The effect of the dependent position upon blood flow in the limbs.

T HE RELATIONSHIP between the blood flow in and the position of the limbs is a matter of some clinical and physiologic interest. Clinically, it is important to know in cases of ischemic arterial disease whether the limbs have a better blood flow when they are kept horizontal on a level with the body, or are allowed to be dependent. Physiologically, it is known that when a horizontal limb is lowered into a dependent position there is an increase in local arterial (hydrostatic) pressure which is opposed (except early after dependency) by an equal increase in hydrostatic venous pressure. The arteriovenous pressure gradient, therefore, is unaltered. Nevertheless, it is possible that changes may occur in the caliber of the vessels in the dependent limb which may affect the vascular resistance and alter the blood flow. Thus, there may be passive dilatation consequent to the increased intravascular pressure, or there may be active constriction, either as a reflex or a local response to increased blood pressure or flow, or there may be complicated effects that cannot be predicted. The measurement of changes in blood flow when a limb is lowered entails considerable technical difficulty. The use of a venous occlusion plethysmograph does not seem feasible because the veins of a dependent limb are so distended by hydrostatic pressure that they are not suitable for the further quantitative trapping of blood. Skin temperature measurements are of limited applicability because they do not reflect accurately the blood flow in the deeper