Vagal cardiovascular reflexes in young persons with syncope

Blood-injury phobia with and without a history of fainting: disgust sensitivity does not explain the fainting response

OBJECTIVE

Individuals diagnosed with blood-injury phobia respond to venipuncture with strong psychophysiological responses. We investigated whether disgust sensitivity contributes to the fainting response and is associated with parasympathetic activation, as suggested by previous research.

METHODS

Twenty individuals diagnosed with blood-injury phobia (9 with a history of fainting to the sight of blood, 11 without such a fainting history) and 20 healthy controls were compared. Psychophysiological responses and self-report measures of anxiety, disgust, and embarrassment were monitored during rest, a paced breathing task, and venipuncture. In addition, trait disgust sensitivity and blood-injury fears were assessed.

RESULTS

Blood-injury phobics reported enhanced anxiety, disgust, and embarrassment during venipuncture. They also experienced heightened arousal, as indicated by heart rate, respiration rate, and minute ventilation. Blood-injury phobics without a fainting history tended toward higher anxiety and disgust scores. There was no evidence for increased parasympathetic activation in either blood-injury phobic subgroup or of an association of disgust and parasympathetic activation.

CONCLUSION

The tendency to faint when exposed to blood-injury stimuli may suffice as a conditioning event leading into phobia, without specific involvement of disgust sensitivity and parasympathetic activation.

Hemodynamic significance of heart rate in neurally mediated syncope

BACKGROUND

Vasovagal and vasodepressor syncope are used interchangeably in the literature to describe the common faint syndrome, now collectively named neurally mediated syncope. The significance of heart rate (HR) in these reflex-induced reactions remains unclear.

HYPOTHESIS

The study was undertaken to investigate the hemodynamic significance of HR in tilt-induced neurally mediated syncope.

METHODS

In all, 113 patients with syncope of unknown etiology were studied by head-up tilt test with invasive hemodynamic monitoring. Thirty-five patients (15 women, 20 men, age range 21 to 72 years) developed syncope and were enrolled for analysis. The hemodynamic data were compared between patients who developed bradycardia (vasovagal group, n = 15) and those without bradycardia (vasodepressor group, n = 20).

RESULTS

The baseline hemodynamic data (mean +/- standard deviation) and the hemodynamic responses after 10-min headup tilt were similar between patients in the vasovagal and vasodepressor groups. During syncope, patients with vasovagal reaction developed hypotension and paradoxical bradycardia (HR = 52.4 +/- 5.9 beats/min), while patients with vasodepressor reaction developed a precipitous drop in arterial blood pressure with inappropriate HR (105 +/- 21 beats/min) compensation. Patients with vasovagal syncope manifested a significantly lower cardiac index and a significantly higher systemic vascular resistance index than patients with vasodepressor syncope (1.47 +/- 0.29 vs. 1.97 +/- 0.41 1/min/m2, p < 0.001 and 2098 +/- 615 vs. 1573 +/- 353 dynes x s x cm(-5) x m2, p < 0.003, respectively). A positive correlation existed between HR and cardiac index (r = 0.44, p = 0.008) during syncope in the patients studied.

CONCLUSIONS

These findings suggest that the hemodynamic characteristics of vasovagal and vasodepressor reactions are different, and that HR plays a significant role in neurally mediated syncope.

Autonomic cardiovascular responses are impaired in women with irritable bowel syndrome

GOALS

This study characterizes cardiovascular autonomic function in women with irritable bowel syndrome (IBS), using standardized techniques.

BACKGROUND

Autonomic dysfunction is believed to contribute to abnormal gastrointestinal motility and visceral hypersensitivity in IBS. There is mounting evidence of generalized impairment of autonomic activity in patients with IBS.

STUDY

Thirty women aged 39 years (95% C.I. 25-53 years) diagnosed with IBS, and 30 age-matched healthy women were studied. The ratio of low frequency to high frequency heart rate variability domains (LF:HF ratio) was used to represent cardiac sympathovagal activity, and orthostatic testing and sustained isometric handgrip exercise were used as sympathetic stimuli. Parasympathetic activity was represented by the expiratory to inspiratory R-R interval (E:I) ratio during deep breathing at 6 minutes.

RESULTS

LF:HF responses to handgrip exercise (316%, C.I. 134% to 498% vs. 107%, C.I. 15% to 153%; P < 0.05) and orthostatic testing (648%, C.I. 520% to 904% vs. 330%, C.I. 140% to 520%; P < 0.05) were higher in IBS patients than controls, and the E:I ratio was significantly lower (1.47, C.I. 1.33-1.61 vs. 1.20, C.I. 1.14-1.26; P < 0.01).

CONCLUSIONS

Autonomic cardiovascular function is impaired in IBS, manifest as attenuated cardio-vagal tone, and relative sympathetic excess during stimulated conditions.

Head-up tilt test without intravascular cannulation in children and adolescents

Previous studies of head-up tilt test have shown that testing at high degrees lacks specificity in children. We suspected that the high false positive rate might be related to the intravascular catheter and other maneuvers incorporated in the test and therefore studied the sensitivity and specificity of standing and HUT at 80 degrees without any invasive procedure and other maneuvers in children and adolescents. Twenty three patients (11.8+/-2.7 years) with recurrent typical neurally mediated syncope and 35 normal control children (11.6+/-3.0 years) underwent motionless standing for 15 min and tilting to 80 degrees for 30 min. Continuous finger arterial pressure monitoring and ECG were performed during the test. Eight (35%) of the 23 patients developed symptoms of near syncope during motionless standing. Thirteen (57%) of them had positive results at 80 degrees tilting for 30 min. The symptoms of syncope were not always corresponding to excessive haemodynamic changes. None of the controls developed any symptoms or excessive hemodynamic changes. Without intravascular instrumentation and other autonomic maneuvers, active motionless standing or HUT at 80 degrees for 30 min is highly specific but of limited sensitivity for the investigation of vasovagal syncope.

Safety and tolerability of an aggressive tilt table test protocol in the evaluation of patients with suspected neurocardiogenic syncope

Safety and tolerability of a one-step tilt table test with high dose (5 micrograms/min) isoproterenol (ISO) without intermediate stages were evaluated in a symptomatic population of 300 patients referred for clinical syncope, near syncope, or dizziness. ISO has been used as a provocative test but remains controversial. A population of 118 male and 182 female patients with a mean age of 45 (range 5-90) years underwent 300 tests. Heart rate and blood pressure were monitored continuously. A positive test was one in which clinical symptoms were reproduced or hemodynamic criteria met. Patients were initially supine for 5 minutes followed by head upright tilt (HUT) to an angle of 80 degrees for 10 minutes. Negative tests were repeated with an infusion of ISO at a rate of 5 micrograms/min. HUT was positive in 133 (44.3%) of 300 tests. With a 10-minute HUT alone, only 17 (5.7%) of 300 of tests were positive. Of the initial negative tests, 273 of 283 were tested with ISO. With ISO, 116 (42.5%) of 273 were positive. ISO in high dose (5 micrograms/min) was used in 264 of 273 patients, while low dose (1.0-2.5 micrograms/min) was used in 9 of 273 under special circumstances. High dose ISO was tolerated in 164 (62.1%) of 264 patients, reduced in 87 (33%) of 264, and discontinued in 11 (4.2%) of 264. Reasons for reduction included tachycardia (40 patients), nausea (31 patients), chest pain (2 patients), arrhythmia (5 patients), or other (9 patients). Adverse effects resolved within 1 minute of dose reduction. This one-step high dose ISO protocol reproduced neurocardiogenic syncope in symptomatic patients who tested negative without ISO and was safe, tolerated, and expeditious.

Neurally mediated cardiac syncope: autonomic modulation after normal saline infusion

OBJECTIVES

This study assessed the heart variability response to orthostatic stress during tilt table testing before and after normal saline administration.

BACKGROUND

The efficacy of sodium chloride and mineralocortoid in the treatment of neurally mediated cardiac syncope is attributed to intravascular volume expansion; however, their modulation of autonomic nervous system activity has not been evaluated.

METHODS

Heart rate variability analysis was performed on 12 adolescents with a history of syncope or presyncope (mean age 15.2+/-0.7 years) during tilt table testing. Subjects were upright 80 degrees for 30 min or until syncope. After normal saline administration, the patient was returned upright for 30 min. Heart rate variability analysis data were analyzed by an autoregression model (Burg method).

RESULTS

All subjects reproducibly developed syncope during control tilt table testing; median time to syncope was 9.4+/-2.1 min. After normal saline infusion, none of the subjects developed syncope after 30 min upright. In the control tilt, there was an initial increase followed by a progressive decrease in low frequency power until syncope. Repeat tilt after normal saline administration demonstrates that low frequency power increased but the magnitude of initial change was blunted when compared with control. In addition, low frequency power increased during normal saline tilt sequence compared with the control tilt, during which it decreased.

CONCLUSIONS

Normal saline blunted low frequency power stimulation and prevented paradoxical low frequency power (sympathetic) withdrawal. Increasing intravascular volume with normal saline alters autonomic responses that may trigger neurally mediated syncope reflexes.

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.

Complex demodulation of cardiorespiratory dynamics preceding vasovagal syncope

BACKGROUND

The dynamic autonomic processes leading to vasovagal syncope are poorly understood.

METHODS AND RESULTS

We used complex demodulation to continuously assess changes in respiration, R-R interval, and arterial pressure (blood pressure) variability during 60 degree head-up tilt in 25 healthy subjects with tilt-induced vasovagal syncope and 25 age-matched nonsyncopal control subjects. Coherence and transfer function analyses were used to examine the relation between respiration and R-R interval variability before syncope. Baseline blood pressure, R-R, and ventilation were similar between syncope subjects and control subjects. Syncope subjects experienced an increase in tidal volume and decrease in BP beginning 3 minutes before impending syncope (systolic blood pressure <80 mm Hg) necessitated termination of tilt. Approximately 90 seconds before syncope there was a sudden prolongation of R-R interval and increase in amplitude of high and low frequency R-R interval variability, indicating an abrupt enhancement of vagal tone. The increase in respiratory amplitude between 180 and 90 seconds before syncope was not accompanied by changes in R-R interval or R-R variability, suggesting a dissociation between respiration and the respiratory sinus arrhythmia. The coherence analysis showed fewer syncope subjects with coherence between respiratory and R-R interval variabilities and lower transfer magnitudes in syncope subjects compared with control subjects. Nonsyncopal subjects had no change in respiratory, R-R interval, or blood pressure dynamics during matched time periods before the time of syncope.

CONCLUSIONS

Vasovagal syncope is preceded by a period of hyperpnea and cardiorespiratory decoupling followed by an abrupt increase in cardiovagal tone. Respiratory pumping without inspiratory cardiac slowing may partially counteract preload reduction until sudden bradycardia precipitates syncope.

Review article: heart rate and blood pressure control in vasovagal syncope

Vasovagal syncope is characterized by transient failure of usually reliable physiologic mechanisms responsible for maintaining both systemic arterial pressure and cerebral blood flow. Two circulatory phenomena are almost universally present: systemic arterial vasodilation and bradycardia. A third phenomenon, cerebrovascular constriction, has also been described but its contribution to the faint is less well established. The neural reflex pathways responsible for triggering the circulatory changes in the vasovagal faint are incompletely understood, but have recently been the subject of renewed interest. In part, this interest probably stems from the frequency with which vasovagal symptoms are now recognized to be the cause of fainting spells. Additionally, however, there is an increasingly recognized need to develop treatment strategies for those affected patients in whom recurrent vasovagal symptoms are particularly troublesome. It is the goal of this discussion to focus on those aspects of circulatory control, and in particular on potential interactions among certain neural and humoral systems, which may contribute to the inappropriate physiologic responses associated with the vasovagal faint.

Syncope: current diagnosis and treatment

The patient with syncope often poses a formidable diagnostic challenge. A large number of underlying causes must be considered, ranging in severity from benign to life-threatening. A careful, systematic clinical evaluation beginning with a history, physical examination, and ECG will establish the diagnosis in most patients, and the judicious use of specialized testing will confirm or uncover the cause in many of the remaining cases. Further basic and clinical research into the pathogenesis and treatment of neurocardiogenic syncope, the role of HUT testing in neurally mediated syncope, and the optimal use of EPS in patients with cardiac disease will markedly improve our management of these patients in the future.

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.

Incidence and hemodynamic characteristics of near-fainting in healthy 6- to 16-year old subjects

OBJECTIVES

We studied the incidence and hemodynamic characteristics of near-fainting under orthostatic stress in healthy children and teenagers.

BACKGROUND

Orthostatic stress testing is increasingly used to identify young subjects with unexplained syncope. However, the associated incidence of syncope and hemodynamic responses in normal young subjects are not well known.

METHODS

Eighty-four healthy subjects 6 to 16 years old performed forced breathing, stand-up and 70 degrees tilt-up tests. An intravenous line to sample blood for biochemical assessment of sympathetic function was introduced between the stand-up and tilt-up tests. Finger arterial pressure was measured continuously. Left ventricular stroke volume was computed from the pressure pulsations.

RESULTS

Sixteen of the 84 subjects were excluded because of technical problems. The incidence of a near-fainting response in the remaining 68 subjects was 10% (7 of 68) for the stand-up test and 40% (29 of 68) for the tilt-up test. Baseline parasympathetic and sympathetic activity of nonfainting and near-fainting subjects was not different. Near-fainting was characterized by attenuated systemic vasoconstriction and exaggerated tachycardia that occurred as early as 1 min after return to the upright position. On tilt-up, plasma adrenaline levels increased by a factor of 2, with slightly higher increments in the near-fainting subjects.

CONCLUSIONS

Inadequate vasoconstriction is the common underlying mechanism of near-fainting in young subjects. The remarkably high incidence of near-fainting during the tilt-up test after intravascular instrumentation raises serious doubts about the utility of this procedure in evaluating syncope of unknown origin in young subjects.