Sequential changes in cerebral blood flow and distribution of flow within the brain during hemorrhagic shock.

Sequential changes in cerebral blood flow as well as in regional blood flow to the brain (brain stem, cerebellum, hypothalamus, white matter and grey matter) were measured in unanesthetized dogs subjected to gradual prolonged hemorrhage according to a protocol which stimulates the most commonly encountered type of clinical hemorrhagic shock. Microspheres labeled with five different radioactive isotopes were injected into a left atrial catheter at five different times: control, early hypotension (immediately after hemorrhage), late hypotension (just before reinfusion of the shed blood), as well as one and eight hours after reinfusion of the shed blood. Immediately after hemorrhage, the total cerebral blood flow decreased slightly, but increased when calculated as a percent of the cardiac output. In the late hypotensive, hypovolemic stage, there was decreased flow calculated both as percentages of cardiac output and absolute flow as compared with the initial response to hemorrhage. Immediately after reinfusion of the shed blood, there were further reductions of flow. Eight hours subsequently, flow rose to values slightly above control. The patterns of each region was almost identical to that of the total cerebral flow. Since each of the major regions of the brain are approximately equally affected, changes in the level of consciousness and other cerebral functions occurring with hypovolemic shock may reflect circulation of the white matter as well as that of the whole brain.