Intracranial volume--pressure relationships during experimental brain compression in primates. 3. Effect of mannitol and hyperventilation.

In 10 anaesthetized and ventilated baboons a steady level of raised intracranial pressure was achieved by graduated inflation of an extradural balloon. Measurements were made of the ventricular fluid pressure, and of the change in this pressure after injection of 0·05 ml into the ventricle, the volume-pressure response. This response was studied at normocapnia and at hypocapnia (induced by hyperventilation), and before and after the intravenous administration of mannitol under normocapnic and hypercapnic conditions. During hypocapnia, ventricular fluid pressure and the volume-pressure response were reduced equally. Mannitol, however, caused a greater reduction in the volume-pressure response than in ventricular fluid pressure. The intravenous administration of mannitol therefore produces a beneficial effect on intracranial capacitance which is greater than observation of intracranial pressure alone indicates.