Lack of relevance of the Bohr effect in optimally ventilated patients with acute brain trauma.

The relationship between jugular bulb oxyhemoglobin saturation (SjO2) and oxygen tension (PjO2) during hyperventilation was prospectively evaluated in 37 adults with acute brain injuries. Hyperventilation was optimized in all patients, based on measured values of the arteriojugular oxyhemoglobin saturation difference, or cerebral extraction of oxygen (CEO2). Most patients initially had variable CEO2 values, in 578 observations carried out in the acute phase. Overall, there was a strong positive correlation between SjO2 and PjO2 at all levels of arterial pH. In 31 (5.3%) of the 578 observations, where the arterial pH was greater than 7.6, the SjO2 was disproportionally higher than the PjO2, despite a good SjO2-PjO2 correlation (moderate Bohr effect). In only 5 of these 31 observations (0.8%) did the SjO2 and PjO2 largely change in opposite directions (marked Bohr effect) during profound hypocapnia. The present findings support the current practice of continuous or intermittent SjO2 monitoring and management, and of optimized hyperventilation for control of intracranial hypertension, provided that hyperventilation is optimized according to the CEO2 values. Only on rare occasions (arterial pH greater than 7.6) is it advisable to replace SjO2 measurements by those of PjO2, because the latter is not affected by the Bohr effect.