Reduced functional residual capacity and abnormal oxygenation in patients with severe head injury

Severe head injury often results in hypoxemia, but the pathophysiology of this phenomenon is unclear. We studied 24 patients hospitalized after severe head injury to determine whether the abnormality of oxygen transfer as measured by venous admixture (Qs/Qt) was associated with a reduction in functional residual capacity (FRC) and also what changes in these variables could be induced by positive end-expiratory pressure (PEEP). Mean FRC was 68 percent of the value predicted for the upright position, and mean Qs/Qt was 0.196. The FRC and Qs/Qt were significantly related so that patients with the lowest FRC had the highest Qs/Qt (p less than 0.001). The FRC was small enough to expect closure of small airways in many of these patients. The addition of 10 cm H2O of PEEP resulted in an increase in FRC of 28 percent (+/- 15 percent [SD] of the value predicted for upright posture; Qs/Qt declined by 0.05 (+/- 0.05 [SD] ). Most of these patients had no spontaneous breathing due either to the severity of the brain injury or to the therapeutic hyperventilation and muscular paralysis employed to control intracranial pressure. We conclude that FRC is often reduced in patients hospitalized after severe head injury and that associated abnormalities of ventilation-perfusion matching often lead to an elevated Qs/Qt. Therapy with PEEP in the range of 10 to 15 cm H2O was well tolerated by our patients and can be used safely to reduce Qs/Qt.