Echocardiographic demonstration of decreased left ventricular dimensions and vigorous myocardial contraction during syncope induced by head-up tilt

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.

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.

Left ventricular geometry and function preceding neurally mediated syncope

BACKGROUND

Neurally mediated syncope has been associated with increased left ventricular (LV) fractional shortening (FS) during tilt testing, which is consistent with the hypothesis that the stimulation of LV mechanoreceptors leads to reflex hypotension and/or bradycardia. However, FS does not represent true LV contractility because of its dependence on afterload and preload.

METHODS AND RESULTS

To elucidate the role of increased contractility in the mediation of neurally mediated syncope, we compared echocardiographic measures of LV performance corrected for end-systolic stress (ESS) in 21 patients (13 women and 8 men) with unexplained syncope who had either positive (n=10) or negative (n=11) responses to a tilt-table test. Two-dimensional echocardiographic LV imaging was performed at baseline and during the initial 5 minutes of upright tilt. In the supine position, both groups had similar LV end-diastolic volume indexes, stroke volumes, FS, circumferential ESS, and afterload-independent measures of LV performance (stress-corrected midwall and FS). However, after 5 minutes of upright tilt, patients who subsequently had a positive test had a lower stroke volume, lower stress-corrected midwall shortening, and endocardial FS. The tilt-positive group also had a greater fall in ESS and FS early during upright tilt.

CONCLUSIONS

Reduced ESS, LV volume, and chamber function during initial upright tilt are associated with a subsequent positive tilt response in patients with unexplained syncope. These data suggest that if paradoxic activation of LV mechanoreceptors has a role in mediating neurally mediated syncope, it is not triggered by LV hypercontractility or increased systolic wall stress during the initial period of upright tilt.

A review of pathophysiology and therapy of patients with vasovagal syncope

Vasovagal syncope is a common disorder that can compromise quality of life and lead to significant morbidity. It is characterized by an initial exaggerated sympathetic output followed by parasympathetic activation and sympathetic withdrawal, as shown by diagnostic head-up tilt (HUT) table testing. Numerous drugs have been evaluated for treating this disorder. beta-Blockers are well studied and commonly administered but are specifically more efficacious in patients with isoproterenol HUT than in those with regular HUT. The role of the serotonergic system has captured new interest. Selective serotonin reuptake inhibitors show promising results in preventing vasovagal syncope in treatment-refractory patients. Also, new investigations suggest that serotonin receptor antagonism may be beneficial. Despite these findings, definitive treatment does not exist.

Influence of autonomic tone on the filtered QRS duration from signal averaged electrocardiograms in healthy volunteers

We recently reported that signal averaged electrocardiograms (SAECG) measurements possess a circadian rhythm and are closely related to heart rate or heart rate variability in healthy volunteers. This study determines the influence of autonomic tone on the filtered QRS (f-QRS) duration from SAECG by using pharmacologic autonomic blockade and exercise in healthy volunteers. Eleven healthy male volunteers were studied. Three protocols were designed to study the effects of exercise (Ex) under control conditions, beta adrenergic blockade or double blockade. SAECGs and heart rate variability (LF and HF: low and high frequency power, LF/HF ratio) were determined from Holter recordings. Ex significantly decreased the f-QRS duration and HF and significantly increased heart rate and LF/HF. Ex during beta adrenergic blockade significantly increased heart rate and decreased f-QRS duration and HF, but did not change LF/HF. Ex during double blockade did not affect the f-QRS duration, HF, or LF/HF. The changes in f-QRS duration induced during Ex, autonomic blockade, or both were inversely correlated with changes in heart rate and LF/HF and positively correlated with changes in HF. These data suggest that f-QRS duration in healthy subjects is shortened by Ex with increased sympathetic tone or decreased parasympathetic tone or the combination.

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.

Exercise-induced neurocardiogenic syncope: clinical data, pathophysiological aspects, and potential role of tilt table testing

The evaluation of syncope occurring during exercise or occurring spontaneously in highly trained individuals presents a unique diagnostic challenge. It is of critical importance to exclude potential life-threatening disorders such as hypertrophic cardiomyopathy, long QT syndrome, right ventricular dysplasia, anomalous coronary artery distribution, valvular heart disease, myocarditis, or exercise-induced arrhythmia. This review is not directed towards identifying, treating, or determining athletic eligibility of individuals with such disorders. Rather, we endeavour to discuss the pathophysiology of exercise-induced neurocardiogenic syncope and to address the role of head upright tilt testing in evaluating syncope in athletic individuals in whom proper evaluation has excluded the presence of ischaemic heart disease or primary structural or electrical heart disease.

Atrioventricular dissociation as a cause of syncope determined by head-up tilt test

This report describes a patient with syncopal attacks in a sitting position on a reclining seat, in whom atrioventricular dissociation due to accelerated ventricular rhythm was determined to be the cause by recording of the electrocardiogram, blood pressure, and mitral and aortic Doppler flow during a 60 degree head-up tilt test.

Midodrine hydrochloride in the treatment of vasovagal syncope

Therapy of vasovagal syncope is still a subject of debate. Various pharmacotherapies were proposed. However, they are often not tolerated or ineffective. The purpose of this prospective, nonrandomized study was to evaluate the usefulness of alpha-agonist midodrine hydrochloride in the treatment of vasovagal syncope. Forty-one patients (mean age 34 years, 18 men) with history of recurrent syncope and positivity of head-up tilt testing were included (28 patients with type 1, 10 patients with type 2, 3 patients with type 3 according to VASIS classification). In all patients oral therapy with midodrine was started. Initial dose was 2.5 mg two times daily. When necessary, the dose was increased to 5 mg two times daily. Efficacy of treatment was assessed by repeated head-up tilt testing after 1-2 weeks of therapy and by long-term follow-up. After midodrine hydrochloride treatment, 39 of 41 patients (95%) had no inducible presyncope or syncope on repeated tilt table testing. Effective dose was 2.5 mg two times daily in 25 patients and 5 mg two times daily in 16 patients. During a mean follow-up period 19+/-9 months, 38 of 39 patients (97%) with negative repeated tilt table test remained free of syncope recurrence.

Syncope in the pediatric patient. The cardiologist's perspective

The evaluation of syncopal children or adolescents relies heavily on a thorough, detailed history and physical examination. All syncope associated with exercise or exertion must be considered dangerous. The ECG is mandatory, but other laboratory tests are generally of limited value unless guided by pertinent positives or negatives in the history and physical examination. The ECG allows screening for dysrhythmias, such as Wolff-Parkinson-White syndrome, heart block, and long QT syndrome, as well as hypertrophic cardiomyopathies and myocarditis. Tilt table testing can be useful in selecting therapy by demonstrating the physiologic response leading to syncope in an individual patient. The most common type of syncope in otherwise healthy children and adolescents is neurocardiogenic or vasodepressor syncope, which is a benign and transient condition. Because syncope can be a predictor of sudden cardiac death, it must be taken seriously, and appropriate screening must be performed.

Effects of paroxetine hydrochloride, a selective serotonin reuptake inhibitor, on refractory vasovagal syncope: a randomized, double-blind, placebo-controlled study

OBJECTIVES

The purpose of the study was to determine whether the well tolerated serotonin reuptake inhibitor paroxetine hydrochloride could prevent vasovagal syncope in patients resistant to or intolerant of previous traditional therapies.

BACKGROUND

Serotonergic mechanisms play a major role in the processes leading to neurocardiogenic vasovagal syncope, and serotonin reuptake inhibitors have been reported to be effective in preventing refractory syncope.

METHODS

Sixty-eight consecutive patients (26 men and 42 women, mean age 44.7+/-16.5 years) with recurrent syncope and positive head-up tilt test and in whom standard therapies with beta-adrenergic blocking agents, vagolytic, negative inotropic or mineral corticoid agents were ineffectual or poorly tolerated were referred for study. Patients randomly received either paroxetine at 20 mg once a day or a placebo. A head-up tilt test was then reperformed after one month of treatment, and the clinical effect was noted over a mean follow-up of 25.4+/-7.9 months.

RESULTS

The response rates (negative tilt test) after one month of treatment were 61.8% versus 38.2% (p < 0.001) in the paroxetine and placebo groups, respectively. During follow-up spontaneous syncope was reported in six patients (17.6%) in the paroxetine group as compared to 18 patients (52.9%) in the placebo group (p < 0.0001). Only one patient (2.9%) asked to be discontinued from the drug for severe side effects.

CONCLUSIONS

Paroxetine was found to significantly improve the symptoms of patients with vasovagal syncope unresponsive to or intolerant of traditional medications and was well tolerated by patients.

Neurally mediated syncope: pathophysiology and implications for treatment

Neurally mediated syncope may occur in patients whose hemodynamic picture does not fit the characteristics of orthostatic intolerance as described elsewhere in this issue. Nonetheless, patients who suffer from neurocardiogenic or vasovagal syncope may be seriously incapacitated by their episodes of syncope or presyncope. Although it has been assumed that vagal activation as a result of stimulation of ventricular mechanoreceptors is essential to the production of these episodes, several critical observations are presented that suggest that other mechanisms may also be operative in some patient subsets. In addition, evidence is presented that the sympathetic responses of many of these patients may be reduced rather than increased and that abnormal baroreflex responsiveness may also play an causative role. These findings suggest new avenues for therapy in this field in which carefully controlled, randomized, double-blind trials are scarce.

The use of metoprolol and glycopyrrolate to prevent hypotensive/bradycardic events during shoulder arthroscopy in the sitting position under interscalene block

Sudden profound hypotensive and/or bradycardic events (HBE) have been reported in >20% of patients undergoing shoulder arthroscopy in the sitting position under interscalene block anesthesia. Retrospective studies suggest that the administration of beta-blockers is safe and may decrease the incidence of these episodes. We performed a randomized, prospective study to evaluate prophylaxis of these events. One hundred fifty patients were randomized to one of three groups (placebo; prophylactic metoprolol to achieve a heart rate <60 bpm or a maximal dose of 10 mg; or prophylactic glycopyrrolate to achieve a heart rate >100 bpm or a maximal dose of 6 microg/kg) immediately after the administration of the interscalene block. Blood pressure control was achieved with IV enalaprilat as needed. The incidence of HBE was 28% in the placebo group versus 5% in the metoprolol group (P = 0.004). The rate of 22% in the glycopyrrolate group was not significantly different from placebo. Preoperative heart rate and arterial blood pressure, intraoperative sedation score, IV fluids, and enalaprilat use were similar in those patients who had a HBE compared with those who did not. Many aspects of this clinical setting are similar to tilttable testing for patients with recurrent vasovagal syncope, in which beta-adrenergic blockade with metoprolol has also been shown to be effective. We conclude that the Bezold-Jarisch reflex is the most likely mechanism for these events.

IMPLICATIONS

Episodes of acute hypotension and bradycardia occur during shoulder arthroscopy in the sitting position under interscalene block. In this study, we demonstrate that metoprolol, but not glycopyrrolate, markedly decreases the incidence of these episodes when given prophylactically immediately after the administration of the block.

Use of sublingual nitroglycerin during head-up tilt-table testing in patients >60 years of age

Previous work had demonstrated a reduced specificity associated with head-up tilt protocols using high-dose isoproterenol in patients between 20 and 50 years of age. We evaluated the specificity of head-up tilt testing using different isoproterenol infusion doses and administration of nitroglycerin in patients aged >60 years. In addition, whether the same protocols have impact on the sensitivity of the test was also assessed. One hundred sixty subjects were included in this study. Seventy-six were volunteers randomized to either head-up tilt test with low-dose, 3- and 5-microg/min of isoproterenol (group I) or to a protocol including 0.4 mg of sublingual nitroglycerin (group II). In addition, after an upright tilt drug-free state, 58 patients with a history of syncope underwent repeat head-up tilt with increasing doses of isoproterenol infusion, followed by sublingual nitroglycerin if the test result remained negative. The remaining 33 patients were subjected to the nitroglycerin protocol after the drug-free state phase. In the control groups, the incidence of false-positive responses was 88% and 95%, respectively. In patients with syncope after a negative test result during 5 microg of isoproterenol infusion, nitroglycerin administration increased the number of positive responses from 45% to 79%. The percentage of positive tilt in patients undergoing nitroglycerin administration after the drug-free state part of the protocol was 78%. Administration of nitroglycerin was the most significant predictor of a positive upright tilt in patients with syncope. In subjects aged >60 years, head-up tilt protocols with high-dose isoproterenol infusion and nitroglycerin maintained an adequate specificity. In this subset of patients, nitroglycerin seemed to provide a better sensitivity than isoproterenol.

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.

Long-term effects of pharmacological therapy for vasovagal syncope on the basis of reproducibility during head-up tilt testing

The purpose of this study was to determine the efficacy of long-term pharmacological therapy selected on the basis of a head-up tilt test (HUT) in patients in whom reproducibility of the HUT response was demonstrable in the initial study. The HUT (80 degrees upright) was performed for 15 min with or without an infusion of isoproterenol (0.01-0.03 microgram/kg per min) in 54 patients with recurrent unexplained syncope. When vasovagal syncope was induced (positive response), the HUT was repeated to examine the test reproducibility. Vasovagal syncope was induced in 24 patients during HUT alone, and in 30 patients during the HUT with isoproterenol. Acute reproducibility was observed in 49/54 (91%) patients. In the tilt-positive patients, HUT was repeated after an intravenous administration of propranolol (0.1 mg/kg) or disopyramide (1 mg/kg) (acute test). Propranolol proved effective in 21 (80%) of 26 patients, and disopyramide in 13 (56%) of 23 patients. Thereafter, evaluation was done on the long-term clinical follow-up of the pharmacological intervention selected on the basis of the acute test in the 34 patients in whom the HUT could not induce vasovagal syncope after the oral administration of the pharmacological agent (propranolol 60 mg/day, disopyramide 300 mg/day). Thirty-two of 34 patients (94%) did not develop syncopal attacks during a 44 +/- 12-month period. Thus, in patients with unexplained syncope, HUT appears to have a high degree of acute reproducibility, and the acute drug response guided by HUT may be used to develop an effective long-term pharmacological therapy.

Head-up tilt test combined with isoproterenol infusion provokes coronary vasospastic angina

The association of the autonomic nervous system with coronary vasospasm has been controversial. The aim of the present study was to examine the involvement of the autonomic nervous system in coronary vasospasm by applying the head-up tilt (HUT) test to patients with coronary vasospastic angina. Fifteen consecutive patients with coronary vasospastic angina and without significant organic coronary stenoses underwent the HUT test. Prior to the test, coronary spasm was documented angiographically by using an intracoronary injection of acetylcholine or ergonovine. The HUT test was performed in the early morning and repeated in the afternoon if the test was positive in provoking angina pectoris and syncope or presyncope. If the test was negative, it was repeated under intravenous infusion of isoproterenol at a rate of 1-2 microg/min. The HUT test under isoproterenol infusion in the morning provoked vasospastic angina with syncope or presyncope in 9 of the 15 patients. In the test-positive group, heart rate was significantly reduced (104+/-17 beats/min to 84+/-25 beats/min, p<0.05), which preceded a reduction in systolic blood pressure (158+/-25 mmHg to 125+/-17 mmHg, p<0.001), angina attack and syncope. The HUT test without isoproterenol infusion in the morning and the HUT test in the afternoon with or without isoproterenol infusion failed to provoke angina. The heart rate reduction preceding reduced systemic blood pressure and anginal attack suggested that parasympathetic nerve excitation plays an important role in coronary vasospasm. The results also implied that the HUT test combined with isoproterenol infusion is useful for the provocation of coronary spasm.

[Diagnosis and treatment of vasovagal syncope in the child and adolescent]

Recurrent unexplained syncope is a common and often frustrating clinical problem in paediatrics. Over the last decade, head upright tilt table testing has emerged as an important diagnostic method for the identification of patients whose syncope is likely to be neurocardiogenic in origin. At the same time, tilt table testing, by providing syncopal episodes in a controlled setting, has allowed for a greater understanding of their physiopathology. Treatment strategies remain controversial but beta-blocker therapy appears to be very efficient.

Influence of body position on defibrillation thresholds of nonthoracotomy implantable defibrillators: a prospective randomized evaluation

INTRODUCTION

Defibrillation thresholds (DFTs) usually are determined with the patient in the supine position. However, patients may be in the upright position when a shock is delivered during follow-up, which may explain some first shock failures observed clinically. This study investigated whether body posture affects defibrillation energy requirements of nonthoracotomy implantable cardioverter defibrillators with biphasic shocks.

METHODS AND RESULTS

Using a step up-down protocol, DFTs were compared intraindividually in 52 patients ("active-can" sytems in 41 patients, two-lead systems in 11 patients) for the supine and upright positions as achieved by a tilt table. The mean DFT was 7.3 +/- 4.2 J in the supine versus 9.2 +/- 4.8 J in the upright position (P = 0.002). Repeated comparison in reversed order 3 months after implantation in 22 patients revealed thresholds of 6.2 +/- 2.5 J (supine) versus 8.4 +/- 3.7 J (upright; P < 0.03) 1 week and 4.4 +/- 2.4 J (supine) versus 6.2 +/- 4.1 J (upright; P < 0.04) 3 months after implantation. DFTs decreased significantly for both body positions from 1 week to 3 months after implantation (P < 0.04).

CONCLUSION

(1) DFTs for biphasic shocks delivered by nonthoracotomy defibrillators are higher in the upright compared to the supine body position. (2) Differences remain significant 3 months after implantation. For both body positions, DFT decreases significantly from 1 week to 3 months after implantation. These findings have important implications for programming first shock energy to lower than maximal values or for development of devices with lower maximal stored energy.

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.

Efficacy of intravenous granisetron in suppressing the bradycardia and hypotension associated with a rabbit model of the Bezold-Jarisch reflex

This study investigated whether granisetron, a 5-HT3 receptor antagonist, can alter the Bezold-Jarisch reflex (i.e., hypotension and inappropriate heart rate slowing). A hemorrhagic rabbit model that has been shown to induce the Bezold-Jarisch reflex was used. In 11 rabbits (3.8 kg), catheters were placed in the carotid arteries one day before experimental hemorrhage. On the day of the study, the rabbits were given intravenous granisetron (50 micrograms/kg) or an equal volume of saline. Five minutes after administration of granisetron or saline, hemorrhage was induced by continuous blood withdrawal at 5 mL/min and blood pressure (BP) and heart rates were obtained at frequent intervals until systolic BP declined to 80 mmHg. Six rabbits received saline and five granisetron. An average of 77.6 mL +/- 16.4 mL of blood was removed in the group receiving granisetron (compared with 56.5 mL +/- 13.1 mL for the saline group) before achieving the target systolic BP of 80 mmHg. The group receiving granisetron demonstrated the same ability to increase their heart rate from baseline as the saline group. However, the granisetron group had a final heart rate that was closer to their maximal heart rate than the saline group. In this animal model, granisetron was significantly more effective at preventing inappropriate heart rate slowing and allowed significantly more blood to be removed before reaching the target blood pressure. This implies that granisetron may be effective in preventing vasovagal syncope, although further study should be carried out to verify these potentially interesting findings.

A sensor-based evaluation of heart contractility in patients with head-up tilt-induced syncope

Studies using the head-up tilt test (HUT) suggest that a reflex increase in sympathetic activity resulting in vigorous myocardial contractions precedes neurally-mediated syncope (NMS). The aim of this study was to evaluate heart contractility changes during positive HUT. Ten patients with recurrent NMS and positive HUT were investigated. Before HUT we temporarily placed a standard right ventricular pacing electrode incorporating in its tip a recently developed microaccelerometer (Sorin Biomedica, Italy) that measures the peak endocardial acceleration (PEA) during the isovolumetric phase as an index of heart contractility. PEA potential amplitude, heart rate and mean blood pressure were continuously studied during HUT. Syncope occurred 16.7 +/- 10.3 min after 60 degrees tilt, either at baseline (8 patients) or after sublingual nitrate administration (2 patients). PEA value was stable at 0.62 +/- 0.34 (1G = 9.8 m/sec2) during the supine phase. It slightly increased to 0.72 +/- 0.44 G (p = NS) during the first minutes of 60 degrees tilt and then remained unchanged until a further increase of 71 +/- 79% (range 10 to 266%) as compared to tilt value (p = 0.004) at 2.8 +/- 2.4 min (range 0.25 to 6.5 min) before the syncope in 9 patients. The latter increase was not observed in the patient with dilated cardiomyopathy. In conclusion, a significant increase in heart contractility was observed in 9 patients in the minutes preceding HUT-induced NMS. These changes might be used for driving a rate adaptive pacemaker when cardiac pacing is indicated to prevent NMS.

Different humoral responses during head-up tilt testing among patients with neurocardiogenic syncope

Neurocardiogenic dysfunction is believed to result from activation of ventricular mechanoreceptors. To asses other humoral and circulatory mechanisms activated during vasovagal syncope, epinephrine, norepinephrine, renin, and aldosterone levels were measured during head-up tilt testing. Twenty-three patients referred because of vasovagal syncope underwent passive head-up tilt testing (80 degrees). Blood samples were taken at baseline, after 30 minutes of supine rest and at syncope. Five patients (four men, one woman; mean age 46 +/- 27 years) had cardioinhibitory syncope. Seven patients (five men, two women; mean age 40 +/- 12 years) had vasodepressor syncope. Eleven patients (eight men, three women; mean age 55 +/- 21 years) had negative results of head-up tilt tests. Among patients with cardioinhibitory syncope, norepinephrine concentration rose significantly from baseline to syncope (0.44 +/- 0.12 ng/ml versus 1.14 +/- 0.72 ng/ml; p < 0.05), whereas no significant change was observed in epinephrine (0.08 +/- 0.03 ng/ml versus 2.74 +/- 2.85 ng/ml; p = not significant [NS]), renin (5.68 +/- 3.03 pg/ml versus 19.58 +/- 11.47 pg/ml; p = NS), or aldosterone concentration (66.60 +/- 16.10 ng/ml versus 109.00 +/- 44.70 ng/ml; p = NS). Patients with vasodepressor syncope had a significant rise in renin (9.03 +/- 4.56 pg/ml versus 52.53 +/- 41.63 pg/ml; p < 0.05) and aldosterone concentration (95.43 +/- 103.03 ng/ml versus 249.57 +/- 191.54 ng/ml; p < 0.05), whereas no change in level of epinephrine (0.12 +/- 0.12 ng/ml versus 0.28 +/- 0.33 ng/ml; p = NS) or norepinephrine (0.60 +/- 0.26 ng/ml versus 0.86 +/- 0.53 ng/ml; p = NS) was detected. Among patients with negative results of tilt tests, levels of renin (7.94 +/- 7.19 pg/ml versus 27.71 +/- 18.50 pg/ml; p < 0.01) and aldosterone (64.64 +/- 28.33 ng/ml versus 160.91 +/- 79.58 ng/ml; p < 0.01) rose significantly, whereas no change was seen in epinephrine (0.12 +/- 0.14 ng/ml versus 0.23 +/- 0.31; p = NS) or norepinephrine concentration (0.54 +/- 0.21 ng/ml versus 0.82 +/- 0.52; p = NS). Patients with cardioinhibitory syncope were characterized by a rise in norepinephrine level and blunted activation of the renin-angiotensin-aldosterone axis at syncope. Unlike patients with cardioinhibitory syncope, the renin-angiotensin-aldosterone axis is activated in patients with vasodepressor syncope and patients with a negative result of head-up tilt test without a statistically significant increase in catecholamine levels. Patients with cardioinhibitory syncope have higher epinephrine levels at syncope compared with patients with a negative result of head-up tilt test and patients with vasodepressor syncope.

Hemodynamic analysis during isoproterenol-induced vasovagal syncope

An isoproterenol-mediated increase in cardiomotor tone and a decrease in afterload contribute to the induction of vasovagal syncope. Contrary to conventional belief, a significant decrease in preload is not observed immediately before isoproterenol-induced syncope.

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.

Prediction of head-up tilt test result by analysis of early heart rate variations

BACKGROUND

Head-up tilt testing is a useful test for investigating vasovagal syncope. The determination of early, accurate, predictive criteria for a negative result would permit a reduction in the duration of the tilt test.

METHODS AND RESULTS

Patients with no drug use and no illnesses other than recurrent unexplained syncope were recruited. In an initial study (110 consecutive patients), we aimed to determine a predictive criterion based on heart rate variations during the first minutes of upright tilting that could distinguish between patients with positive and negative tilt tests (patients with an early continual decrease in heart rate or blood pressure were excluded). Then we tested the predictive value of the established criterion in a second independent sample of patients with unexplained syncope (109 consecutive patients). An early sustained increase in heart rate < or = 18 bpm during the first 6 minutes of upright tilting at a 60 degree angle allowed us to predict negative tilt tests with 100% specificity, 100% positive predictive value, and 88.6% sensitivity. This criterion was validated in the second, prospective arm of the study (96.4% specificity, 98.4% positive predictive value, and 87.3% sensitivity), even with subsequent use of isoproterenol in low doses.

CONCLUSIONS

In patients with no drug use and no illnesses other than recurrent unexplained syncope, a simple clinical criterion identifies patients who will not develop syncope during a prolonged upright tilt.

Comparison of sensitivity and specificity of tilt protocols with and without isoproterenol in children with unexplained syncope

Head-up tilt testing with or without isoproterenol is extensively used in the evaluation of patients with unexplained syncope. However, sensitivity and specificity of tilt protocols with and without isoproterenol have not been clarified in children, due to lack of age matched control subjects. This study was designed to assess and to compare the sensitivity and specificity of tilting alone and tilting in conjunction with isoproterenol. Thirty children with unexplained syncope (group I) and 15 age-matched control subjects (control group I) underwent successive 60 degrees head-up tilts for 10 minutes during infusions of 0.02, 0.04, and 0.06 microgram/kg/min of isoproterenol, after a baseline tilt to 60 degrees for 25 minutes. Also, 35 children (group II) with unexplained syncope and 15 healthy control subjects (control group II) were evaluated by head-up tilt to 60 degrees for 45 minutes without an infusion of isoproterenol. In response to tilt protocol with graded isoproterenol, 23 (76.6%) of the patients in group I and 2 of the 15 (13.3%) control subjects developed syncope. Accordingly, the sensitivity of tilt testing with isoproterenol was 76.6%, and its specificity was 86.7%. Tilt testing without isoproterenol was positive in 17 (48.5%) of the patients in group II but in only 1 of the 15 (6.6%) control subjects. Thus, sensitivity and specificity of tilt testing without isoproterenol were 48.5% and 93.4%, respectively. The mean heart rate and systolic blood pressure decreased significantly (P < 0.001) in all tilt positive patients during syncope. In conclusion, the head-up tilt test is a valuable diagnostic test in the evaluation of children with unexplained syncope, and isoproterenol is likely to increase the sensitivity of the test without decreasing its specificity.

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.

Syncope in the elderly

Age-related physiologic changes and disease-related abnormalities predispose older adults to syncope. It is important to know the physiologic changes that occur with normal aging to understand better their interaction with disease processes and to facilitate better evaluation and treatment of syncope when it occurs. Attention to situational stresses, such as posture changes, meals, or medications, is also likely to increase the diagnostic yield and improve therapeutic strategies that can reduce morbidity and potential mortality of recurrent episodes. Therapy should be directed toward minimizing multiple factors that contribute to syncope, avoiding iatrogenic medication effects, and treating specific contributory diseases.

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 circulatory control in vasovagal syncope

The orthostatic volume displacement associated with the upright position necessitates effective neural cardiovascular modulation. Neural control of cardiac chronotropy and inotropy, and vasomotor tone aims at maintaining venous return, thus opposing gravitational pooling of blood in the lower part of the body. The present concept of the vasovagal response or "common faint" implicates the development of inappropriate cardiac slowing due to sudden augmentation of efferent vagal activity, and arteriolar dilatation by sudden reduction or cessation of sympathetic activity. The venous pooling associated with lasting orthostatic stress results in development of central hypovolemia. At a certain point during the ongoing reflex adaptation to the hypovolemia in progress, a depressor reflex is set in train. The depressor reflex input along this second "peripheral" afferent pathway is postulated to originate from various sites in the cardiovascular system but remains uncertain. The common faint in humans is of both vaso- and vagal origin; the pure vagal response is less common than its vasodepressor variant. There is strong evidence for an early loss of vasomotor tone in the majority of fainting subjects. Blocking the vagus nerve or cardiac pacing is not of much help in preventing vasovagal syncope; though atropine or pacing may prevent bradycardia in vasovagal fainting, they have never been proven to prevent hypotension. Baroreflex modulation of autonomic outflow remains present during the presyncopal stages until it becomes offset by an opposing depressor reflex with relative bradycardia and relaxation of arterial resistance vessels. The nature of the vasodilatation associated with the vasovagal response has still not been settled.

Response of neurocardiac syncope to beta-blocker therapy: interaction between age and parasympathetic tone

Beta-blockers are a first line therapy for neurocardiac syncope, but are not always effective. The purpose of this study was to determine whether differential autonomic responses to orthostasis predict the response of patients with neurocardiac syncope to beta-adrenergic blockade. We computed the RMS successive difference of the RR intervals (RMSSD: a measure of cardiac parasympathetic tone) during supine and upright phases of the initial tilt test in 28 patients with syncope and positive tilt tests who were treated with atenolol. Follow-up tilt testing was performed to assess the efficacy of the drug in preventing tilt induced neurocardiac syncope. RMSSD did not differ at baseline (supine) between those who did (n = 20) and did not (n = 8) respond to beta-blockade. However, withdrawal of parasympathetic tone in response to tilt varied inversely with age (r = -0.69; P < 0.01). Reduced age adjusted parasympathetic withdrawal during orthostasis was associated with a 47% versus 8% risk of beta-blockade failure (odds ratio = 11; P = 0.01). Patients with diminished age adjusted parasympathetic withdrawal during orthostatic stress are less likely to respond to beta-blocker therapy of neurocardiac syncope than their counterparts. This may reflect a correspondingly greater sympathetic response to orthostasis in these patients, but the mechanism for this interaction is undetermined.

Neurocardiogenic syncope and Prinzmetal's angina associated with bronchogenic carcinoma

A clinical case is presented illustrating a previously unreported association of (1) neurocardiogenic syncope of new onset in a 57-year-old man, (2) Prinzmetal's angina, and (3) bronchogenic carcinoma of the lung. Initiation of aggressive chemotherapy resulted in immediate suppression of both cardiac manifestations. This newly described paraneoplastic syndrome is discussed.

Response to beta blockers in patients with neurocardiogenic syncope: how to predict beneficial effects

No definitive data are available about the possibility of predicting improvement in patients with neurocardiogenic syncope treated with beta blockers. Among 112 patients with syncope and a positive head-up tilt test (HUT), independent predictors for prevention of symptoms with beta blockers were determined using the Cox proportional hazards model. Each patient underwent HUT at 70 degrees for 20 minutes both in the drug-free state and during isoproterenol infusion given to increase the heart rate by at least 25%. Fifty-nine patients had a positive HUT during isoproterenol infusion and 53 in the drug-free state. All patients were then given esmolol infusion at 500 micrograms/kg per minute for 3 minutes followed by 300 micrograms/kg per minute maintenance dose. HUT was then repeated as previously described with or without isoproterenol, depending upon the initial positive response. Regardless of the response during esmolol, all patients were treated with metoprolol 50 to 100 mg twice daily. At follow-up, 36 patients experienced symptom relapse. Four of them had negative HUT on esmolol, whereas the remaining 32 did not respond to the acute infusion of esmolol. Only four patients with positive HUT on esmolol had a favorable response to metoprolol. Patients responding to metoprolol were older (55 +/- 12 years vs 42 +/- 15 years, P < 0.05). Response to metoprolol was predicted by a negative test on esmolol (P < 0.0001) and a positive HUT on isoproterenol (P < 0.001). Age older than 42 years was also associated with a higher likelihood of metoprolol success (P < 0.02).

CONCLUSION

Acute challenge with esmolol infusion appears to be an accurate predictor of response to chronic beta blockers, together with age and a positive HUT during low-dose isoproterenol infusion.

Changes in plasma epinephrine concentration and in heart rate during head-up tilt testing in patients with neurocardiogenic syncope: correlation with successful therapy with beta-receptor antagonists

INTRODUCTION

Tilt table testing is widely used in the management of patients with neurocardiogenic syncope. However, the exact pathophysiologic mechanism of this disorder is still under debate. Likewise, therapy of these patients continues to represent a challenge in many cases. Therefore, the present study aimed to gain further insight into the pathophysiology of this syndrome and to examine easily accessible clinical parameters that can improve therapy selection.

METHODS AND RESULTS

In 16 patients with neurocardiogenic syncope, changes in endogenous catecholamine concentrations were determined during repeated tilt table testing before and during treatment with metoprolol. Tachycardia preceded syncope in 8 of 10 responders compared to only 1 of 6 nonresponders (P < 0.05). In responders, the relative increase in epinephrine levels averaged 197% +/- 51% during drug-free tilting and 75% +/- 33% during repeated testing while on beta-blocker therapy (P < 0.05). In nonresponders, there was a smaller relative increase in epinephrine averaging 137% +/- 35% at baseline tilt. During repeated tilt testing, a similar increase was observed in these patients with recurrent syncope (156% +/- 104%; P = NS compared to baseline).

CONCLUSION

In patients with neurocardiogenic syncope who show both an increase in epinephrine concentration during tilt test and sinus tachycardia prior to the onset of symptoms, beta-blocker treatment is very effective. These findings confirm the major role of sympathetic activation as a trigger of syncope. Particularly, heart rate changes at the onset of syncope may allow early identification of patients responding to antiadrenergic therapy.

Comparative study of the efficacy of metoprolol and verapamil in patients with syncope and positive head-up tilt test response

Head-up tilt testing is a reliable diagnostic tool for patients with recurrent unexplained syncope. It has also been used to assess the drug efficacy for the treatment of patients with syncope. The objective of this study was to assess the efficacy of verapamil compared with metoprolol in a randomized crossover design for patients with syncope and a positive tilt test response. Twenty-eight patients with a positive head-up tilt test were randomized to receive either metoprolol or verapamil; the test was repeated after 7 days of therapy. Of the 15 patients initially given metoprolol, 2 (13.3%) did not respond; both were crossed over to verapamil, to which they remained unresponsive. Of the 13 patients who initially received verapamil, 8 (61%) did not respond; after crossing over to metoprolol therapy, 1 remained unresponsive. Overall, 20 of 23 patients receiving metoprolol had negative results on repeat tilt testing, whereas only 5 of 15 patients receiving verapamil had negative results (p < 0.001). In conclusion, the results of this study suggest that verapamil is considerably less efficacious than metoprolol in managing neurocardiogenic syncope.

Is the heart "empty' at syncope?

Neurally-mediated syncope (NMS) is thought to be reflexly triggered by vagal cardiac ventricular afferents that are activated by impaired cardiac filling. If this hypothesis is true then maneuvers that increase venous pooling should progressively diminish cardiac volume triggering syncope once a threshold decrease in cardiac filling is reached. Beat-to-beat recordings of heart rate, blood pressure (Finapres) and stroke volume (impedance cardiograph) were made at rest and during head-up tilt (80 degrees) in twenty controls and in fourteen patients with recurrent NMS (group 1). Hemodynamic profiles of controls and group 1 were compared. In eleven additional patients with NMS (group 2) we measured cardiac chamber volume from apical two or four-chamber views or stroke volume from Doppler measurements of the left ventricular outflow tract at rest and during tilt. Baseline values and initial response to head-up tilt of controls and group 1 patients were similar. A small negative trend in blood pressure and total peripheral resistance was present for at least 250 s before the onset of syncope. Stroke volume remained stable during this presyncopal period and increased at syncope. The profile of stroke volume changes using impedance cardiography mirrored those obtained using Doppler (5 subjects). Reliable echocardiographic measurements of cardiac chamber size were obtained in five subjects and did not change during tilt, presyncope or syncope. These data show that there is no significant decrease in cardiac volume before syncope that could serve as a trigger of syncope.

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.

Autonomic control of asystolic vasovagal syncope

A 30 year old woman with a lifelong history of severe, recurrent, vasovagal syncope became asystolic for 30 seconds after 37 minutes of 60 degrees head-up tilt. During early tilt, sympathetic activity, heart rate, left ventricular contractility, and cardiac output increased. Mean blood pressure was initially maintained. Presyncope was associated with maximal contractility and bradycardia despite sustained sympathetic activity. Subsequently, asystole occurred associated with complete withdrawal of muscle nerve sympathetic activity. In asystolic vasovagal reactions, presyncope may be triggered by increased left ventricular contractility and is associated with increased levels of parasympathetic and sympathetic activity. Asystole and peripheral vasodilatation may be caused by sudden and complete withdrawal of the increased sympathetic activity.

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.

Head-up tilt test: a highly sensitive, specific test for children with unexplained syncope

Unexplained syncope may cause diagnostic and therapeutic problems in children. The head-up tilt test has been shown to be a useful tool for investigating unexplained syncope, especially for diagnosis of neurally mediated syncope. In this study 20 patients aged 9-18 years (12.0 +/- 2.5 years) with syncope of unknown origin and 10 healthy age-matched children were evaluated by head-up tilt to 60 degrees for 25 minutes. The test was considered positive if syncope or presyncope developed in association with hypotension, bradycardia, or both. If tilting alone did not induce symptoms (syncope or presyncope), isoproterenol infusion was administered with increasing doses (0.02-0.08 mu g/kg per minute). During the tilt test, symptoms were elicited in 15 (75%) of the patients with unexplained syncope but in only one (10%) of the control group (p < 0.001). The sensitivity of the test was 75% and its specificity 90%. Three patterns of response to upright tilt were observed in symptomatic patients: vasodepressor pattern with an abrupt fall in blood pressure in 67%; cardioinhibitory pattern with profound bradycardia in 6%; and mixed pattern in 27%. In patients with positive head-up tilt, there were sudden decreases in systolic blood pressure (from 130 +/- 15 to 61 +/- 33 mmHg) and in mean heart rate (from 147 +/- 26 to 90+/-38 beats per minute) (p < 0.001) during symptoms. Treatments with atenolol 25 mg/day has shown complete suppression of syncope in positive responders during a mean follow-up period of 18 +/- 6 months. The head-up tilt test is a noninvasive, sensitive, specific diagnostic tool for evaluating children with unexplained syncope.

Excessive myocardial contraction in vasovagal syncope demonstrated by echocardiography during head-up tilt test

The purpose of this study was to gain insight into the mechanism of vasovagal syncope and to test the efficacy of theophylline to prevent syncope. Twenty-six patients with vasovagal syncope underwent two-dimensional echocardiography and theophylline test during head-up tilt test. A standard parasternal short-axis view of echocardiography at the level of the papillary muscle was recorded to measure fraction shorting of the left ventricle, and left ventricular end-diastolic and end-systolic dimensions. Heart rate, blood pressure, and symptoms were recorded. There were three groups; Group 1: no syncope with and without isoproterenol (n = 5); Group 2: syncope only after the infusion of isoproterenol (n = 16); Group 3: syncope without isoproterenol (n = 5). Groups 2 and 3 proceeded to theophylline injection (250 mg). The study showed that the 80 degrees tilt induced an increase in heart rate of 6.6 +/- 4.0, 12.4 +/- 6.6, and 25.4 +/- 4.5 beats/min in Groups 1, 2, and 3, respectively (p < 0.05 in Groups 1 and 2, p < 0.05 in Groups 1 and 3). The addition of isoproterenol during posture change from supine to an 80 degrees tilt made the significant change of fractional shortening from 0.4 +/- 5% to 6 +/- 13% in Group 2 (p = 0.05), compared with no significant change in Group 1. There were no significant differences in left ventricular end-diastolic dimension and end-systolic dimension in each group between baseline and isoproterenol infusion during posture change. Vasovagal syncope was associated with vigorous myocardial contraction, rather than with contraction against an empty left ventricle. The acute loading of theophylline was not effective in preventing vasovagal syncope.

Changes in left ventricular volume during head-up tilt in patients with vasovagal syncope: an echocardiographic study

We tested the hypothesis that patients who have vasovagal syncope during head-up tilt have a greater decrease in their left ventricular volume in response to tilt than do normal subjects. Measurements were done in the supine position and during graded tilt by using two-dimensional echocardiography. We compared seven patients with vasovagal syncope with nine normal volunteers. The rate of reduction of end-diastolic volume index during tilt was faster in the vasovagal group than in normal subjects. A more significant reduction of stroke index and ejection fraction during tilt was found in the vasovagal group than in normal subjects, possibly because of more peripheral translocation of blood volume in the venous system during tilt and an early vagal effect on ventricular contraction.

Randomized double-blind, placebo-controlled trial of oral atenolol in patients with unexplained syncope and positive upright tilt table test results

The objective of this investigation was the assessment of the response rate of oral atenolol in patients with vasovagal syncope after 1 month of treatment. We randomized into two groups all patients referred to our unit who had had at least one episode of syncope or two episodes of presyncope 1 month before presentation and had a positive isuprel Tilt Table Test (TTT). Group 1 (Gr 1) received oral atenolol, and group 2 (Gr 2) received placebo medication. After a 1-month period patients were reassessed for degree of their symptoms and underwent repeated TTT. Forty-two patients were enrolled in the study. Gr 1 and Gr 2 were comparable in age (38 +/- 13 years vs 43 +/- 14 years, p = 0.216 and sex (male/female = 6:15 vs 10:11, p = 0.204). The severity of attack was similar in both groups. Eight patients in Gr 1 and six patients in Gr 2 had mitral valve prolapse (p = 0.5). No significant differences were seen in systolic blood pressure (122 +/- 17 vs 117 +/- 16 mmHg, p = 0.334), diastolic blood pressure (70 +/- 11 vs 72 +/- 11 mm Hg, p = 0.677), and heart rate (79 +/- 12 vs 79 +/- 13, p = 0.98) between the two groups. The response rates (negative TTT) after 1 month of treatment were 62% versus 5% (p = 0.0004) in the atenolol and control group, respectively. Moreover, patients who received atenolol reported feeling better compared with those who received placebo (71% vs 29%, p = 0.02). In conclusion, atenolol significantly improved symptoms of patients with vasovagal syncope.(ABSTRACT TRUNCATED AT 250 WORDS)