Effects of head-up tilting on baroreceptor control in subjects with different tolerances to orthostatic stress

During orthostatic stress, an increase in peripheral vascular resistance normally results in arterial blood pressure being well maintained, despite a decrease in cardiac output. The present study was undertaken to determine whether the sensitivity of the carotid baroreceptor reflex was increased during orthostatic stress and whether failure to develop this increase was associated with poor orthostatic tolerance. Three groups of subjects were studied: asymptomatic controls; patients investigated for suspected posturally related syncope but who had normal responses to an orthostatic stress test (normal patients); and patients who were shown to have low orthostatic tolerance (early fainters). We determined responses of R–R interval and forearm vascular resistance (mean arterial pressure/brachial artery velocity by Doppler ultrasonography) to the loading and unloading of carotid baroreceptors by application of pressures of -30 and +30mmHg to a chamber fitted over the neck. Responses were determined after 20min of supine rest and after 10min of head-up tilt at 60°. Responses of cardiac interval were not significantly different between the three groups, and they were not altered by the postural change. Vascular responses also did not differ between the groups during supine rest. However, in healthy volunteers and in normal patients, responses to both neck suction and pressure were significantly enhanced during head-up tilt. In controls, responses to suction were increased by tilt from 0.04±0.1 to -1.01±0.2%·mmHg^-1 (means±S.E.M.; P<0.001) and those to neck pressure from -0.6±0.3 to -3.1±1.1%·mmHg^-1 (P<0.05). In the normal patients, the corresponding changes were: during suction, from -0.2±0.1 to -0.7±0.1%·mmHg^-1 (P<0.05); during pressure, from -0.7±0.1 to -1.5±0.3%·mmHg^-1 (P<0.05). In contrast, in patients with low orthostatic tolerance, posture had no effect on the reflex (neck suction, from -0.3±0.1 to -0.3±0.1%·mmHg^-1; neck pressure, from -1.0±0.3 to -0.9±0.2%·mmHg^-1). We suggest that an increase in the sensitivity of the carotid baroreceptor/vascular resistance reflex may be important in the maintenance of blood pressure during orthostatic stress, and that failure of this to occur in patients with posturally related syncope may go some way towards explaining their poor orthostatic tolerance.