Hypoxaemia following sustained low-volume venous air embolism in sheep

In six upright (head above thorax) anaesthetised sheep, serial blood gas measurements were made over a 100-minute period during which repeated small-volume air emboli were injected intravenously to lower and maintain the end-tidal CO2 concentration approximately 0.5% below its initial baseline level. With constant volume ventilation and an inspired N2O:O2 ratio of 2:1, the arterial PCO2 progressively increased and the arterial PO2 progressively decreased with significant arterial hypoxaemia ensuing in three out of the six animals. It is suggested that during neurosurgery performed in the sitting position and with an inspired oxygen concentration of 33%, the degree of cardio-respiratory disturbance caused by venous air embolism should be assessed by continuous monitoring not only of end-tidal CO2 concentration but also of arterial oxygen saturation using pulse oximetry.