CO2 is a potent cerebral vasodilator which induces major changes in cerebral blood flow and volume. Acute hypocapnia decreases cerebral blood volume; thus the intracranial pressure can be reduced rapidly, thereby improving cerebral hemodynamics. Hypocapnia, rapidly achieved by controlled hyperventilation, is therefore a valid treatment in cerebral resuscitation, especially when associated with elevated intracranial pressure. However, the effect of hypocapnia is short and unusually does not exceed a few hours. Consequently the indications for prolonged hyperventilation are very limited, if they exist at all. Intracranial pressure and cerebral blood flow changes induced by PEEP are still controversial.