Decreased CO2 or bicarbonate in blood. Hypocapnia, even when marked, is normally well tolerated; transient induction of hypocapnia can lead to life-saving physiologic changes in patients with severe intracranial hypertension or neonatal pulmonary-artery hypertension, but hypocapnia of longer duration in critically ill patients adversely affects outcomes. Prophylactic induction of hypocapnia has no clinical role.