Received November 13, 2004
Revised December 21, 2004
Accepted after revision January 21, 2005

¹ University of Leeds

^* To whom correspondence should be addressed. E-mail: giosuegulli{at}yahoo.it.

	Abstract

In addition to the gain, the time delay in the input-output response in a feedback system is crucial for the maintenance of its stability. Patients with posturally related (vasovagal) syncope have inadequate control of blood pressure and one possible explanation for this could be prolonged latency of the baroreflex. We studied 14 patients with histories of syncope and poor orthostatic tolerance (assessed by a progressive orthostatic stress test) and 16 healthy controls. We performed spontaneous sequence analysis of the fluctuations of R-R period (ECG) and systolic arterial pressure (SAP, Finapres) recorded during a 20 min supine period and during 20 min 60° head-up-tilt (HUT). The baroreflex latency was determined by identifying the lag between the changes in SAP and in R-R interval from which the highest correlation coefficient was obtained. During supine, 74% of sequences in controls and 54% in patients occurred with zero beats of delay (i.e. R-R interval changed within the same R-R interval). The remaining sequences occurred with delays of up to 4 beats. HUT shifted the baroreflex delay to be approximately one heartbeat slower and again patients showed more sequences with prolonged response. The delay in heartbeats was transformed into delay in time. In controls, 75% of baroreflex responses occurred within one second. In patients, 75% of baroreflex responses took more than 2 seconds to occur. The results showed that syncopal patients with poor orthostatic tolerance have increased baroreflex latency. This may lead to instability and inadequate blood pressure control and may predispose to vasovagal syncope.

Key Words: Arterial baroreflex, Autonomic nervous system, Hypotension