Hemodynamics of dogs in histamine shock, with special reference to splanchnic blood volume and flow.

• It is well-known that the administration of a large dose of histamine to experimental animals can produce severe and persistent hypotension, a condition often referred to as histamine shock. Because of the increase in hematocrit observed under such circumstances, the cause of the decrease in arterial pressure was once attributed to an escape of plasma from the dilated capillaries, with ensuing reductions in blood volume and cardiac output. Using T-1824 to measure directly the plasma volume in unanesthetized dogs before and after the subcutaneous injection of large doses of histamine, however, Deyrup found no decrease in the plasma volume when the arterial pressure was markedly reduced for many hours. Furthermore, the increase of hematocrit usually found in dogs during histamine hypotension cannot be demonstrated after splenectomy.' Thus, in the dog at least, the so-called hemoconcentration is actually due to a contraction of the spleen. In unanesthetized dogs, Deyrup and Eoot showed that the cardiac output decreased drastically after the subcutaneous injection of large doses of histamine. Since the heart rate increased under such conditions, it seems likely that the reduction in cardiac output is caused by a decrease in venous return. Although the total blood volume does not decrease during histamine shock,' a change in the distribution of blood occurs such that the Fceiis value (overall cell percentage/large vessel cell percent-