A likely mechanism responsible for sustained early cerebral hypoperfusion relates to impaired cerebrovascular auto-regulation. Such impaired cerebral auto-regulation may make even a normal arterial carbon dioxide tension (PaCO2) (the major physiological regulator of cerebral blood flow) insufficient to achieve and maintain adequate cerebral perfusion and, consequently, cerebral oxygenation. However, PaCO2 is the major determinant of cerebral blood flow and an increased PaCO2 (hypercapnia) markedly increases cerebral blood flow. Moreover, arterial carbon dioxide is modifiable and, as such, is a potential therapeutic target.