Haemodynamic changes during neck pressure and suction in seated and supine positions.

We sought to quantify the contribution of cardiac output (Q) and total vascular conductance (TVC) to carotid baroreflex-mediated changes in mean arterial pressure (MAP) in the upright seated and supine positions. Acute changes in carotid sinus transmural pressure were evoked using brief 5 s pulses of neck pressure and neck suction (NP/NS) via a simplified paired neck chamber that was developed to enable beat-to-beat measurements of stroke volume using pulse-doppler ultrasound. Percentage contributions of Q and TVC were achieved by calculating the predicted change in MAP during carotid baroreflex stimulation if only the individual changes in Q or TVC occurred and all other parameters remained at control values. All NP and NS stimuli from +40 to -80 Torr (+5.33 to -10.67 kPa) induced significant changes in Q and TVC in both the upright seated and supine positions (P < 0.001). Cardiopulmonary baroreceptor loading with the supine position appeared to cause a greater reliance on carotid baroreflex-mediated changes in Q. Nevertheless, in both the seated and supine positions the changes in MAP were primarily mediated by alterations in TVC (percentage contribution of TVC at the time-of-peak MAP, seated 95 +/- 13, supine 76 +/- 17 %). These data indicate that alterations in vasomotor activity are the primary means by which the carotid baroreflex regulates blood pressure during acute changes in carotid sinus transmural pressure.