A closed-loop approach to the study of the baroreflex dynamics during posture changes at rest and at exercise in humans

We hypothesized that during rapid up-tilting at rest, due to vagal withdrawal, arterial baroreflex sensitivity (BRS) may decrease promptly and precede the operating point (OP) resetting, whereas different kinetics are expected during exercise steady state, due to lower vagal activity than at rest. To test this, eleven subjects were rapidly (< 2s) tilted from supine (S) to upright (U) and vice versa every 3 minutes, at rest and during steady state 50W pedaling. Mean arterial pressure (MAP) was measured by finger cuff (Portapres) and R-to-R interval (RRi) by electrocardiography. BRS was computed with the sequence method both during steady and unsteady states. At rest, BRS was 35.1ms∙mmHg-1 (SD17.1) in S and 16.7ms∙mmHg-1 (SD6.4) in U (p<0.01), RRi was 901ms (SD118) in S and 749ms (SD98) in U (p<0.01), and MAP was 76mmHg (SD11) in S and 83mmHg (SD8) in U (p<0.01). During up-tilt, BRS decreased promptly [first BRS sequence was 19.7ms∙mmHg-1 (SD5.0)] and was followed by an OP resetting (MAP increase without changes in RRi). At exercise, BRS and OP did not differ between supine and upright positions [respectively, BRS was 7.7ms∙mmHg-1 (SD3.0) and 7.7ms∙mmHg-1 (SD3.5), MAP was 85mmHg (SD13) and 88mmHg (SD10), and RRi was 622ms (SD61) and 600ms (SD70)]. The results support the tested hypothesis. The prompt BRS decrease during up-tilt at rest may be ascribed to a vagal withdrawal, similarly to what occurs at exercise onset. The OP resetting may be due to a slower control mechanism, possibly an increase in sympathetic activity.