Venous air in the bypass circuit: a source of arterial line emboli exacerbated by vacuum-assisted drainage

Background. Arterial emboli cause neurocognitive deficits in cardiac surgical patients. Carotid artery emboli, detected ultrasonically, have been observed after venous air entrainment into the cardiopulmonary bypass circuit. We investigated in vitro the extent to which venous air affected emboli detected in the arterial line downstream from a 40-μm filter. Methods. Using salvaged clinical cardiopulmonary bypass circuits, fixed volumes of air were introduced into the venous return line at unrestricted rates and at fixed rates using gravity venous drainage and vacuum-assisted venous drainage. Emboli counts were recorded distal to the arterial line filter using a 2-MHz pulsed-wave Doppler monitor. Emboli counts were similarly recorded after the introduction of carbon dioxide into the venous return line instead of air. Results. The number of emboli rose with increasing volumes of entrained venous air (p< 0.001), and there was an almost tenfold increase with vacuum-assisted venous drainage (p< 0.0001) compared with gravity venous drainage. Venous air was entrained at a significantly faster rate under vacuum-assisted venous drainage (p< 0.0001). When the entrainment rate of venous air was fixed, the difference in emboli numbers recorded for gravity and assisted venous drainage was not significant. There was a significant reduction in arterial line emboli when carbon dioxide rather than air was entrained under both vacuum-assisted and gravity drainage (p< 0.001). Conclusions. Entrained venous air during cardiopulmonary bypass is a potential hazard, particularly during vacuum-assisted venous drainage. Every effort should be made to avoid venous air entrainment.