Central volume expansion is pivotal for sustained decrease in heart rate during seated to supine posture change

During prolonged, static carotid baroreceptor stimulation by neck suction ( NS) in seated humans, heart rate (HR) decreases acutely and thereafter grad ually increases. This increase has been explained by carotid baroreceptor a daptation and/or buffering by aortic reflexes. During a posture change from seated to supine (Sup) with similar carotid stimulation, however, the decr ease in HR is sustained. To investigate whether this discrepancy is caused by changes in central blood volume, we compared (n = 10 subjects) the effec ts of 10 min of seated NS (adjusted to simulate carotid stimulation of a po sture change), a posture change from seated to Sup, and the same posture ch ange with left atrial (LA) diameter maintained unchanged by lower body nega tive pressure (Sup + LBNP). During Sup, the prompt decreases in HR and mean arterial pressure (MAP) were sustained. HR decreased similarly within 30 s of NS (65 +/- 2 to 59 +/- 2 beats/min) and Sup + LBNP (65 +/- 2 to 59 +/- 2 beats/min) and thereafter gradually increased to values of seated. MAP de creased similarly within 5 min during Sup + LBNP and NS (by 7 +/- 1 to 9 +/ - 1 mmHg) and thereafter tended to increase toward values of seated subject s. Arterial pulse pressure was increased the most by Sup, less so by Sup LBNP, and was unchanged by NS. TA diameter was only increased by Sup. In co nclusion, static carotid baroreceptor stimulation per se causes the acute ( <30 s) decrease in HR during a posture change from seated to Sup, whereas t he central volume expansion (increased LA diameter and/or arterial pulse pr essure) is pivotal to sustain this decrease. Thus the effects of central vo lume expansion override adaptation of the carotid baroreceptors and/or buff ering of aortic reflexes.