To elucidate the role of increased basal vagal activity in
vasovagal syncope, we compared patients with
bradyarrhythmia due to increased vagal tone and patients with
vasovagal syncope using an upright-tilt (60 degrees) positioning test with
isoproterenol infusion. Eight patients with unexplained recurrent
syncope after clinical and electrophysiological investigations and 5 patients without
syncope who had
bradyarrhythmias due to increased vagal tone were studied. All 8 patients with recurrent
syncope had some prodrome suggestive of
vasovagal syncope. The upright-tilting test was considered positive if
syncope developed in association with
hypotension or
bradycardia, or both. If 10 min of control tilting was negative, the patient was lowered to the supine position. Upright-tilting was then repeated during continuous intravenous
isoproterenol infusion at successive incremental doses of 0.01 to 0.03 microgram/kg/min. During the control upright-tilting test, none of the patients had positive responses. During the upright-tilting with
isoproterenol infusions, all patients with
vasovagal syncope had positive responses; whereas, all patients with
bradyarrhythmia due to increased vagal tone had negative responses. In patients with
vasovagal syncope, the heart rate (HR) and the mean blood pressure (mBP) were higher at the time of supine positioning than at the time of
syncope (
HR: 109 +/- 16-->88 +/- 16 bpm, p < 0.05) (mBP: 86 +/- 5-->53 +/- 6 mmHg, p < 0.01). However, in patients with
bradyarrhythmia there was no significant change in HR and mBP between the supine and 10 min of the upright-tilting with
isoproterenol infusion.(ABSTRACT TRUNCATED AT 250 WORDS)