Vascular capacitance and fluid shifts in dogs during prolonged hemorrhagic hypotension

The mean circulatory pressure (Pmc) in dogs anesthetized with chloralose-urethane was estimated from 0.5 to 150 minutes after hemorrhages of 0, 17, or 34 ml/kg, or that volume giving an arterial pressure (Pa) of 40 mm Hg. The Pmc was determined by fibrillating the heart and then rapidly pumping blood from aorta to vena cava until Pa=venous pressure (Pv)=Pmc. Within about 10 seconds, the heart was defibrillated. Vascular compliance was estimated as the ratio of a test blood volume change (0, +/- 8.5, or 17 ml/kg) to the change in Pmc, determined 0.5 minute after the start of the test volume change. Erythrocyte and plasma volumes were measured by 51Cr-erythrocyte and 125I-albumin dilution. In response to prolonged hemorrhage: (1) the total vascular compliance apparently decreased; (2) most of the changes in capacity vessels occurred within 5 minutes, and indeed a large part of the response probably had occurred by the time of the first measurement at 30 seconds; (3) the progressive recovery in Pmc after 5 minutes was primarily from fluid shifting into the vasculature; (4) even after 2 hours of severe hemorrhagic hypotension, the venoconstriction was not lost; but (5) after an hour of severe hemorrhagic hypotension (arterial pressure of 40 mm Hg) there was water loss from the vasculature, because plasma protein and erythrocyte concentrations and plasma oncotic pressure increased.