Mechanisms of initial heart rate response to postural change.

We explored in 43 healthy subjects the afferent mechanisms of the initial heart rate response to standing by comparing free standing, 70 degrees head-up tilt, handgrip, and contraction of abdominal and leg muscles. The results indicate the following. 1) Standing evokes an immediate, large, bimodal increase of heart rate (HR) of about 20 s duration that far exceeds the gradual HR rise induced by 70 degrees head-up tilt. 2) The immediate HR increase with active standing is due to the exercise reflex and results in a first peak about 3 s after standing briskly. 3) The secondary, more gradual HR increase after 5 s of standing and the subsequent rapid decrease of HR between about 12 and 20 s corresponds through the baroreceptor reflex with a striking fall, recovery, and sometimes overshoot of arterial pressure. 4) The maximum HR increase found after about 12 s of standing is augmented and delayed after rest. 5) The time course of the initial HR response is not modified by physical training. We conclude that active and passive changes of posture result in fundamentally different cardiovascular effects for about 20 s and that "central command," muscle receptors, high-pressure receptors, low-pressure receptors, and the plasma catecholamine level are probably all involved in the initial HR response to standing.