Global cerebral blood flow and CPP after severe head injury: a xenon-CT study

Objective: To evaluate the association between global cerebral blood flow and different cerebral perfusion pressure ranges in severe head injury. Design: A retrospective study Setting: Neurosurgical and trauma patients in an intensive care unit in a regional hospital. Patients and participants: Out of a series of 237 consecutive patients with severe head injuries (GCS <= 8), 162 were submitted to ICP monitoring and 89 of them underwent 180 xenon-CT studies and cerebral perfusion pressure (CPP) measurement. The xenon-CT studies did not include any unsalvageable patients nor any mistakenly diagnosed as severe on entry. Interventions: None. Measurements and results: Most (95.6%) of xenon-CT studies were obtained with CPP values within the 50-90 mmHg range. Perfusion data were grouped according to CPP values: (1) below 50 mmHg, (2) 50-60 mmHg, (3) 60-70 mmHg, (4) above 70 mmHg. Global cerebral blood flow did not differ among the groups (p = 0.49). No differences in physiological variables were found among the CPP groups, except for intracranial pressure, higher in the group with CPP below 50 mmHg, and mean arterial pressure, higher in the CPP above 70 mmHg group (p < 0.0001). No differences were found for cerebral metabolic rate of oxygen and lactate. Conclusions: There was little correlation between CPP values and global cerebral blood flow levels in our selected patients, probably because pressure autoregulation was preserved. Global metabolism measurements were constant within the groups, suggesting that in patients with controlled physiological variables an interplay between cerebral blood flow and metabolism might be more relevant than the relationship between CPP and cerebral blood flow.