Sequential changes in distribution of cardiac output in hemorrhagic shock

The distribution of cardiac output was measured in various stages of hemorrhagic shock in unanesthetized dogs by microspheres labeled with five different isotopes. The microspheres were injected sequentially through a previously implanted left atrial catheter at the following times: control, early hypotension, late hypotension, and one hour as well as eight hours after return of the shed blood. Cardiac output was independently measured by the indicator dilution method at the time of each microsphere injection. Regional blood flows measured with microspheres were expressed in terms of the absolute values and as the percent distributions of the cardiac output. The early response to hemorrhage consisted of a redistribution of flow favoring the heart and brain, which have the least alpha adrenergic innervation. Later in hypotension, the percent cardiac output to the heart and arterial inflow to the liver and diaphragm increased further; this secondary change may be attributed to metabolic vasodilatory factors. Immediately after the return of the shed blood, organ blood flow changed in the opposite direction to the initial response. Eight hours after infusion, there were further increases in the blood flow to organs which had not immediately returned to control values. The data support the concept of participative roles of metabolic vasodilatory influences, increased metabolic requirements, and continued operation of neural and hormonal factors in the posttransfusion period.