No reduction in cerebral metabolism as a result of early moderate hyperventilation following severe traumatic brain injury

Object. Hyperventilation has been used for many years in the management of patients with traumatic brain injury (TBI). Concern has been raised that hyperventilation could lead to cerebral ischemia; these concerns have been magnified by reports of reduced cerebral blood flow (CBF) early after severe TBI. The authors tested the hypothesis that moderate hyperventilation induced early after TBI would not produce a reduction in CBF severe enough to cause cerebral energy failure (CBF that is insufficient to meet metabolic needs). Methods. Nine patients were studied a mean of 11.2 ± 1.6 hours (range 8-14 hours) after TBI occurred. The patients' mean Glasgow Coma Scale score was 5.6 ± 1.8 and their mean age 27 ± 9 years; eight of the patients were male. Intracranial pressure (ICP), mean arterial blood pressure, and jugular venous oxygen content were monitored and cerebral perfusion pressure was maintained at a level higher than 70 mm Hg by using vasopressors when needed. Measurements of CBF, cerebral blood volume (CBV), cerebral metabolic rate for oxygen (CMRO₂), oxygen extraction fraction (OEF), and cerebral venous oxygen content (CvO₂) were made before and after 30 minutes of hyperventilation to a PaCO₂ of 30 ± 2 mm Hg. Ten age-matched healthy volunteers were used as normocapnic controls. Global CBF, CBV, and CvO₂ did not differ between the two groups, but in the TBI patients CMRO₂ and OEF were reduced (1.59 ± 0.44 ml/100 g/minute [p < 0.01] and 0.31 ± 0.06 [p < 0.0001], respectively). During hyperventilation, global CBF decreased to 25.5 ± 8.7 ml/100 g/minute (p < 0.0009), CBV fell to 2.8 ± 0.56 ml/100 g (p < 0.001), OEF rose to 0.45 ± 0.13 (p < 0.02), and CvO₂ fell to 8.3 ± 3 vol% (p < 0.02); CMRO₂ remained unchanged. Conclusions. The authors conclude that early, brief, moderate hyperventilation does not impair global cerebral metabolism in patients with severe TBI and, thus, is unlikely to cause further neurological injury. Additional studies are needed to assess focal changes, the effects of more severe hyperventilation, and the effects of hyperventilation in the setting of increased ICP.