Intravenous cannulation alters the specificity of head-up tilt testing for vasovagal syncope in elderly patients

Age and Ageing, July, 1994 by S.J. McIntosh, J. Lawson, R.A. Kenny

Summary

Prolonged head-up tilt is increasingly used as a diagnostic test for vasovagal syncope. Its sensitivity is reported to increase with the concurrent administration of intravenous isoprenaline. False-positive responses are common in young controls particularly following intravascular instrumentation. We studied the influence of intravenous cannulation alone on responses to head-up tilt in ten healthy elderly subjects. All remained asymptomatic during tilt when non-cannulated whilst five developed symptomatic hypotension following cannulation. Thus, intravascular instrumentation influences responses to head-up tilt in elderly subjects; the significance of positive responses obtained using intravenous isoprenaline in this age group requires further evaluation.

Introduction

Vasovagal responses are increasingly recognized as a cause of unexplained syncope (1), (2). Reflex-induced hypotension with or without bradycardia may result in clinical manifestations ranging from mild dizziness to life-threatening asystole (3). Although it has long been recognized that vasovagal responses are provoked centrally by stimuli such as fear and nausea, it has only recently become apparent that they may also be triggered peripherally by the activation of left ventricular mechanoreceptors. These respond to deformation such as that which occurs when the near empty ventricle contracts vigorously (4). Afferent impulses pass to the brainstem cardiovascular centre from which efferent pathways project to the heart and peripheral vasculature. The bradycardic component of the vasovagal response is vagally mediated whilst vasodilatation (and hence hypotension) is believed to result from sympathetic withdrawal (4).

Prolonged head-up tilt has recently been introduced as a diagnostic test for vasovagal syncope (1), (2). It is thought to activate ventricular receptors in susceptible individuals by reducing cardiac volumes and enhancing catecholamine release (2). The concurrent administration of intravenous isoprenaline is reported to increase the sensitivity and reduce the duration of this test presumably by further augmenting catecholamine levels (5). Whether positive responses to head-up tilt with isoprenaline reflect increased activation of ventricular receptors or whether cannulation alone without durg administration enhances the response is undefined.

Subjects and Methods

Ten healthy elderly subjects (70 [ or -]6 years; 8 male) were recruited by poster advertising. All gave informed consent and the study was approved by the local ethical committee. Subjects had no history of dizziness, falls or syncope, had no intercurrent illness and were not taking any medications. All underwent two tilt studies one without (study A) and one with (study B) intravenous cannulation performed in a random order on separate days.

Studies were carried out at 09 h 00 following an overnight fast. Subjects initially rested supine on the tilt table for 30 min after which phasic blood pressure (BP: digital photoplethys-mography), heart rate (HR: surface electrocardiogram) and forearm blood flow (FBF: strain-gauge plethysmography) were measured until steady baseline readings were obtained. They were then tilted to 70[degrees] (Akron automated tilt table; footplate support) for 30 min. BP and HR readings were documented at 2, 5, 10, 15 and 30 minutes of tilt as were FBF readings. Subjects were returned to supine if symptomatic hypotension occurred. BP, HR and (where possible) FBF were documented at the time of the symptomatic episode. A 19 gauge venflon was inserted into the antecubital vein 30 min prior to study B.

Results

Study A: No subject was symptomatic during tilt when non-cannulated. Systolic and diastolic BP were unchanged at 30 min compared with baseline (-4.3 mmHg; 95% CI - 15.6 to 24.2 and 2.9mmHg; 95% CI - 18.2 to 12.4; NS) as was HR ( 6.1 bpm; 95% CI -16.0 to 3.8; NS) (Table). Baseline systolic BP was significantly lower in those subjects who subsequently became symptomatic in study B (-16 mmHg; 95% CI 1.8 to 30.2; p>0.05) but no other differences were noted.

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Study B: Five subjects became presyncopal at 22[ or -]7 min (range 15-30 min) during head-up tilt when cannulated (Figure). Baseline systolic BP, diastolic BP and HR were similar for symptomatic and asymptomatic subjects in Study B. In presyncopal subjects both systolic and diastolic BP fell markedly during symptoms compared with baseline ( - 39.2 mmHg; 95% CI 11.0 to 67.1 and - 15.8 mmHg; 95% CI 1.2 to 39.7; p < 0.05) whilst the change in HR was not significant ( 8.3 bpm; 95% CI - 22.2 to 8.0; NS). HR increased or remained unaltered during symptoms in four subjects; only one subject developed a bradycardia. Responses in asymptomatic individuals in study B were similar to those observed in study A. Baselie FBF did not differ for subjects with and without presyncope. Symptomatic individuals experienced a rise in FBF (vasodilatation) compared with baseline during the hypotensive episode ( 0.53 ml/100 ml/min; 95% CI - 1.96 to 0.91; NS).

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Discussion

Published data suggest that head-up tilt results in a vasovagal response in up to 17% of healthy controls (6) and in 27-74% of patients with unexplained syncope (2), (7). This marked range in positive response rates for patients probably reflects inter-study differences notably in the ages of the individuals tested and in the use of invasive monitoring.