Evaluating cerebral oxygenation using jugular venous oximetry in head injuries

When caring for the critically ill patient, a primary objective is to maintain an adequate delivery of oxygen to the tissues. Monitoring and ensuring satisfactory cerebral oxygen delivery in patients at risk for cerebral ischemia presents a significant challenge to the clinician. Cerebral oxygen content usually is determined using blood gas analysis, whereas blood flow to cerebral tissues is inferred from monitoring of the cerebral perfusion pressure. There is an assumption that cerebral oxygen delivery is adequate so long as the cerebral perfusion pressure exceeds 50-60 mmHg in the normal brain and the cerebral arterial oxygen content is normal. The cerebral perfusion pressure, however, is an estimated value calculated as the difference between the mean systemic arterial pressure and the intracranial pressure and provides merely an indirect index of the adequacy of cerebral oxygen delivery. The validity of the cerebral perfusion pressure as an index for cerebral oxygen delivery becomes questionable when the autoregulatory mechanisms of the brain are disturbed, as in severe head injuries. Jugular bulb oxygen monitoring provides a method of evaluating cerebral oxygenation status for early identification of impending ischemia by measuring oxygenation of the mixed cerebral venous blood. In this article, the authors review the physiology of cerebral oxygenation, describe a method of monitoring cerebral oxygenation using jugular bulb oximetry, and review a case study to show how these assessments can affect the nursing care of the head-injured patient.