Hypocapnia and the injured brain: more harm than benefit

Hypocapnia is used in the management of acute brain injury and may be life-saving in specific circumstances, but it can produce neuronal ischemia and injury, potentially worsening outcome. This review re-examines the rationale for the use of hypocapnia in acute brain injury and evaluates the evidence for therapeutic and deleterious effects in this context.

A MEDLINE/PubMed search from 1966 to August 1, 2009, was conducted using the search terms "hyperventilation," "hypocapnia," "alkalosis," "carbon dioxide," "brain," "lung," and "myocardium," alone and in combination. Bibliographies of retrieved articles were also reviewed.

Hypocapnia--often for prolonged periods of time--remains prevalent in the management of severely brain-injured children and adults. Despite this, there is no proof beyond clinical experience with incipient herniation that hypocapnia improves neurologic outcome in any context. On the contrary, hypocapnia can cause or worsen cerebral ischemia. The effect of sustained hypocapnia on cerebral blood flow decreases progressively because of buffering; subsequent normocapnia can cause rebound cerebral hyperemia and increase intracranial pressure. Hypocapnia may also injure other organs. Accidental hypocapnia should always be avoided and prophylactic hypocapnia has no current role.

Hypocapnia can cause harm and should be strictly limited to the emergent management of life-threatening intracranial hypertension pending definitive measures or to facilitate intraoperative neurosurgery. When it is used, Paco2 should be normalized as soon as is feasible. Outside these settings hypocapnia is likely to produce more harm than benefit.