Estimate of the maximum absorption rate of microscopic arterial air emboli after entry into the arterial circulation during cardiac surgery.

Some clinicians place patients in the Trendelenburg position during aortic unclamping to decrease the incidence of microscopic cerebral air embolism. Experimental studies have shown that use of the Trendelenburg position does not prevent air emboli from reaching the brain. Nevertheless, the position can decrease the velocity at which bubbles approach the brain, giving more time for nitrogen in the bubbles to be absorbed. We applied a validated mathematical model to estimate that the maximum rate of absorption of microscopic air emboli in blood during and after cardiopulmonary bypass (CPB) equals 1.57 microns/s. This rate is very small, since clinically relevant microbubbles have radii 50-1500 microns, and travel from the aorta to the brain within seconds. The result holds for all usual conditions of CPB. We conclude that absorption does not affect the disposition of air introduced into the arterial circulation. Use of the Trendelenburg position cannot decrease neurologic injury from cerebral air embolism by permitting greater bubble absorption.