Changes in forelimb weight and segmental vascular resistances following severe hemorrhage.

Abstract

The forelimb was used to study capillary fluid fluxes following hemorrhage in dogs anesthetized with sodium pentobarbital. Arterial hemorrhage (48 ml/kg) and 4 hours of hypovolemia produced decreases in forelimb weight, arterial and venous pressures, and skin and skeletal muscle blood flows. Segmental resistances (large artery, small vessel, large vein) in both skin and skeletal muscle increased markedly, and the increase was either completely or largely maintained throughout the hypovolemic period. The large arteries and large veins constricted proportionately more than the small vessels following hemorrhage. In animals that survived the hypovolemic period, the large arteries and large veins constricted almost proportionately, whereas in the animals that died, the large veins constricted proportionately more than the large arteries. The weight loss exceeded forelimb vascular volume, indicating that net extravascular fluid reabsorption occurred. In fact, the weight loss from 60-240 minutes after hemorrhage began appears to be largely attributable to extravascular fluid reabsorption, since resistance and, inferentially, blood volume were relatively constant in the forelimb capacitance vessels. These data fail to support the hypothesis that fluid filtration is a determinant of irreversibility but rather suggest that the compensatory responses to blood loss in the forelimb, i.e., vasoconstriction and net extravascular fluid reabsorption, persist during severe prolonged hemorrhagic hypotension.