Determinants of cardiac output during experimental therapeutic hemodilution.

In 20 dogs, blood loss of 20 ml/kg during thoracotomy was immediately replaced with 40 ml/kg of 2.5% dextran-40 or dextran-70 in saline. An immediate rise in blood volume, associated with a decrease in hematocrit and blood viscosity, produced a marked rise in cardiac output. This effect was slightly greater with dextran-40. During the first four hours after infusion of dextran-40, both blood volume and cardiac output fell below control levels. Blood volume and cardiac output remained at or near control levels during this period in the group receiving dextran-70. In both groups, blood viscosity and hematocrit remained low throughout the study, but cardiac output increased only in response to an increase in blood volume. Twenty-four hours after infusion of either dextran-40 or dextran-70, hematocrit had decreased further, presumably due to postoperative blood loss while blood volume and cardiac output remained at or near control levels. During isovolemic hemodilution in this experimental setting, cardiac output responded primarily to changes in blood volume rather than blood viscosity. Therefore, decreased oxygen availability during hemodilution was compensated only by a rise in oxygen extraction.