Hemodynamic responses in orthostasis following 5 hours of sitting.

Six young men were administered an orthostatic test, 20 min of quiet standing before and after 5 h of quiet sitting, to determine the effect of sitting fatigue on orthostatic responses. Hemodynamic responses were recorded with an impedance plethysmograph. During the 5 h sitting period, there was a 15.6% decrease in calf blood flow, a 19.4% increase in venous pooling in the calf, and an increase in blood pressure. A comparison between the two standing tests showed increases in blood pressure and in total peripheral resistance, decreases in heart rate (7 beats/min), cardiac output (12.7%) and calf blood flow (18.5%), and 70% increase in calf venous pooling after sitting. The results show that prolonged sitting constitutes a mild orthostatic stress where an increase in venous pooling is compensated by an increase in blood pressure. When this is followed by standing, insufficient compensations are made to the large amounts of pooling resulting from both postures. Despite the maintenance of a stable blood pressure (same as at the end of the sitting period) there is an insufficient increase in total peripheral resistance and heart rate resulting in a relatively low cardiac output. Thus, prolonged sitting constitutes a liability for subsequent performance in an upright position unless methods to minimize venous pooling are employed.