Hyperventilation is commonly used in neurological patients to decrease elevated intracranial pressure (ICP) or relax a tense brain.

However, the potentially deleterious effects of hyperventilation may limit its clinical application.

The aim of this review is to summarize the physiological and outcome evidence related to hyperventilation in neurological patients.

Physiologically, hyperventilation may adversely decrease cerebral blood flow (CBF) and the match between the cerebral metabolic rate and CBF.

In patients with severe traumatic brain injury (TBI), prolonged prophylactic hyperventilation with arterial carbon dioxide tension (PaCO2) less than 25 mmHg or during the first 24 h after injury is not recommended.

Most patients (>90%) with an aneurysmal subarachnoid hemorrhage undergo hyperventilation (PaCO2 <35 mmHg); however, whether hyperventilation is associated with poor outcomes in this patient population is controversial.

Hyperventilation is effective for brain relaxation during craniotomy; however, this practice is not based on robust outcome evidence.