R–R interval–blood pressure interaction in subjects with different tolerances to orthostatic stress

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 deg 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 the supine period, 74% of sequences in control subjects 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 four 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 control subjects, 75% of baroreflex responses occurred within 1 s. In patients, 75% of baroreflex responses took more than 2 s 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.