Cerebrovascular effects of high intracranial pressure after moderate hemorrhage.

Patients with head injuries often develop increased intracranial pressure after hemorrhage. The authors studied the effect of moderate hemorrhage followed by elevated intracranial pressure on cerebrovascular variables. Cerebral blood flow in 13 pigs was measured with laser Doppler flowmetry, and cerebral venous blood gases were taken from the sagittal sinus. High intracranial pressure (80% of mean arterial pressure) was induced by infusion of artificial cerebrospinal fluid into the cisterna magna, and blood pressure was reduced by bleeding to a mean of 78% of the prebleeding values in eight pigs. Five pigs served as secondary controls. High intracranial pressure before hemorrhage caused a decrease in cerebral blood flow to 34% of the baseline values, a decrease in sagittal sinus oxygen saturation to 46%, and a decrease in cerebral perfusion pressure to 36%, but did not change cerebrovascular resistance. High intracranial pressure after hemorrhage decreased cerebral blood flow to 14% of baseline values. Sagittal sinus oxygen saturation decreased to 22%, cerebral perfusion pressure decreased to 30%, and the cerebrovascular resistance increased by 355%. The moderate hypotension after hemorrhage caused a considerable enhancement of the effects of high intracranial pressure on cerebral hemodynamics.