THRESHOLDS FOR CEREBRAL-ISCHEMIA AFTER SEVERE HEAD-INJURY - RELATIONSHIP WITH LATE CT FINDINGS AND OUTCOME

Cerebral ischemic insults occur in at least 30% of severely head injur ed patients at a very early stage following trauma and are associated with early death. To date, the threshold for ischemia of 18 mL/100 g/m in used in human head injury studies has been adopted from animal stud ies (by temporary occlusion of the middle cerebral artery). Since the traumatized brain becomes more susceptible to irreversible damage if a ccompanied by ischemia one may question whether the threshold for isch emic vulnerability is higher than 18 mL/100 g/min. Cerebral ischemia c an cause atrophy. Therefore, the authors obtained computerized tomogra phy (CT) scans in 33 comatose head-injured patients (Glasgow Coma Scor e of 8 or less) at least 3 months following injury and compared ventri cle sizes (as a reflection of atrophy) with cerebral blood flow (CBF) obtained within 4 h (average 2.3 +/- 0.8 h) after injury. Ventricular measurements were performed in three fashions: the third ventricular s ize (cm), the bicaudate cerebral ventricular index (BCVI), and the hem ispheric ventricular index (HCVI). No significant correlation was foun d between early CBF and any of the ventricle sizes. Applying a multipl e correlation analysis with four independent parameters [CBF, CBF/time postinjury, CBF/(time postinjury)(2), age], only age emerged as a sig nificant indicator for predicting ventricle size (p < 0.001). We also compared CBF data, obtained within 4 h after trauma, from survivors at 3 months after injury (mean CBF of 32 mL/100 g/min) with CBF data fro m nonsurvivors (CBF 20 mL/100 g/min). The difference in CBF between su rvivors and nonsurvivors was significant at p < 0.001 (Wilcoxon rank-s um test). The proportion of patients with CBF less than or equal to 20 mL/100 g/min was 56% in the nonsurvivors and only 5% in survivors. Th e difference in the proportions was significant at p < 0.001 (chi-squa re test). We conclude that a measure of atrophy does not correlate wit h ultra-early CBF. However, based on the clear distinction between sur vivors and nonsurvivors, we suggest the threshold for ischemia after h ead injury be redefined as a CBF of 20 mL/100 g/min.