Raised intracranial pressure and cerebral blood flow. 3. Venous outflow tract pressures and vascular resistances in experimental intracranial hypertension.

Pressure changes within the venous outflow tract from the brain were studied in anaesthetized baboons. Segmental vascular resistance changes were also calculated and the results correlated with the changes in cerebral blood flow, measured by the (133)Xenon clearance method. Three different methods were used to raise intracranial pressure: cisterna magna infusion, a supratentorial subdural balloon, and an infratentorial subdural balloon. A close correlation was found between the cortical vein pressure and intracranial pressure with all methods of raising intracranial pressure: the overall correlation coefficient was 0·98. In the majority of animals sagittal sinus pressure showed little change through a wide range of intracranial pressure. In three of the six animals in the cisterna magna infusion group, however, sagittal sinus pressure increased to levels approaching the intracranial pressure during the later stages of intracranial hypertension. Jugular venous pressure showed little change with increasing intracranial pressure. The relationship between cerebral prefusion pressure and cerebral blood flow differed according to the method of increasing intracranial pressure. This was due to differing patterns of change in prevenous vascular resistance as venous resistance increased progressively with increasing pressure in all three groups. The present results confirm, therefore, the validity of the current definition of cerebral perfusion pressure-that is, cerebral perfusion pressure is equal to mean arterial pressure minus mean intracranial pressure-by demonstrating that intracranial pressure does represent the effective cerebral venous outflow pressure.