Elsevier

Archives of Medical Research

Volume 30, Issue 3, May–June 1999, Pages 203-211
Archives of Medical Research

Original articles
Differences in Heart Rate Variability Between Cardioinhibitory and Vasodepressor Responses to Head-Up Tilt Table Testing

https://doi.org/10.1016/S0188-0128(99)00022-6Get rights and content

Abstract

Background

Patients with syncope show different responses to head-up tilt (HUT) test, which may be due to different pathophysiological mechanisms.

Methods

HUT (70°) was performed in 24 patients who experienced recurrent syncope. Nine patients had a cardioinhibitory (CI) response, 7 patients had a vasodepressor (VD) response, and 8 patients had a mixed (MX) response. Heart rate variability was analyzed at 60-sec periods during HUT.

Results

Total spectrum (TS) was greater at rest and 1 min after syncope in the CI and MX groups as compared to the VD group. Low frequency spectrum (LF) was significantly greater during rest and the first minute after syncope in the CI groups as compared with the VD group. After the rest period, the CI and MX groups showed more elevated high frequency spectrum (HF) values than the VD group (p <0.01). One minute after syncope, the HF increased in the CI and MX groups but not in the VD group (p <0.01). The VD group showed higher LF/HF ratio from the beginning of rest (3.9 ± 4.1) as compared to the CI and MX groups (p <0.01). This difference was most significant 2 min before syncope occurred. The CI and MX groups showed greater pNN50 and rMSSD as compared to the VD group.

Conclusions

Our results suggest that vagal tone is higher in subjects showing cardioinhibitory and mixed responses to HUT. In contrast, patients with a vasodepressor response showed predominantly sympathetic activity. These findings suggest that there are different pathophysiological mechanisms underlying syncope.

Introduction

Syncope is defined as a sudden temporary loss of consciousness associated with a loss of postural tone with spontaneous recovery 1, 2. Neurally mediated syncope is the term used to refer to syncope that results from reflex mechanisms associated with inappropriate vasodilatation and/or bradycardia. Head-up tilt (HUT) testing has assumed an important role in evaluating patients with unexplained syncope 3, 4. Patients with a positive response can be classified into mixed, cardioinhibitory, and vasodepressor categories (5).

Currently, these responses are believed to have a common pathophysiological mechanism. Head-up tilt leads to pooling of venous blood in the lower limbs, resulting in decreased venous return. The normal compensatory response to head-up posture is reflex tachycardia, more forceful contraction of the left ventricle, and vasoconstriction. However, in individuals with neurally mediated syncope, this forceful ventricular contraction in the setting of a relatively empty cavity may activate the cardiac mechanoreceptors. An afferent pathway consisting of unmyelinated left ventricular vagal C fibers transmits signals to specific central nervous system sites. This reduces the efferent sympathetic tone and results in reflex hypotension and/or bradycardia (Bezold-Jarisch reflex) 6, 7, 8.

However, provocation of syncope with HUT testing after heart transplantation has raised questions about this mechanism, because no evidence of reinnervation has been found in these patients 9, 10. This suggests the existence of a peripheral element in the pathophysiology of neurally mediated syncope. Based on the central role of the autonomic nervous system in the genesis of neurally mediated syncope, heart rate variability analysis has been used to explore the cardiac autonomic tone in response to head-upright tilt 11, 12, 13 in normal subjects, HUT results in a significant increase of the low frequency power spectrum in addition to withdrawal of the high frequency or parasympathetic tone. Controversial results have been obtained with heart rate variability analysis in neurally mediated syncope. Heart rate variability has been found to be decreased in normal adults (14) and elderly people (15), while other investigators have found increased parasympathetic activity 16, 17. We hypothesized that the response of the autonomic nervous system to head-up tilt is different in patients showing cardioinhibitory and vasodepressor responses. We, therefore, used heart rate variability analysis to explore the differential responses of cardiac autonomic tone to HUT in a group of patients with a positive non-pharmacological tilt-table test.

Section snippets

Materials and Methods

Twenty-four consecutive patients with a history of ≥2 episodes of syncope were included in the study. Patients were sent from the Outpatient Clinic to the National Institute of Cardiology Electrophysiology Department for additional evaluation. The study group consisted of 14 male and 10 female patients aged 10 to 62 years. All had a positive response during a non-pharmacological HUT test, manifesting different types of syncope during the tilt test. The group showing a mixed response included

Results

Table 1 shows the distribution of age and gender of patients manifesting different types of syncope during the tilt test. The patients in the CI and MX groups were significantly younger than those in the VD group (p <0.001). The mean time syncope the occurred during the tilt test was 12 ± 9 min and was similar in all groups.

Discussion

The classic theory of the pathophysiology of syncope relates bradycardia and hypotension to the afferent stimulation of the parasympathetic nervous system. However, syncope can be induced in patients with heart transplantation without autonomic innervation; therefore, alternative mechanisms have been postulated.

There are few reports on heart rate variability in neurally mediated syncope, and the results have been discrepant (19). Pagani et al. (14) assessed heart rate variability in healthy

Conclusions

The present study provides new findings and supports previous observations in the study of patients with neurally mediated syncope, showing the existence of two main types of autonomic behavior: one in the cardioinhibitory and mixed response groups with predominant parasympathetic activity, and the other in the vasodepressor group, with predominantly sympathetic activity. This suggests that there are different pathophysiological mechanisms involved in neurally mediated syncope that may be

Acknowledgements

The authors wish to thank Yolanda Ortega for her help in the preparation of the manuscript.

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