Influence of posture and prolonged head-down tilt on cardiovascular reflexes.

We investigated the influence of ten days 6 degrees head-down tilt (HDT) on short-term cardiovascular control. To help differentiate between the effects of HDT-induced fluid redistribution and changed autonomic cardiovascular modulation under prolonged HDT, the effect of acute posture changes was investigated as well. Six healthy male volunteers were studied. Continuous finger blood pressure was measured non-invasively by means of Finapres. Heart rate (HR) was derived from the electrocardiogram. Responses to forced breathing (FRSA), Valsalva's manoeuvre (VM), Mental Stress (MS) and Sustained Handgrip (SHG) were measured. Changing posture from HDT to standing enhanced the BP and HR responses to VM, both during straining and after release. During prolonged HDT, responses to VM changed toward the pattern seen in the upright posture before HDT, suggesting a strong influence of fluid redistribution. Neither posture nor prolonged HDT influenced HR variation during FRSA and responses to MS and SHG. BP variation during FRSA was influenced by posture but not by prolonged HDT. Thus, cardiovascular reflex tests which reflect the parasympathetic (FRSA) or the sympathetic (MS and SHG) efferents to the heart were not influenced by posture or prolonged HDT. Only the responses to VM were affected by both posture and prolonged HDT. These results are probably due to a decrease in blood volume and stroke volume under prolonged HDT, an increase in venous distensibility and, to a lesser extent, to inadequate cardiovascular regulatory responses.