VVS, 2.9 ± 0.3 ml · 100 ml⁻¹, P = 0.70). In controls, shorter pooling_time correlated to higher LBNP-tolerance (r = –0.550, P < 0.05) as well as better maintained stroke volume (r = –0.698, P < 0.01) and cardiac output (r = –0.563, P < 0.05). In contrast, shorter pooling_time correlated to lower LBNP-tolerance in VVS (r = 0.821, P < 0.001) and larger decline in stroke volume (r = 0.611, P < 0.05). Furthermore, in controls, shorter pooling_time correlated to baroreflex-mediated hemodynamic changes during LBNP, e.g., increased vasoconstriction (P < 0.001). In VVS, pooling_time was not correlated with LBNP-induced baroreceptor unloading, but rather highly correlated to resting calf blood flow (P < 0.001).

Conclusions: Shorter pooling_time seems to elicit greater sympathetic activation with a concomitant higher orthostatic tolerance in healthy women. The contrasting findings in VVS indicate a deteriorated vascular sympathetic control suggesting well-defined differences already in the initial responses during orthostatic stress.

Reduced venous compliance: an important determinant for orthostatic intolerance in women with vasovagal syncope

The influence of lower limb venous compliance on orthostatic vasovagal syncope (VVS) is uncertain. The most widespread technique to calculate venous compliance uses a nonphysiological quadratic regression equation. Our aim was therefore to construct a physiologically derived venous wall model (VWM) for calculation of calf venous compliance and to determine the effect of venous compliance on tolerance to maximal lower body negative pressure (LBNP). Venous occlusion plethysmography was used to study calf volume changes in 15 women with VVS (25.5 ± 1.3 yr of age) and 15 controls (22.8 ± 0.8 yr of age). The fit of the VWM and the regression equation to the experimentally induced pressure-volume curve was examined. Venous compliance was calculated as the derivative of the modeled pressure-volume relationship. Graded LBNP to presyncope was used to determine the LBNP tolerance index (LTI). The VWM displayed a better fit to the experimentally induced pressure-volume curve (P < 0.0001). Calf blood pooling was similar in the groups and was not correlated to the LTI (r = 0.204, P = 0.30). Venous compliance was significantly reduced at low venous pressures in women with VVS (P = 0.042) and correlated to the LTI (r = 0.459, P = 0.014) in the low pressure range. No correlation was found between venous compliance at high venous pressures and the LTI. In conclusion, the new VWM accurately adopted the curvilinear pressure-volume curve, providing a valid characterization of venous compliance. Reduced venous compliance at low venous pressures may adversely affect mobilization of peripheral venous blood to the central circulation during hypovolemic circulatory stress in women with VVS.

Assessment of human baroreflex function using carotid ultrasonography: what have we learnt?

The arterial baroreflex is critical to both short- and long-term regulation of blood pressure. However, human baroreflex research has been largely limited to the association between blood pressure and cardiac period (or heart rate) or indices of vascular sympathetic function. Over the past decade, emerging techniques based on carotid ultrasound imaging have allowed new means of understanding and measuring the baroreflex. In this review, we describe the assessment of the mechanical and neural components of the baroreflex through the use of carotid ultrasound imaging. The mechanical component refers to the change in carotid artery diameter in response to changes in arterial pressure, and the neural component refers to the change in R-R interval (cardiac baroreflex) or muscle sympathetic nerve activity (sympathetic baroreflex) in response to this barosensory vessel stretch. The key analytical concepts and techniques are discussed, with a focus on the assessment of baroreflex sensitivity via the modified Oxford method. We illustrate how the application of carotid ultrasound imaging has contributed to a greater understanding of baroreflex physiology in humans, covering topics such as ageing and diurnal variation, and physiological challenges including exercise, postural changes and mental stress.

Orthostatic tolerance is difficult to predict in recurrent syncope patients

OBJECTIVES

We tested the hypothesis that detailed anthropometric and hemodynamic measurements predict orthostatic tolerance in neurally mediated syncope patients. In addition, we tested whether orthostatic tolerance is related to syncope frequency in real life.

BACKGROUND

Earlier studies in patients with neurally mediated syncope suggested that orthostatic heart rate and blood pressure responses predict the tilt table responses with high sensitivity and specificity.

METHODS

We analyzed data from 157 consecutive patients (n = 100 exploratory cohort, n = 57 confirmatory cohort) with recurrent syncope in whom orthostatic tolerance was quantified as the time to (pre)syncope during head-up tilt testing combined with lower body negative pressure. We measured heart rate, brachial blood pressure, cardiac stroke volume, heart rate and blood pressure variability, and spontaneous baroreflex sensitivity supine and early during head-up tilt.

RESULTS

The orthostatic heart rate increase showed the strongest correlation with orthostatic tolerance. The best multivariate model including age, supine diastolic blood pressure, supine blood pressure variability, as well as tilt-induced changes in diastolic blood pressure and heart rate explained no more that 40% of the variability in orthostatic tolerance. The model failed to predict orthostatic tolerance in the confirmatory cohort. Frequency or number of free-living syncopal episodes were only weakly related to orthostatic tolerance.

CONCLUSIONS

In patients with neurally mediated syncope, orthostatic tolerance in the clinical laboratory is difficult to predict with a wide range of anthropometric and cardiovascular measurements and correlates poorly with syncope occurrence in real life.

Sympatho-vagal responses in patients with sleep and typical vasovagal syncope

Sleep syncope is a recently described form of vasovagal syncope that interrupts sleep. The pathophysiology of this condition is uncertain but a ‘central’ non-baroreflex-mediated trigger has been suggested. In the present study, we tested the hypothesis that patients with sleep syncope have abnormal sympatho-vagal responses to non-baroreflex, but normal responses to baroreflex stimuli. We collected historical data from SS patients (patients with vasovagal syncope with sleep syncope; n=16) and NSS patients (patients with vasovagal syncope without sleep syncope; n=35), including demography, and triggers and symptoms during syncope. MBP (mean blood pressure), HR (heart rate) and MSNA (muscle sympathetic nerve activity) in SS patients were compared with NSS patients and matched controls (n=16) during HG (handgrip), CPTs (cold pressor tests), HUT (head-up tilting) and tilt-induced pre-syncope. Patients and controls were of similar age and gender distribution [SS patients, age 46.0±4 years (69% female); NSS patients, 47.3±4 years (63% female); controls, 43.7±5 years (69% female)]. Compared with NSS patients, SS patients reported more fainting episodes: (i) triggered by phobias (75 compared with 37%; P=0.001); (ii) while in the horizontal position (44 compared with 6%; P=0.001); and (iii) associated with abdominal symptoms (69 compared with 9%; P=0.001). Compared with controls, the MBP response to HG was attenuated in SS patients (P=0.016), and MSNA (burst frequency and incidence) responses to CPT were attenuated in both syncope groups (SS, P=0.011 and 0.003 respectively; NSS, P=0.021 and 0.049 respectively). MSNA responses to HUT did not differ. For both non-baroreflex and baroreflex responses, there were no differences in any of the MSNA indices between the syncope groups. Patients with vasovagal syncope, with or without sleep syncope, have very similar sympatho-vagal responses to both non-baroreflex and baroreflex stimuli. This is consistent with sleep syncope being a subform of vasovagal syncope. Attenuation of sympathetic responses to non-baroreflex pathways may be important in the mechanism of vasovagal syncope.

Is vasovagal syncope a disease?

Vavovagal syncope (VVS) is not generally associated with cardiovascular, neurological or other diseases, and, therefore, represents an isolated manifestation. Isolated VVS cannot be regarded as a disease for several reasons: spontaneous syncope occurs in about half of individuals during their lives, and the unidentified neural pathways involved in the vasovagal response are probably present in all healthy humans, with individual differences in susceptibility; VVS is induced during tilt testing in several subjects with no history of syncope; during haemorrhagic shock, the vasovagal reaction can be observed in subjects with no history of syncope; about 20% of astronauts, who are selected on the basis of their great resistance to orthostatic stress, experience syncope or presyncope on landing after a short-duration space flight; to date, no genetic basis of VVS has been demonstrated; subjects with VVS are generally normotensive and, importantly, have normal blood pressure regulation apart from the episodes of syncope; hormonal disorders or a generalized state of autonomic involvement, although frequently investigated, have never been clearly demonstrated. Isolated VVS should be distinguished from those forms that start in old age and which are often associated with cardiovascular or neurological disorders, and other dysautonomic disturbances such as carotid sinus hypersensitivity, post-prandial hypotension, and symptoms of autonomic dysfunction. In these subjects, VVS appears as an expression of a pathological process, i.e. a disease, mainly related to a generalized involvement of the autonomic nervous system, which is not yet well-defined from a nosological point of view.

How to avoid a misdiagnosis in patients presenting with transient loss of consciousness

Daily in the UK, frontline medical and paramedical staff are required to manage patients with "collapse ?cause". This universal colloquialism refers to patients who have had an abrupt loss of postural tone. Some of these patients would have had a "blackout" or a transient loss of consciousness (T-LOC). The three most important causes of T-LOC are syncope, epilepsy and psychogenic blackouts. Determining the correct cause is an important challenge; if the initial clinical diagnosis is wrong, investigations may be misdirected, and the final diagnosis and treatment incorrect. Syncope is much more common than epilepsy and may present with symptoms akin to the latter. This fact is not well appreciated and often leads to misdiagnosis. This article deals with the clinical features of the three main causes of blackouts, the value of investigations in arriving at a diagnosis and the problem of misdiagnosis. Pathways for managing patients presenting with blackouts are suggested.

Autonomic Nervous System Changes in Vasovagal Syncope:

Spectral analysis of heart rate variability was used to compare the changes in autonomic function during tilting in young and older patients with vasovagal syncope. Twenty-four young (age 28 +/- 8 years) and 31 older (56 +/- 5 years) patients with unexplained syncope and a positive tilt test and 25 controls (age 48 +/- 12 years) were included in the study. Frequency-domain measurements of the low (LF) (0.06-0.15 Hz) and high (HF) (0.15-0.40 Hz) frequency bands and the ratio of LF to HF were computed from Holter recordings for 4-minute intervals before and immediately after tilting and just before the end in all groups. Syncopal patients showed a different pattern of response to tilting from controls in all spectral indexes. Young and older patients showed the same pattern of changes in all measurements, even though certain differences were observed. The LF after tilting reduced more in the older (-20 +/- 7% vs -14 +/- 5%, P < 0.001), while HF reduced more in young patients (-17 +/- 8% vs -8 +/- 3%, P < 0.001). Young patients showed mainly a cardioinhibitory type (71%) of response whereas a vasodepressor type response predominated (68%) in the older patients. The autonomic nervous system appears to play an important role in the pathophysiological mechanism of vasovagal syncope. This role is similar in young and in older patients and this should be taken into account in the therapeutic approach to the condition. Specific differences between age groups may be related to the type of vasovagal syncope.

Responses to Tilt test in young and elderly patients with syncope of unknown origin

The aim of the present study was to assess the cardiovascular autonomic function and responses to tilt test in young and elderly patients with syncope of unknown origin. We evaluated two groups of patients with previous unexplained syncope: 192 older subjects (112 males, 80 females, mean age 67.2 +/- 6.8 years) and 188 young subjects (102 males, 86 females, mean age 25 +/- 9 years). All patients underwent ambulatory electrocardiogram (ECG) monitoring, to evaluate time domain indices of heart rate variability (HRV), and head-up tilt test in the morning after an overnight fast. The responses of positive tilt test were classified using the VAsovagal Syncope International Study (VASIS) criteria: mixed (VASIS-1), cardioinhibition (VASIS-2A), severe cardioinhibition/asystole (VASIS-2B), pure vasodepression (VASIS-3). All the time-domain HRV indexes were lower in the older than in young subjects. The rate of positive responses was not different in the two groups. In elderly group the positive head-up tilt test responses showed: a pure vasodepressive response (VASIS 3) in 126 patients (65%), a mixed (VASIS-1) response in 25 patients (13%), a cardioinhibitory (VASIS-2A) response in 13 patients (7%). Only 28 (14.6%) of elderly group patients had negative head-up tilt test response. In contrast, in young group the positive head-up tilt test responses showed: 114 patients (61.2%) a mixed (VASIS-1) vasovagal response, 40 patients (22.3%) a cardioinhibitory (VASIS-2A) response, four (2.1%) patients a severe cardioinhibitory (VASIS-2B) and four (2.1%) patients a pure vasodepression (VASIS-3) response, respectively. The tilt test was negative in response in 26 young patients (12.2%). Our results confirm that the head-up tilt test may be useful in assessing unexplained syncope, since it is seen to be positive in 85% of elderly patients and 86% in young patients. In our subjects, vasodepressive response was the most frequent cause of syncope in older subjects, while vasovagal response is the commonest cause of syncope of young patients. This different behaviour in the elderly may be is explained with physiological aging, which is associated with a reduction of sympathetic-parasympathetic control on the cardiac rhythm, demonstrated by reduction in all the time domain HRV indices.

Insulin sensitivity in young women with vasovagal syncope

BACKGROUND

Insulin, in addition to its known metabolic effects, has sympatho-excitatory and vasodilatory actions on muscular blood vessels. The goal of this study was to evaluate insulin sensitivity in young women with vasovagal syncope and positive tilt test results (HUT+) and to compare it with that in patients with negative tilt test results (HUT-) and in control subjects without a history of syncope.

METHODS

Different indices of insulin sensitivity were obtained by an oral glucose tolerance test (OGTT) in 13 young women with syncope and HUT+ (age 26.8 +/- 9.1 years, body mass index 20.4 +/- 2.1), 8 patients with HUT- (age 26 +/- 5.6 years, body mass index 21.9 +/- 2.4), and 13 control subjects without syncope and HUT- (age 28.9 +/- 8.8 years, body mass index 23.1 +/- 1.7). The following parameters were assessed: fasting glucose and insulin levels (G(0), I(0)); G(0)/I(0) ratio; G(0) x I(0); areas under the curve for glucose and insulin; homeostatic model assessment (HOMA); quantitative insulin sensitivity check index (QUICKI); and composite whole-body insulin sensitivity index (ISI).

RESULTS

G(0) and I(0) values were significantly lower in patients with HUT+ than in control subjects (G(0) 4.9 vs 81.9, P <.05, I(0) 4.7 vs 9.1, P <.005). All the fasting values-based indices (ie, HOMA 0.9 vs 1.9, P <.005) and the ISI (12.8 vs 7.1, P =.01) differed significantly in both groups. None of the parameters showed significant differences between patients with HUT- and control subjects. Sixty-one percent of patients with HUT+ had a vasovagal reaction during OGTT.

CONCLUSIONS

Young women with vasovagal syncope and HUT+ have a greater insulin sensitivity. They have a propensity to reproduce symptoms during the OGTT. This hypersensitivity could be one of the predisposing factors for vasovagal episodes.

Role of endogenous adenosine in vasovagal syncope

Adenosine may be a potential mediator in the pathogenesis of vasovagal syncope. Intravenous adenosine increases sympathetic discharge and provokes vasovagal syncope in sensitive subjects. No data are available for endogenous adenosine. The authors compared the results of head-up tilt-table testing (HUT) (45 minutes at 60 degrees) of three arbitrary groups of subjects: sensitive (n = 25, age 34 y, vasovagal syncope, positive HUT), moderately sensitive (n = 28, age 34 y, vasovagal syncope, negative HUT), and nonsensitive (n = 19, age 30 y). A positive test result produced syncopal symptoms with hypotension and/or bradycardia. Single-lead electrocardiogram (ECG) was recorded, and arterial pressure was measured noninvasively. Fourier transform was used for power-spectral heart rate variability (HRV) analysis of 5-minute ECG data. In the nonsensitive and moderately sensitive groups, HUT was repeated with intravenous dipyridamole, an adenosine transport blocker. In the sensitive group, HUT was repeated with oral theophylline, an adenosine receptor blocker, or placebo. In the moderately sensitive group, a third HUT was performed with dipyridamole and oral theophylline. If adenosine plays a role in vasovagal syncope, then dipyridamole would induce more positive HUT responses, a positive HUT response would be prevented by theophylline, and hemodynamic and HRV data in positive HUT responses induced by dipyridamole should reproduce those observed during spontaneous positive HUT responses. Dipyridamole induced positive HUT responses in 57% of the moderately sensitive group and 21% of the nonsensitive group (p < 0.05). Theophylline treatment was not efficient in preventing HUT-induced syncope in sensitive subjects; however, it prevented dipyridamole-induced syncope in 75% of the moderately sensitive group. Dipyridamole immediately increased arterial pressure, heart rate, and total HRV in all (p <0.05). In sensitive subjects, these responses were different: small for arterial pressure and for total and low-frequency HRV, and large for heart rate. It is concluded that endogenous adenosine, like exogenous adenosine, may induce vasovagal syncope. However, the mechanism of adenosine-induced syncope is probably different from that of HUT-induced vasovagal syncope.

Elevated parasympathetic nerve tone in isoproterenol-induced neurally mediated syncope during head-up tilt testing

To clarify the autonomic nerve mechanisms whereby isoproterenol induces vasovagal reaction, 32 patients with recurrent syncopal episodes of unknown origin underwent head-up tilt testing. Syncope was induced in 11 patients by a control tilt (control/+ group). The remaining 21 patients (control/- group) underwent the procedure during isoproterenol infusion. Syncope was induced in 13 (isoproterenol/+ group), but not in the remaining 8 patients (isoproterenol/- group). High-frequency component (HF) and low-frequency component (LF)/HF, which are indices of heart rate variability, were assessed for 2-min intervals at baseline (T1) during the initial phase (T2) and at the end (T3) of tilting in each test. The ratios of HF and LF/HF at T2/T1 and T3/T2 were calculated. The HF value at T3 in the control/+ group was greater than that in the control/- group (1.47+/-0.91 vs 0.75+/-0.34; p<0.05). The HF value at T3 in the isoproterenol/+ group was significantly greater than those in the isoproterenol/- group (1.19+/-1.04 vs 0.43+/-0.23; p<0.05). The HF value at T2 in the isoproterenol/+ tended to be greater than in the isoproterenol/- group (1.30+/-0.85 vs 0.66+/-0.53; p=0.07). The ratio of HF for T2/T1 in isoproterenol/+ was greater than in the other groups (2.04+/-1.26 in isoproterenol/+ vs 0.58+/-0.39 in control/-, 0.47+/-0.18 in control/+ and 0.71+/-0.45 in isoproterenol/-; p=0.0001), whereas that of HF for T3/T2 in the control/+ group was greater than in the other groups (1.48+/-0.62 in control/+ vs 0.97+/-0.33 in control/-; p<0.05, 0.79+/-0.35 in isoproterenol/- and 0.88+/-0.37 in isoproterenol/+; p<0.01). No difference was observed in either LF/HF or the ratio of LF/HF among the groups. The early and persistent increase in parasympathetic nerve tone provoked by isoproterenol plays an important role in enhancing the vasovagal reaction during head-up tilt testing.

Autonomic nervous system dysfunction in adolescents with postural orthostatic tachycardia syndrome and chronic fatigue syndrome is characterized by attenuated vagal baroreflex and potentiated sympathetic vasomotion

The objective was to determine the nature of autonomic and vasomotor changes in adolescent patients with orthostatic tachycardia associated with the chronic fatigue syndrome (CFS) and the postural orthostatic tachycardia syndrome (POTS). Continuous electrocardiography and arterial tonometry was used to investigate the heart rate and blood pressure responses before and 3-5 min after head-up tilt in 22 adolescents with POTS and 14 adolescents with CFS, compared with control subjects comprising 10 healthy adolescents and 20 patients with simple faint. Heart rate and blood pressure variability, determined baroreceptor function using transfer function analysis, and measured cardiac vagal and adrenergic autonomic responses were calculated using timed breathing and the quantitative Valsalva maneuver. Two of 10 healthy controls and 14 of 20 simple faint patients experienced vasovagal syncope during head-up tilt. By design, all CFS and POTS patients experienced orthostatic tachycardia, often associated with hypotension. R-R interval and heart rate variability were decreased in CFS and POTS patients compared with control subjects and remained decreased with head-up tilt. Low-frequency (0.05-0.15 Hz) blood pressure variability reflecting vasomotion was increased in CFS and POTS patients compared with control subjects and increased further with head-up tilt. This was associated with depressed baroreflex transfer indicating baroreceptor attenuation through defective vagal efferent response. Only the sympathetic response remained. Heart rate variability declined progressively from normal healthy control subjects through syncope to POTS to CFS patients. Timed breathing and Valsalva maneuver were most often normal in CFS and POTS patients, although abnormalities in select individuals were found. Heart rate and blood pressure regulation in POTS and CFS patients are similar and indicate attenuated efferent vagal baroreflex associated with increased vasomotor tone. Loss of beat-to-beat heart rate control may contribute to a destabilized blood pressure resulting in orthostatic intolerance. The dysautonomia of orthostatic intolerance in POTS and in chronic fatigue are similar.

Distinct hemodynamic profiles in patients with vasovagal syncope: a heterogeneous population

OBJECTIVE

The objective was to investigate mechanisms of vasovagal syncope by identifying laboratory techniques that characterize cardiovascular profiles in patients with vasovagal syncope.

BACKGROUND

The triggering mechanisms of vasovagal syncope are complex. The patient population is likely heterogeneous. We hypothesized that distinct hemodynamic profiles are definable with provocative maneuvers.

METHODS

Three groups of subjects were matched for age and gender: 16 patients with a history of syncope and an inducible vasovagal response during passive tilt table testing (70 degrees, 45 min, group I), 16 with a history of syncope, negative passive tilt table testing but positive isoproterenol tilt table testing (0.05 microg/kg per min, 70 degrees, 10 min, group II), and 16 control subjects. Beat-to-beat hemodynamic functions were determined noninvasively by photo-plethysmography and impedance cardiography.

RESULTS

At baseline, hemodynamic functions were not different among the three groups (supine). In response to tilt before any symptoms developed, total peripheral resistance decreased 9% +/- 14% in group I from baseline supine to tilt position but increased 27% +/- 18% in group II and 28% +/- 17% in controls (p < 0.001). Responses to isoproterenol were not significantly different between group II and controls in supine position. In response to tilt during isoproterenol infusion before any symptoms developed, total peripheral resistance decreased 24% +/- 20% in group II and increased 20% +/- 48% in controls (p = 0.002).

CONCLUSIONS

Group I patients may have impaired ability to increase vascular resistance during orthostatic stress. The inability to overcome isoproterenol-induced vasodilatation during tilt is important in triggering a vasovagal response in group II patients. These data suggest that the population with vasovagal response is heterogeneous. Distinct hemodynamic profiles in response to various provocative maneuvers are definable with noninvasive, continuous monitoring techniques.

24-hour heart rate variability in patients with vasovagal syncope

Since alterations in the autonomic nervous system are thought to play a major role in the pathogenesis of vasovagal syncope, we characterized the chronic autonomic profile of 44 patients with syncope and 20 healthy subjects by means of heart rate variability using 24-hour Holter recordings (time- and frequency-domain indexes), and evaluated whether the different types of responses to tilting (vasodepressive versus cardioinhibitory) could be associated with different cardiac autonomic patterns. Twenty-three patients exhibited a positive response to tilting, which was vasodepressive in 11 patients and cardioinhibitory in 12 patients. All vasodepressive patients had a standard deviation of the averages of NN (SDANN) intervals in all 5-minute segments lower than 100 ms. Patients with vasodepressive syncope also had significantly lower values of RMSSD (the 24-hour square root of the mean of the sum of the squares of differences between adjacent normal RR intervals) than those with cardioinhibitory response, and lacked the day-night rhythm of the low frequency/high frequency ratio. However, only SDANN values correctly identified patients with vasodepressive response to tilting. We conclude that (1) the population of patients with vasovagal syncope is heterogeneous, (2) patients with vasodepressive syncope have a peculiar chronic autonomic profile as assessed by 24-hour heart rate variability analysis, and (3) the evaluation of the autonomic profile in 24-hour Holter recordings could be of value in the diagnosis of patients with syncope.

Frequency-domain analysis of heart rate variability during positive and negative head-up tilt test: importance of age

The study of autonomic behavior during a head-up tilt test (HUT) has been deemed important to understand the loss of consciousness mechanism. Though HRV in patients with HUT(+) and HUT(-) has been compared, few trials emphasized the importance of age. HRV in frequency domain was analyzed based on 5-minute samples in the supine position, and between 5 and 10 minutes during early tilt test (R1) in 102 patients with one or more episodes of syncope (mean age 44.3 +/- 20.8, range 15-85 years, 55 women). Two subgroups were selected afterwards: (1) young patients between 15 and 35 years of age (41 patients) and (2) elderly patients aged 60 or more (36 patients). The following parameters were taken into account: the sum of low (LF) and high frequency (HF) (LF and HF in absolute values and in normalized units), the LF/HF ratio (L/H ratio), and the percentage of change between baseline and R1 values. The HRV behavior in young and elderly patients with positive and negative HUT was established. We then analyzed the correlation between HRV and age and HUT outcome. A multiple regression analysis encompassing age, HUT outcome, gender, and number of syncope episodes was performed. In young patients, the LF and HF areas and the L/H ratio changed significantly between baseline and R1. The L/H ratio increases from baseline to R1. Conversely, these differences were not significant in the elderly. No differences between HUT(+) and HUT(-) within the same age group were observed. Age related significantly to practically all HRV parameters analyzed, whereas the tilt test outcome correlates poorly with HF normalized units and LF normalized units during R1, and the L/H ratio changes between baseline and R1. By means of a multivariate analysis, only age shows a significant correlation with the HRV values. Despite an all age triggering of vasovagal syncope during HUT, the young and elderly patients' autonomic behavior differs. The young considerably increase their sympathovagal balance during HUT, whereas the elderly have a mitigated autonomic response. No significant differences were observed during the first minutes of the test between those with a HUT(+) and those with a HUT(-) within the same age group. Age, and not the HUT response, is the major determinant of the autonomic behavior during early HUT.

False positive head-up tilt: hemodynamic and neurohumoral profile

OBJECTIVES

This study examined differences in mechanisms of head-up tilt (HUT)-induced syncope between normal controls and patients with neurocardiogenic syncope.

BACKGROUND

A variable proportion of normal individuals experience syncope during HUT. Differences in the mechanisms of HUT-mediated syncope between this group and patients with neurocardiogenic syncope have not been elucidated.

METHODS

A 30-min 80 degrees HUT was performed in eight HUT-negative volunteers (Group I), eight HUT-positive volunteers (Group II) and 15 patients with neurocardiogenic syncope. Heart rate and blood pressure (BP) were monitored continuously. Epinephrine and norepinephrine plasma levels, as well as left ventricular dimensions and contractility determined by echocardiography, were measured at baseline and at regular intervals during the test.

RESULTS

The main findings of this study were the following: 1) All parameters were similar at baseline in the three groups; and 2) During tilt: a) the time to syncope was shorter in Group III than in group II (9.5 +/- 3 vs. 17 +/- 3 min p < 0.05); b) there was an immediate, persisting drop in mean BP in Group III; c) the decrease rate of left ventricular end-diastolic dimensions was greater in Group III than in Group II or Group I (-1.76 +/- 0.42 vs. -0.87 +/- 0.35 and -0.67 +/- 0.29 mm/min, respectively, p < 0.05); d) the leftventricular shortening fraction was greater in Group III than in the other two groups (39 +/- 1 vs. 34 +/- 1 and 32 +/- 1%, respectively, p < 0.05); and e) although the norepinephrine level remained comparable among the groups, there was a significantly higher peak epinephrine level in Group III than in Group II and Group I (112.3 +/- 34 vs. 77.6 +/- 10 and 65 +/- 12 pg/ml, p < 0.05).

CONCLUSIONS

Mechanisms of syncope during HUT appeared to be different in normal volunteers and patients with neurocardiogenic syncope. In the latter, there was evidence of an impaired vascular resistance response from the beginning of the orthostatic challenge. Furthermore, in the patients there was more rapid peripheral blood pooling, as indicated by the echocardiographic measurements of left ventricular end-diastolic changes, leading to more precocious symptoms. In syncopal patients, the higher level of plasma epinephrine probably mediated the increased cardiac contractility and possibly contributed to the impaired vasoconstrictive response.

Inadequate sympathovagal balance in response to orthostatism in patients with unexplained syncope and a positive head up tilt test

AIM

To analyse the immediate response of heart rate variability (HRV) in response to orthostatic stress in unexplained syncope.

SUBJECTS

69 subjects, mean (SD) age 42 (18) years, undergoing 60 degrees head up tilt to evaluate unexplained syncope.

METHODS

Based on 256 second ECG samples obtained during supine and upright phases, spectral analyses of low (LF) and high frequency (HF) bands were calculated, as well as the LF/HF power ratio, reflecting the sympathovagal balance. All variables were measured just before tilt during the last five minutes of the supine position, during the first five minutes of head up tilt, and just before the end of passive tilt.

RESULTS

Symptoms occurred in 42 subjects (vasovagal syncope in 37; psychogenic syncope in five). Resting haemodynamics and HRV indices were similar in subjects with and without syncope. Immediately after assuming the upright posture, adaptation to orthostatism differed between the two groups in that the LF/HF power ratio decreased by 11% from supine (from 2.7 (1.5) to 2.4 (1.2)) in the positive test group, while it increased by 11.5% (from 2.8 (1.5) to 3.1 (1.7)) in the negative test group (p = 0.02). This was because subjects with a positive test did not have the same increment in LF power with tilting as those with a negative test (11% v 28%, p = 0.04), while HF power did not alter. A decreased LF/HF power ratio persisted throughout head up tilt and was the only variable found to discriminate between subjects with positive and negative test results (p = 0.005, multivariate analysis). During the first five minutes of tilt, a decreased LF/HF power ratio occurred in 33 of 37 subjects in the positive group and three of 27 in the negative group. Thus a decreased LF/HF ratio had 89% sensitivity, 89% specificity, a 92% positive predictive value, and an 86% negative predictive value.

CONCLUSIONS

Through the LF/HF power ratio, spectral analysis of HRV was highly correlated with head up tilt results. Subjects developing syncope late during continued head up tilt have a decrease in LF/HF ratio immediately after assuming the upright posture, implying that although symptoms have not developed the vasovagal reaction may already have begun. This emphasises the major role of the autonomic nervous system in the genesis of vasovagal (neurally mediated) syncope.

Cerebrovascular mechanisms in neurocardiogenic syncope with and without postural tachycardia syndrome

BACKGROUND AND PURPOSE

Recent transcranial Doppler studies in patients with neurocardiogenic syncopes (NCS) have demonstrated that the cerebrovascular response to sudden systemic hypotension is vasoconstriction instead of compensatory vasodilation (autoregulation). We tried to characterize the conditions leading to this unexpected response in NCS patients further by continuously monitoring autoregulation and autonomic parameters during a standardized tilt-table test (TTT).

METHODS

Sixteen patients below the age of 50 years with a history of at least three syncopes of undetermined cause and tilt-table verified NCS and 20 normal controls were studied. Arterial blood pressure (ABP) and heart rate (HR) were monitored by Finapres and cerebral blood flow velocity (CBFV) of the left middle cerebral artery by transcranial Doppler. Baroreflex sensitivity and autoregulation parameters were measured continuously, using cross-spectral analysis of Mayer waves (3-9 cycles per minute oscillations) in ABP, HR and CBFV, respectively. Pulsatility indices (PI) of CBFV and ABP were determined continuously. Measurements were taken during 5 min in supine and during 5 min in tilted position. In patients, tilting was continued for a maximum of 45 min until the onset of syncope or presyncope.

RESULTS

According to the maximum increase in heart rate (deltaHR) during the first 5 min of standing, heart rate responses were classified as postural tachycardia syndrome (POTS) (deltaHR > 35/min) or as normal. Only one out of 20 control subjects showed a POTS (5%) in contrast to seven patients (44%). Patients with a POTS had significantly lower PI values in ABP and higher ratios between the PI of CBFV and the PI of ABP both in supine and in tilted positions. Baroreflex sensitivity during standing decreased significantly in POTS patients when compared to controls. Although autoregulation remained intact during standing, mean CBFV decreased significantly and continuously. The nine patients without a POTS showed almost the same cardiovascular and cerebrovascular responses as the control subjects. All 16 patients showed similar circulatory responses during syncope (sudden hypotension, relative or absolute bradycardia, reduced CBFV and increased PI in CBFV).

CONCLUSIONS

The development of a POTS during tilting indicates a high risk for fainting. The characteristic hemodynamic features in the initial phase of standing in these patients can be interpreted in terms of central hypovolemia (low PI of ABP) with sufficient ABP regulation and increased cerebrovascular resistance (defined as the ratio between PI of CBFV and ABP). Cerebral autoregulation seems not to be affected in patients suffering from NCS.

Sensitivity to orthostatic stress and beta-receptor activation in patients with isoproterenol-induced vasovagal syncope: a case controlled study

Cardiomotor and vasomotor responses were assessed during isoproterenol tilt-induced vasovagal reaction in patients with a history of syncope. In a case controlled study, all patients and controls were subjected to a standard protocol: baseline supine (10 min), baseline tilt (70 degrees, 45 min), isoproterenol supine (0.05 microgram/kg per min, 10 min), and isoproterenol tilt (70 degrees, 10 min). The participants were 11 consecutive patients referred for syncope evaluation (5 men, 6 women; mean age, 34.1 +/- 10.4 years; range, 18-56 years) and 11 age and sex matched controls (5 men, 6 women; mean age, 35.5 +/- 12.2 years; range, 19-63 years). On-line, beat-to-beat measurements of cardiomotor functions (heart rate, stroke volume, and cardiac output) and vasomotor functions (systolic, mean, and diastolic blood pressures and total peripheral resistance [TPR]) were detected noninvasively by volume clamp photoplethysmography and impedance cardiography. Patients and controls had similar cardiomotor and vasomotor responses during passive tilt and during isoproterenol infusion in the supine position. Immediately after tilt during isoproterenol infusion and before the onset of symptoms, decreases in vasomotor functions were significant in study patients when compared with those in controls; whereas responses in cardiomotor functions were similar between the two groups. When compared with baseline supine findings, TPR decreased by 56.5% +/- 10.9% and 29.5% +/- 23.3% in the patient population and controls, respectively (P = 0.005). When compared with isoproterenol supine findings, TPR decreased by 27.5% +/- 22.8% in the study patients and increased by 22.6% +/- 48.1% in the controls (P = 0.005). The inability to overcome isoproterenol-induced vasodilation during orthostatic stress played an important role in the initiation of a vasovagal response. These observations hold the key to early detection of hemodynamic changes and potential therapeutic interventions before patients become symptomatic.

Age-related changes of cardiac parasympathetic modulation after vasovagal syncope

The time-domain analysis of 24-hour electrocardiographic recordings showed that vagal modulation of heart rate is reduced within 48 hours from vasovagal syncope. However, patients with recent vasovagal syncope can be differentiated from healthy subjects only up to the age of 40 years with this analysis, because this parasympathetic modulation physiologically decreases with increasing age.

Rationale for assessing the effects of manipulative therapy on autonomic tone by analysis of heart rate variability

BACKGROUND

For more than 100 years, chiropractors have asserted that overall health can be improved through the use of spinal manipulative therapy. The autonomic nervous system is known to control and regulate all involuntary physiologic activities by controlling the activities of the internal organs, glands, and circulation. Recent studies document a potential relationship between the vertebral subluxation complex, autonomic tone, and cardiac function.

OBJECTIVE

This discussion reviews how it is possible to use heart rate variability analysis to calculate a quantitative index of autonomic function, which accurately reflects the sympathetic and parasympathetic tone and the sympathovagal balance.

DISCUSSION

The technique of heart rate analysis known as heart rate variability could be extremely useful in assessment of treatment outcomes in clinical chiropractic practice. At present, heart rate variability is in widespread use in the fields of neurology, cardiology, psychology, psychophysiology, obstetrics, anesthesiology, and psychiatry.

CONCLUSION

Further studies in this area may lead to a better understanding of the effects of spinal manipulation on (1) the general health of an individual, (2) an individual's susceptibility to lowered immunity and recuperative capacity, and (3) conditions that lie outside the scope of musculoskeletal therapeutics and are more in line with classical chiropractic concepts. This can also contribute to a better-informed interprofessional cooperation between allopathic and chiropractic health care providers.

Relationship between blood pressure and heart-rate variability during graded head-up tilt

To investigate the relationship between change in blood pressure and autonomic nerve activity, two types of head-up tilt experiments were performed. One was a 30 degrees, 45 degrees, 60 degrees, and 90 degrees graded tilt-up, in which tilt angles were changed at 6-min intervals and 5 min were spent at each angle. The other was a 10-min lasting head-up tilt at 60 degrees. Electrocardiogram (ECG) and blood pressure (Finapres) of 18 healthy non-smoking subjects (9 men, 9 women) were recorded during the experiments. Heart-rate variability was examined by general spectral analysis (GSA). The high-frequency/total-area ratio (HF/TO) showed a decrease as the tilt angle increased. Compared with the values at the 0 degrees position, these changes were statistically significant (P < 0.05). The low-frequency/HF ratio (LF/HF) showed a significant (P < 0.05) difference between 0 degrees and 90 degrees, and between 30 degrees and 90 degrees. Some of the subjects could not maintain their blood pressure during either of the head-up tilt experiments, and they showed only a slight change in HF/TO and LF/HF. This result confirmed that immediate responses to head-up tilt reflect autonomic nerve activity. Hence, changes in the frequency components were found to be an index of autonomic nerve activity, and they explained the individual differences observed in the ability to control blood pressure during a transition to upright posture.

Cardiac autonomic patterns preceding occasional vasovagal reactions in healthy humans

BACKGROUND

The wide range of clinical presentation of orthostatic vasovagal syncope suggests different underlying changes in the cardiac autonomic modulation.

METHODS AND RESULTS

To evaluate the beat-by-beat modifications in the neural control of heart period preceding a syncopal event, we studied RR interval variability in 22 healthy subjects who experienced fainting for the first time during a 90 degrees head-up tilt and in 22 control subjects by means of time-variant power spectral analysis. Sympathetic and vagal modulations to the sinoatrial node were assessed by the normalized power of the low-frequency (LF, approximately 0.1-Hz) and high-frequency (HF, approximately 0.25-Hz) oscillatory components of RR variability. When the patients were supine, no differences were observed in the hemodynamic and spectral parameters of the 2 groups. During the tilt procedure, RR, LFNU, and HFNU (NU=normalized units) values were relatively stable in control subjects. During early tilt (T1), subjects with syncope had reduced RR intervals compared with control subjects. In 13 subjects with syncope, RR decreased while LFNU and LF/HF increased in the last minute of tilt before syncope (T2). Conversely, in the remaining 9 fainters, LFNU and LF/HF decreased from T1 to T2 and HFNU increased slightly.

CONCLUSIONS

Two different patterns may be recognized in the cardiac autonomic changes preceding an occasional vasovagal event, namely, one characterized by a progressive increase of the marker of cardiac sympathetic modulation up to the onset of syncope, the other by a sympathetic inhibition with an impending vagal predominance. The recognition of different pathophysiological mechanisms in fainters may have important therapeutic implications.

Abnormality of alpha-adrenergic vascular response in patients with neurally mediated syncope

Although diagnosis of neurally mediated syncope (NMS) using the head-up tilt (HUT) test has been established, the precise etiologic mechanism of NMS is still obscure. Previously, we reported the contribution of impaired alpha-adrenergic vascular response to syncope in patients with various arrhythmias. This study evaluates alpha-adrenergic vascular response in 21 NMS patients with syncope and a positive HUT test (80 degrees, 30 minutes, and low-dose isoproterenol, NMS group, mean age 31 +/- 14 years) and 21 control subjects (C group, 33 +/- 14 years) who had no evidence of syncope and no structural heart disease. After 30 minutes in a recumbent position, pharmacologic total autonomic blockade was attained using atropine and propranolol. Thereafter, increased systolic blood pressure with 0.4 microg/kg/min phenylephrine (designated as deltaBPphenyl) and decreased systolic blood pressure with 0.5 microg/kg/30 seconds of phentolamine (designated as deltaBPphent) were measured as indexes of alpha-adrenergic vascular sensitivity and activity, respectively. DeltaBPphenyl in the NMS group (70.0 +/- 37) was significantly less than that in C group (107 +/- 38, p <0.005). DeltaBPphent was significantly greater in the NMS group than in the C group (33.5 +/- 10 vs 21.0 +/- 14, p <0.005). Thus, decreased alpha-adrenergic vascular sensitivity and elevated alpha-adrenergic vascular tone were observed in patients with NMS. Although it is not known whether the mechanism causing NMS can be attributed to this abnormal alpha-adrenergic vascular response, the abnormality could at least contribute to augmenting the symptoms of NMS.

Neurally mediated hypotension and autonomic dysfunction measured by heart rate variability during head-up tilt testing in children with chronic fatigue syndrome

Recent investigations suggest a role for neurally mediated hypotension (NMH) in the symptomatology of chronic fatigue syndrome (CFS) in adults. Our previous observations in children with NMH and syncope (S) unrelated to CFS indicate that the modulation of sympathetic and parasympathetic tone measured by indices of heart rate variability (HRV) is abnormal in children who faint during head-up tilt (HUT). In order to determine the effects of autonomic tone on HUT in children with CFS we performed measurements of HRV during HUT in 16 patients aged 11-19 with CFS. Data were compared to 26 patients evaluated for syncope and with 13 normal control subjects. After 30 minutes supine, patients were tilted to 80 degrees for 40 minutes or until syncope occurred. Time domain indices included RR interval, SDNN, RMSSD, and pNN50. An autoregressive model was used to calculate power spectra. LFP (.04-.15 Hz), HFP (.15-.40Hz), and TP (.01-.40Hz). Data were obtained supine (baseline) and after HUT. Thirteen CFS patients fainted (CFS+, 5/13 pure vasodepressor syncope) and three patients did not (CFS-). Sixteen syncope patients fainted (S+, all mixed vasodepressor-cardioinhibitory) and 10 did not (S-). Four control patients fainted (Control+, all mixed vasodepressor-cardioinhibitory) and nine did not (Control-). Baseline indices of HRV were not different between Control+ and S+, and between Control- and S-, but were depressed in S+ compared to S-. HRV indices were strikingly decreased in CFS patients compared to all other groups. With tilt, SDNN, RMSSD, and pNN50 and spectral indices decreased in all groups, remaining much depressed in CFS compared to S or control subjects. With HUT, sympathovagal indices (LFP/HFP, nLFP, and nHFP) were relatively unchanged in CFS, which contrasts with the increase in nLFP with HUT in all other groups. With syncope RMSSD, SDNN, LFP, TP, and HFP increased in S+ (and Control+), suggesting enhanced vagal heart rate regulation. These increases were not observed in CFS+ patients. CFS is associated with NMH during HUT in children. All indices of HRV are markedly depressed in CFS patients, even when compared with already low HRV in S+ or Control+ patients. Sympathovagal balance does not shift toward enhanced sympathetic modulation of heart rate with HUT and there is blunting in the overall HRV response with syncope during HUT. Taken together these data may indicate autonomic impairment in patients with CFS.

Autonomic control of vasovagal syncope

In the pathophysiological study of vasovagal syncope, the nature of the interaction between baroreceptor sensitivity (BS), sympathetic withdrawal, and parasympathetic activity has yet to be ascertained. Altered BS may predispose toward abnormal sympathetic and parasympathetic responses to orthostasis, causing hypotension that may progress to syncope if there is sympathetic withdrawal. To examine this hypothesis, we monitored blood pressure (BP), heart rate (HR), BS, forearm blood flow, and muscle nerve sympathetic activity (MNSA) continuously in 18 vasovagal patients during 60 degrees head-up tilt, syncope, and recovery. Results were compared with those of 17 patients who were able to tolerate tilt for 45 min. During early tilt, BP was maintained in both groups by an increase in HR and MNSA from baseline (P < 0.01), but BS decreased more in the syncopal group (P < 0.05). At the start of presyncope (mean 2.7 +/- 0.2 min before syncope and 15.2 +/- 12 min after tilt), when BP fell, HR and sympathetic activity remained increased from baseline (P < 0.01). Thereafter, BP and HR correlated directly with sympathetic activity and regressed in linear fashion until syncope (P < 0.001), whereas BS increased to baseline. At syncope, BP, HR, and sympathetic activity fell below baseline (P < 0.01, P < 0.05, and P < 0.01, respectively), but BS did not increase. During recovery, sympathetic activity increased to baseline and BS increased (P < 0.05), whereas HR and BP remained low (P < 0.01 and P < 0.05, respectively). The mechanism for the initiation of hypotension during presyncope remains unknown, but BS may contribute. Vasodilatation and bradycardia during presyncope appear to be more closely related to withdrawal of sympathetic activity than to increased parasympathetic cardiac activity.

Reproducibility of tilt table testing in patients with vasovagal syncope and its relation to variations in autonomic nervous system activity

To assess the variability of head-up tilt table testing, 35 patients with vasovagal syncope, shown by a positive tilt table test, underwent a second test 1 week later. Also, on the day before each test, spectral and time-domain indexes of heart rate variability were derived from Holter recordings to examine the stability of autonomous nervous system activity in these patients. Fifteen healthy volunteers served as a control group and also underwent two tilt table tests with prior Holter monitoring. Twenty-one (60%) of the 35 patients had a second positive test. None of the patients in the control group experienced syncope during either of the tests. The heart rate variability measures in the control group varied slightly from 1 day to the other, in contrast to the syncopal patients, where only low frequency spectral power and the mean of all 5-minute standard deviations of RR intervals showed comparable behavior. The indexes which reflect parasympathetic activity exhibited significant fluctuations in the syncopal patients. These fluctuations were due entirely to the patients who did not reproduce the outcome of the tilt table test, where high parasympathetic tone was associated with the positive test and normal parasympathetic tone with the negative test. In contrast, the patients with two positive tests had high parasympathetic tone during both test periods, with low individual variability. In conclusion, patients with vasovagal syncope show variations in vagal autonomic tone and appear to be more prone to syncope when their parasympathetic tone is elevated.

Spectral analysis of heart rate variability during tilt-table testing in patients with vasovagal syncope

Spectral analysis of heart rate variability was used to assess changes in autonomic function in 44 patients with vasovagal syndrome and 20 normal controls before and during postural tilt and to attempt to relate such changes to specific types of haemodynamic response to tilt. Frequency domain measurements of the high (HF) and low (LF) frequency bands and the ratio LF/HF were derived from Holter recordings, computed by Fast Fourier Analysis for 4 min intervals immediately before tilt testing, immediately after tilting and just before the end of the test. In the syncopal patients the mean values of LF and HF decreased significantly in response to tilting, while the LF/HF ratio remained constant. All parameters showed a statistically significant increase just before the onset of syncope. In the control group there was an increase in the LF and LF/HF ratio and a decrease in the HF immediately after tilting. The three subgroups of patients had similar patterns of changes in autonomic activity. The results of this study show that syncopal patients have a different pattern of response to the tilting test. The pathological mechanism leading to vasovagal syncope appears to be independent of the specific type of haemodynamic response to tilt testing.

Comparison of heart rate variability in patients with chronic fatigue syndrome and controls

Recent studies have reported a close association between chronic fatigue syndrome and neurally mediated hypotension. We hypothesized that this association may result from an abnormality in autonomic function among patients with chronic fatigue syndrome, which may be detectable using an analysis of heart rate variability. We prospectively studied 19 patients who fulfilled the Centers for Disease Control criteria for chronic fatigue syndrome and 11 controls. Each subject underwent a two-stage tilt-table test while wearing a Holter monitor. Heart rate variability was assessed in the supine baseline position and during upright tilt using frequency domain parameters. In the baseline supine position, high frequency (HF) power, low frequency (LF) power, and the ratio of low frequency power to high frequency power (LF/HF ratio) were similar. In both patient groups, upright tilt resulted in a similar decrease in HF power, increase in LF power, and increase in the LH/HF ratio. In conclusion, autonomic function, as assessed using an analysis of heart rate variability, does not differ in the baseline supine state, nor in response to upright tilt among patients with chronic fatigue syndrome and healthy controls.

Assessment of autonomic function at rest and during tilt testing in patients with vasovagal syncope

This study evaluated autonomic nervous system function in 30 patients with syncope and a positive tilt test result, 20 with a negative test result, and 20 healthy controls. Indexes of heart rate variability were measured during the intervals immediately before and after tilt, while all subjects were asymptomatic, and over a 24-hour period. There were no significant differences among the groups in any of the indexes of heart rate variability over the 24-hour period. In patients with a positive tilt result, tilting caused a decrease in low-frequency (LF) and high-frequency (HF) bands, although the LF/HF ratio did not significantly change. In patients with a negative tilt result there was a decrease in the HF band but no other significant changes. In the controls there was an increase in the LF band and LF/HF ratio and a decrease in the HF band. Our findings showed that patients with vasovagal syncope have no chronic differences from normal subjects in autonomic nervous system activity, but that these patients respond differently to the orthostatic stimulus.

A novel psychophysiological treatment for vasovagal syncope

The objective of this study was to evaluate the efficacy of transactional psychophysiological therapy (TPT) in a patient with recurrent vasovagal syncope (VVS) and to quantify the capacity of human dialogue to effect significant and consistent measurable therapeutic cardiovascular (CV) changes. A 31-year-old nurse with recurrent VVS and a reproducibly abnormal tilt-table test was refractory to pharmacological and conventional psychiatric treatments. She was treated with TPT. Her CV responses during psychotherapy were incorporated into the dialogue as an important source of communicative information, and she was taught psychophysiological techniques to correct exaggerated CV responses. These responses, during 16 weekly and 12 subsequent monthly sessions, were analysed using a one-way multiple analysis of variance. As TPT progressed, the magnitude and lability of CV responses as well as frequency of VVS were reduced. She has been relatively asymptomatic for 14 years posttherapy. In conclusion, (1) TPT may be an effective primary/adjunctive treatment for patients with VVS; (2) TPT may reduce syncopal episodes, perhaps by normalizing limbic input to the brainstem baroreflex system.

Vagal control of heart rate variability in vasovagal syncope: studies based on 24-h electrocardiogram recordings

The autonomic nervous system has an important role in the pathophysiology of vasovagal syncope. The purpose of this investigation was to evaluate to what extent the autonomic system is involved in the mechanism of fainting and to characterize the profile of heart rate variability of individuals who are prone to undergo a critical event. Thirty patients (mean age 41 years) with vasovagal syncope and 15 comparable controls were monitored by 24-h electrocardiography. Heart rate variability was analysed over the whole 24 h and during the daytime and night-time using time domain parameters (average of heart periods, RR; standard deviation of heart periods, SDNN; standard deviation of the average of RR intervals over all the 5-min segments of the entire recording, SDANN; percentage of the total number of all RR intervals of pairs of adjacent RR intervals differing more than 50 ms over the entire recording, pNN50; the square root of the sum of the square of differences between adjacent RR intervals, rMSSD), as indicated by the Task Force for Clinical Use of Standard Measurements of Heart Rate Variability. These parameters explore the influence of the autonomic nervous system on sinus node function and provide information about the vagal control to the heart. Among these parameters, pNN50 and rMSSD were significantly reduced in individuals with vasovagal syncope when compared with controls, over each time period considered. The two parameters are related to high-frequency oscillations in the frequency domain and to the vagal influence of the heart. These results indicate that the vagal tone to the heart is altered in subjects who suffer from vasovagal syncope. The results of this study provide an insight into the pathophysiological mechanism of fainting and may offer another means of evaluating patients with syncope.

Neurally mediated syncope: a review of cardiac and arterial receptors

This article reviews the basic physiology of the reflexogenic areas of the heart and the mechano- and baroreceptors that regulate cardiovascular and autonomic homeostasis, all of which contribute to our understanding of the pathogenesis of neurally mediated syncope. The mechanisms of neurally mediated syncope may involve excessive activation of ventricular receptors that trigger severe hypotension and bradycardia. Thus, neurally mediated syncope may be the clinical expression of the Bezold-Jarisch reflex, which occurs in situations of increased sympathetic activity, perhaps as a result of heightened cardiac receptor sensitivity. The arterial baroreceptors exert a ubiquitous influence on the heart and circulation, and serve primarily to buffer transient changes in arterial pressure by transmitting sensory information regarding their stretch to the central nervous system. This information, in conjunction with cardiac receptor input, elicits alterations in neural efferent output from sympathetic and parasympathetic fibers to provide subtle, continuous regulation of beat-to-beat cardiovascular hemodynamics to an array of physiologic and psychological stressors.

Autonomic nervous system activity during tilt testing in syncopal patients, estimated by power spectral analysis of heart rate variability

Spectral analysis of heart rate variability (HRV) was used to assess changes in autonomic function before and during postural tilt in 28 syncopal patients: 14 (group A) with positive and 14 (group B) with negative tilting test, and 14 normal controls (group C). Frequency-domain measurements of the high (HF) and low (LF) frequency bands and the ratio LF/HF were derived from Holter recordings, computed by Fast Fourier analysis for 4-minute intervals immediately before tilt testing, immediately after tilting, and just before the end of the test. In group A, the mean values of LF and HF decreased slightly in response to tilting while the LF/HF ratio increased, though these changes were not statistically significant. All parameters showed a statistically significant increase just before the onset of syncope. In group B, there were no significant changes in the parameters measured throughout the test. In group C, there was an increase in the LF and LF/HF ratio and a decrease in the HF immediately after tilting. There were no further significant changes in any of the parameters during the test. Syncopal patients have a different pattern of response to the orthostatic stimulus, in that they do not show the increase in sympathetic tone observed in normal individuals immediately after tilting. In the patients with a positive tilt test, there is a shift in the balance of ANS activity towards the sympathetic system shortly before the onset of syncope.

Pathophysiologic basis for vasodepressor syncope

The current knowledge regarding the pathophysiologic basis of the vasodepressor response was reviewed. The balance of evidence indicates that the mechanoreceptor hypothesis seems unlikely to be the sole afferent alteration that leads to the vasodepressor response. Alternative afferent mechanisms should include neurohumoral mediated sympathoinhibition triggered by opioid mechanisms as well as impaired endothelial and NO responses to orthostatic stress in susceptible individuals. It is possible that impaired cardiovagal and sympathetic outflow control of arterial baroreceptors is enhanced by the aforementioned mechanisms. The role of central sympathoinhibition and vagal excitation triggered directly from pathways within the temporal lobe or triggered by alterations in regional cerebral blood flow should be considered as potential alternative mechanisms. Efferent autonomic outflow during vasodepressor syncope include sympathetic neural outflow withdrawal in addition to activation of parasympathetic outflow to the heart and abdominal viscera. Further human research is needed to understand the underlying mechanisms that result in the described neural and vascular responses.

Neural monitoring of vasovagal syncope

Head-up tilt testing has become a valuable and widely accepted diagnostic tool for evaluation of patients with vasovagal syncope. This test has afforded clinical researchers the opportunity to focus on the hemodynamic, humoral, and neural changes that accompany syncope. We review the animal and clinical studies that provide insight into the possible pathophysiological mechanisms involved in vasovagal syncope. Hemodynamic measurements in patients with vasovagal syncope suggest that a relative decrease in ventricular size and increase in cardiac contractility may be seen in many patients with vasovagal syncope. Patients with vasovagal syncope have also demonstrated numerous "exaggerated" neurohumoral responses to syncope. Differential changes in plasma levels of epinephrine, renin, endothelin, vasopressin, cortisol, prolactin, beta endorphins, and substance P have been reported by some investigators either prior to or during a syncopal episode in patients with vasovagal syncope. The precise pathophysiological significance of these measurements is unknown at the present time. Measurements of autonomic tone may be accomplished indirectly with analysis of heart rate variability or baroreflex slope, or directly by sympathetic neural recordings of the peroneal nerve. We have demonstrated decreased baroreflex slopes in patients with vasovagal syncope. Using microneurography, we and others have demonstrated decreased sympathetic nerve activity occurring 11 +/- 3 seconds prior to syncope during head-up tilt table testing. A variety of other abnormal reflexes, including blunted forearm blood flow responses during exercise, have been demonstrated by others. These observations suggest that pacing instituted after the event may not be as helpful as the use of a hemodynamic sensor that will result in the initiation of pacing prior to sympathetic withdrawal or modify the decrease in sympathetic tone that occurs prior to syncope.

Presyncopal sympathetic withdrawal is the same in patients with vasodepressor syncope and controls who faint on head-up tilting

Head-up tilt provokes vasodepressor syncope in patients with this disorder but may also cause fainting in unaffected subjects. The aims of this study were to examine autonomic function and sequential changes in heart rate variability and plasma catecholamines during graded head-up tilt in patients with vasodepressor syncope compared with healthy subjects. Studies were performed in 10 patients and 15 control subjects. Eight negative controls completed the study; presyncope or syncope developed in seven positive controls and all 10 patients. The negative control group showed a progressive increase in mid-frequency from the supine position to end tilt. Patients and positive controls showed significant and similar falls in mid-frequency in the presyncope period. The rise in plasma norepinephrine was blunted in patients and positive controls, whereas plasma epinephrine increased more in these groups compared with the negative control group. In conclusion, the patterns of heart rate variability and catecholamine changes could not be distinguished in patients and positive control subjects.

Baroreflex sensitivity in patients with vasovagal syncope

BACKGROUND

In the present study, we tested the hypothesis that baroreflex sensitivity is reduced in patients with vasovagal syncope compared with normal control subjects.

METHODS AND RESULTS

We investigated 30 patients with vasovagal syncope (mean age, 43.6 +/- 16.7 years; 14 men and 16 women) and 32 normal control subjects (mean age, 41.8 +/- 17.0 years; 24 men and 8 women). Cardiopulmonary baroreceptor sensitivity was assessed by measuring the change in forearm vascular resistance during subhypotensive lower body negative pressure (LBNP). Carotid baroreflex sensitivity was assessed by measuring the change in RR interval during the manipulation of carotid transmural pressure. Phenylephrine baroreceptor sensitivity was assessed on the basis of the linear regression slope of the RR interval versus systolic blood pressure during the increment in blood pressure after intravenous administration of phenylephrine. In patients with vasovagal syncope, during the application of -10 mm Hg LBNP, forearm vascular resistance decreased by 0.7 +/- 11.6 U versus an increase of 8.3 +/- 6.2 U in control subjects (P = .002). Phenylephrine baroreceptor sensitivity was 11 +/- 7 ms/mm Hg in patients versus 14 +/- 6 ms/mm Hg in control subjects (P = NS). Carotid baroreflex sensitivity was 4 +/- 6 versus 4 +/- 2 ms/mm Hg in patients and control subjects, respectively (P = NS).

CONCLUSIONS

In patients with vasovagal syncope, during the application of subhypotensive LBNP, there is impaired forearm vasoconstriction or paradoxical forearm vasodilation. This suggests impaired cardiopulmonary baroreceptor inactivation or paradoxical activation of these receptors and is consistent with reduced cardiopulmonary baroreceptor sensitivity.

Reproducibility of time-domain indexes of heart rate variability in patients with vasovagal syncope

The aim of this study was to examine whether the indexes of heart rate variability (HRV) are stable from day-to-day in patients with vasovagal syncope and whether the stability of the HRV indexes is linked with that of the clinical results of the tilt test. Nineteen patients with a history of syncopal episodes and a positive tilt test underwent a second test 1 week later. Of these, 11 (group P-P) also had a positive second test, whereas 8 (group P-N) had a negative second test. Fifteen healthy volunteers served as a control group. Five time domain indexes were derived: the mean of all coupling intervals between normal beats (mean NN), the SD about the mean of all coupling intervals between normal beats (SDNN), the mean of all 5-minute standard deviations of NNs (SD), the proportion of adjacent normal RR intervals differing by >50 ms (pNN50), the root-mean-square of the difference between successive RRs (rMSSD) and the standard deviations of 5-minute mean NN intervals (SDANN). The control group showed good reproducibility of all HRV indexes (slope 0.86 to 0.97). The syncopal patients taken as a whole had significantly less reproducibility than the controls in the pNN50 parameter. This difference was due entirely to the patients in the P-N group, who had a remarkable lack of reproducibility in both the pNN50 and rMSSD measures (slope pNN50, 0.52; rMSSD, 0.78), whereas the P-P group had a reproducibility of all HRV indexes, which was no different from that in controls (slope 0.83 to 1.04). In patients with vasovagal syncope, certain HRV measures that express parasympathetic tone did not show the high reproducibility found in normal subjects. Syncopal patients who lack reproducibility in these HRV parameters also show a lack of reproducibility in the clinical result of tilt testing.

Vasopressin release during orthostatic hypotension after cardiac transplantation

At the time of cardiac transplantation all nerves from the donor ventricles are cut. These nerves may regrow, but there is no method of measuring any regrowth. Arginine vasopressin (AVP) release was studied during hypotension induced by head-up tilt and lower body negative pressure (LBNP) in transplant recipients and in normal controls. Subjects were tilted to 60 degrees for up to 60 min or until symptomatic. Lower body negative pressure (40 mmHg) was applied for 10 min after 30 min rest. Seven of 17 transplant recipients and 11 of 12 controls became symptomatic during tilt testing, and 9 of 12 controls and 9 of 17 transplant recipients became symptomatic after 10 min of LBNP. Symptoms during tilt did not predict symptoms during LBNP. Resting AVP levels were similar but osmolality was greater in transplant recipients. Resting haematocrit was reduced, and atrial natriuretic peptide increased in transplant recipients, suggesting increased plasma volume. In symptomatic subjects, changes in humoral concentrations were similar when compared between transplant recipients and normals, except that the rise in AVP at the time of symptoms was reduced in transplant recipients, with a comparable drop in blood pressure consistent with persistent cardiac afferent denervation in a subset of transplant recipients.

Head-up tilt table test: how far and how long?

There is great variation between laboratories in the angle and duration of tilt used in the head-up tilt table test. We compared the hemodynamic response to three sequential randomized 20-min tilts of 60 degrees, 80 degrees and 90 degrees in 20 normal subjects. We studied 66 patients to determine the duration of tilt needed to document three types of orthostatic intolerance. Blood pressure and heart rate were monitored non-invasively. Thirty-nine patients with orthostatic hypotension of diverse causes and 17 patients with orthostatic tachycardia plus were tilted to 90 degrees for 10 min. Ten patients with vasovagal syncope were tilted to 90 degrees for 30 min. A multivariate analysis of variance performed to assess the hemodynamic responses to 60 degrees, 80 degrees and 90 degrees tilts did not show a statistically significant difference (F = 0.459). Logistic regression analyses revealed that the duration of tilt required to document orthostatic hypotension was 5 min (73% cases correctly identified), orthostatic tachycardia plus 10 min (86.5% cases correctly identified) and neurally mediated syncope 10 min (86.5% cases correctly identified). Tilt duration is a more important variable than tilt angle and the duration of the tilt should depend on the suspected cause of orthostatic intolerance.

Abnormal reflex venous function in patients with neuromediated syncope

OBJECTIVES

We sought to compare the forearm reflex venous response to mental arithmetic stress in patients with neuromediated syncope and in normal subjects.

BACKGROUND

Patients with neuromediated syncope have a paradoxic arterial vasodilation in response to stressors that usually provoke vasoconstriction. Given the postulated role of diminished preload in provoking the reflex responses resulting in syncope, we hypothesized that mental stress might provoke paradoxic reflex venodilation in patients with neuromediated syncope.

METHODS

Twelve normal subjects (mean age [+/-SD] 47 +/- 9 years) and 27 patients with neuromediated syncope (mean age 42 +/- 13 years) were studied before and during a mental arithmetic stress test. Forearm venous pressure-volume relations were determined by using radionuclide plethysmography.

RESULTS

During mental arithmetic stress, heart rate and systolic and diastolic blood pressure increased significantly and similarly both in normal subjects and in patients with neuromediated syncope. The heart rate and blood pressure changes were qualitatively similar in both groups. However, with mental arithmetic stress, forearm venoconstriction of 13 +/- 2% (mean +/- SEM) was noted in normal subjects (p < 0.001) but not in patients with neuromediated syncope (mean 2%, p = NS). This group response of patients with neuromediated syncope did not result from a lack of individual responses but occurred because these patients had a wide range of responses. The normal physiologic and methodologic variability of the method was +/- 4%. Thirteen of the 27 patients with neuromediated syncope had forearm venoconstriction of 14.5 +/- 6.8% during mental arithmetic stress, whereas 7 had paradoxic forearm venodilation of 14.6 +/- 8.8%, and 7 were considered nonresponders (-1.3 +/- 3.4%). Thus, 14 (52%) of the 27 patients with syncope did not have normal vasoconstriction in response to mental stress.

CONCLUSIONS

Patients with neuromediated syncope have an abnormal range of forearm venomotor responses to mental arithmetic stress. Reflex control of the veins may play an important role in the pathogenesis of neuromediated syncope.

Failure of reflex venoconstriction during exercise in patients with vasovagal syncope

BACKGROUND

In this study, we tested two hypotheses. First, we tested the hypothesis that reflex constriction of the venous capacitance beds in patients with vasovagal syncope is impaired during both subhypotensive lower-body negative pressure. Second, we proposed that splenic venoconstriction may be impaired during exercise in patients with vasovagal syncope.

METHODS AND RESULTS

We evaluated 25 patients with vasovagal syncope (age, 45.0 +/- 15.9 years; 12 men, 13 women) and 24 control subjects (age, 41.3 +/- 13.7 years; 16 men, 8 women). A nuclear technique was used to assess changes in forearm venous tone during lower-body negative pressure and in splenic venous volume during cycle exercise. Changes in forearm vascular resistance (FVR) during cycle exercise were assessed with a strain-gauge plethysmography technique. The percentage reduction in unstressed forearm vascular volume during lower-body negative pressure was similar in patients and control subjects (9.0 +/- 8.0% versus 9.7 +/- 5.9%, P=NS). During exercise, splenic venous volume decreased less in patients than in control subjects (15.8 +/- 21.7% versus 42.6 +/- 12.6%, P < .0001). FVR decreased by 2 +/- 32% in patients but increased 108 +/- 90% in control subjects (P < .0001). There was no relation between percentage change in splenic volume and percentage change in FVR during exercise in either patients or control subjects (r= -.06, P=NS and r= -.18, P=NS, respectively).

CONCLUSIONS

Patients with vasovagal syncope exhibit a failure of the normal increase in tone in the splenic capacitance bed and in forearm resistance vessels during dynamic exercise. Forearm venous tone increases normally during lower-body negative pressure.

How the two sides of the heart adapt to graded impedance to venous return with head-up tilting

OBJECTIVES

The study sought to probe whether the adaptation of the right ventricle to reduced preload may influence that of the left ventricle (interdependence) and whether and how this mechanism contributes to maintain an adequate pump function.

BACKGROUND

A study like this requires that subjects be normal, restraint of venous return be gradual, systolic function and diastolic filling and dimensions of either ventricle be monitored.

METHODS

Of 30 healthy men (mean [+/- SD] age 35 +/- 7 years) studied with Doppler echocardiography, 20 were studied in the supine position and after 20 degrees, 40 degrees and 60 degrees tilting for 10 min; the remaining 10 subjects were also studied at the same levels of tilting for 45 min.

RESULTS

At 20 degrees, heart rate, blood pressure and stroke volume were steady; the diastolic right ventricular area was reduced (p < 0.001); and the end-diastolic dimension of the left ventricle did not vary. Tilting at 40 degrees and 60 degrees increased heart rate and diastolic pressure, decreased systolic pressure and stroke volume and reduced the diastolic dimensions of both ventricles. At any tilting level, right and left peak early inflow velocities (E) were decreased, peak late velocities (A) were unchanged, and E/A ratios were reduced, suggesting that the atrial-ventricular pressure difference was diminished bilaterally and that the atrial contribution to ventricular filling was maintained. Tachycardia at 40 degrees and 60 degrees tilting was not associated with enhancement of left ventricular fiber fractional shortening or mean velocity of shortening for any corresponding end-systolic wall stress; changes in heart rate also did not correlate with those in fiber fractional shortening and velocity of shortening. The adaptive responses to the same degrees of tilting for a duration of 45 min were comparable to those at 10 min.

CONCLUSIONS

With moderate restraint of venous return, the left ventricle maintains filling and output in response to a reduction in right ventricular diastolic volume, which increases left ventricular compliance (interdependence), and to the pulmonary blood reservoir, which compensates for an immediate decrease in right ventricular stroke volume. The decreased lung blood volume would facilitate right ventricular ejection, resulting in a normal stroke output despite the reduced preload. Thus, mechanical adjustments fully compensate for moderate reduction of venous return. A more severe reduction requires chronotropic support for the maintenance of cardiac output. With prolongation of tilting time to 45 min, adaptive mechanisms do not become exhausted in normal persons.