CIRCULATORY RESPONSE EVOKED BY A 3-S BOUT OF DYNAMIC LEG EXERCISE IN HUMANS

1. The mechanisms underlying the pronounced transient fall in arterial blood pressure evoked by a 3 s bout of bicycle exercise were investig ated in twenty healthy young adults and four patients with hypoadrener gic orthostatic hypotension. 2. In healthy subjects a 3 s bout of upri ght cycling induced a 28 +/- 3 mmHg fall in mean arterial pressure at 12 s. The fall in mean arterial pressure was preceded by a 12 +/- 2 mm Hg rise in right atrial pressure at 3 s and accompanied by a 54 +/- 7% increase in left ventricle stroke volume at 6 s. Systemic vascular re sistance dropped 48 +/- 2% at 7 s after tile start of the manoeuvre to remain at that level for approximately 5 s. The total response lasted about 20 s. During sustained upright cycling the initial fall in mean arterial pressure was also present, but less pronounced (17 +/- 2 vs. 26 +/- 3 mmHg). A 3 s bout of supine cycling in four patients with hy poadrenergic orthostatic hypotension also elicited a pronounced fall i n mean arterial pressure (22 +/- 4 mmHg) and in systemic vascular resi stance (38 +/- 4%). 3. A bout of exercise with a large muscle mass ind uces two main effects. First, it mechanically increases filling of the heart due to activation of the muscle pump, resulting in an increase in cardiac output. Second, it induces a drop in systemic vascular resi stance. The increase in cardiac output is not sufficient to compensate fully for the pronounced fall in systemic vascular resistance and the result is a transient fall in arterial pressure at the onset of whole -body exercise. The rise in right atrial pressure evoked by 3 s cyclin g is abrupt and large, but the almost immediate onset and rapid fall o f the systemic vascular resistance is too fast for sympathetically med iated reflex effects due to stimulation of the cardiopulmonary afferen ts. An important factor involved in the drop in systemic vascular resi stance appears to be local, non-autonomically mediated vasodilatation in exercising muscles, since it also occurs inpatients with autonomic failure.