Controlled hyperventilation (induced hypocapnia) is frequently recommended following head trauma. The therapeutic consequences of this therapy include

Controlled hyperventilation to a PaCO2 of 25 kPa raises tissue pH, increases cerebrovascular resistance, decreases cerebral blood flow, and consequently reduces intracerebral pressure (ICP). In the effort to avoid brain swelling by lowering cerebral blood flow and ICP, the clinician must be wary of causing ischemic brain damage through hypoperfusion. The metabolic compensation to induced hypocapnia leads to normalization of the pH by loss of bicarbonate (metabolic acidosis), and over 8-24 h the beneficial effects of the hypocapnia will have been lost. The partial pressures of carbon dioxide should be allowed to slowly return to normal and should be held in reserve in case unanticipated increases in ICP require another pulse of short-term reduction. It is important to monitor the patient while the PaCO2 is rising because untoward or rapid increases in ICP may occur in response to the rising cerebral blood flow.