Prolonged latency in the baroreflex mediated vascular resistance response in subjects with postural related syncope.

In addition to the gain, the delay of the baroreflex response plays an important role in the maintenance of cardiovascular system stability. Additionally when postural changes induce sudden drops in blood pressure, a delayed response may fail to maintain sufficient cerebral perfusion pressure. We tested the hypothesis that the delay of the carotid baroreceptor reflex is impaired in subjects with poor orthostatic tolerance. An orthostatic test with 60 degrees head-up tilt, and progressive lower-body negative pressure was performed on 27 patients with histories of unexplained syncope and 13 control subjects. The test was stopped at the onset of presyncope and time to presyncope was taken as a measure of orthostatic tolerance. Twelve patients had normal tolerance and thirteen patients had low tolerance. We measured beat-to-beat blood pressure (Finapres) and brachial artery blood flow velocity (Doppler ultrasonography). Before the test, we determined the response of forearm vascular resistance (mean arterial pressure/mean brachial artery velocity) to loading/unloading of carotid baroreceptors by the application of neck suction/pressure (-/+30 mmHg) to a chamber fitted overlying the carotid sinus. We measured the gain in the response (maximum percentage change from baseline value in vascular resistance divided by the neck collar pressure) and the latency in the response (delay of the maximum change in vascular resistance after neck-collar stimulation). Results are reported as means +/- SEM. In the three groups, there were no differences in the sensitivity of the vascular resistance response after baroreceptor loading/unloading. Following baroreceptor unloading, the latency of the response was 14.0+/-1.3 seconds in early fainters, 9.3+/-0.7 seconds in normal patients and 10.1+/-1.2 seconds in controls. The latency in blood pressure rise was 11.1+/-1.3 seconds in early fainters, 7.9+/-0.9 seconds in normal patients and 7.2+/-1.0 seconds in controls. The results following baroreceptor loading were more scattered. The early fainters still had a tendency to show prolonged latency. These results suggest that the delay in the baroreflex response plays an important role in postural related syncope.