Cerebral circulatory effects of acutely induced hypervolemia in human subjects.

• Hemodynamie alterations incident to acute changes in intravaseular volume have been extensively investigated in man as well as in animals. From these studies have emerged well-defined concepts of the responses elicited by acute reductions in blood volume. On the other hand, studies of the effects of expansion of the intravascular volume have yielded no clear hemodynamie sequence whereby the cardiovascular system reacts or adjusts to its expanded contents. Thus, many observers" have noted an increase in right atrial pressure and cardiac output; others have noted the pressure increase but have found no consistent change in cardiac output. Fowler, Bloom, and Ward have adduced evidence indicating that selective expansion of the plasma volume (dextrau) induces a consistent increase in cardiac output, whereas expansion of the iutravascular contents with whole blood fails to elicit this response. In addition, several groups" have investigated the effects of plasma volume expansion on renal blood flow and have largely agreed that renal perfusion is substantially increased under these circumstances. There is no evidence that blood flow to the brain is influenced by expansion of the intravascular volume. Cerebral vascular resistance is largely governed by alterations in the CO? tension of the blood. There is evidence, however, which would suggest that cerebral blood flow may vary with alterations in perfusion pressure, as with acute hypotension, or with changes in viscosity and/or O2 capacity, as with anemia" or polycythemia.