[Mechanisms of vasovagal syncope elucidated by upright-tilt with isoproterenol infusion].

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)