EFFECT OF HYPERVENTILATION ON REGIONAL CEREBRAL BLOOD-FLOW IN HEAD-INJURED CHILDREN

Objectives: To study cerebral blood flow and cerebral oxygen consumpti on in severe head-injured children and also to assess the effect of hy perventilation on regional cerebral blood flow, Design: Prospective co hort study. Setting: Pediatric intensive care unit at a tertiary-level university children's hospital. Patients: Twenty-three children with isolated severe brain injury, whose admission Glasgow Coma Scores were <8. Interventions: Paco(2) was adjusted by altering minute ventilatio n, Cerebral metabolic measurements were made at three levels of Paco(2 ) (>35, 25 to 35, and <25 torr [>4.7, 3.3 to 4.7, and <3.3 kPa]) after allowing 15 mins for equilibrium. Measurements and Main Results: Thir ty eight studies (each study consisting of three sets of measurements at different levels of Paco(2)) were performed on 23 patients, At each level of Paco(2), the following measurements were made: xenon-enhance d computed tomography scans; cerebral blood flow; intracranial pressur e; jugular venous bulb oxygen saturation; mean arterial pressure; and arterial oxygen saturation, Derived variables included: cerebral oxyge n consumption; cerebral perfusion pressure; and oxygen extraction rati o, Cerebral blood flow decreased below normal after head injury (mean 49.6 +/- 14.6 mL/min/100 g). Cerebral oxygen consumption decreased out of proportion to the decrease in cerebral blood flow; cerebral oxygen consumption was only a third of the normal range (mean 1.02 +/- 0.59 mL/min/100 g). Neither cerebral blood flow nor cerebral oxygen consump tion showed any relationship to time after injury, Glasgow Coma Score at the time of presentation, or intracranial pressure, The frequency o f one or more regions of ischemia (defined as cerebral blood flow of < 18 mL/min/ 100 g) was 28.9% during normocapnia, This value increased t o 73.1% for Paco(2) at <25 torr. Conclusions: Severe head injury in ch ildren produced a modest decrease in cerebral blood flow but a much la rger decrease in cerebral oxygen consumption, Absolute hyperemia was u ncommon at any time, but measured cerebral blood flow rates were still above the metabolic requirements of most children, The clear relation ship between the frequency of cerebral ischemia and hypocarbia, combin ed with the rarity of hyperemia, suggests that hyperventilation should be used with caution and monitored carefully in children with severe head injuries.