Cerebral microemboli during cardiopulmonary bypass: increased emboli during perfusionist interventions

Background. Microemboli to the cerebral circulation occur during cardiopulmonary bypass (CPB) and can contribute to postoperative neurologic dysfunction. Cerebral microemboli are known to occur during specific surgical interventions, but the source of a large proportion of emboli remains unexplained. We investigated whether interventions by the perfusionist could account for the appearance of cerebral microemboli. Methods. Transcranial Doppler ultrasonography was used to continuously monitor the middle cerebral artery of 18 patients undergoing coronary artery bypass grafting. The CPB circuit consisted of a softshell venous reservoir, a hollow-fiber membrane oxygenator, and a 32-μm arterial filter. The mean embolic rate was calculated for three time periods: (1) during surgical interventions (aortic cannulation and decannulation, cross-clamp application and removal, CPB start and end, and start of cardiac ejection); (2) during perfusionist interventions (blood sampling and drug administration into the venous reservoir); and (3) during baseline (all other time periods during CPB). Results. Microemboli were detected in all patients (mean ± standard deviation, 207 ± 142 per patient, median, 132). The number of emboli per minute was significantly (p< 0.001) higher during perfusionist interventions (6.9 ± 4.5) than during surgical interventions (1.5 ± 1.5) or during baseline (0.4 ± 0.5). Drug administration resulted in a higher embolic rate than blood sampling. Conclusions. Interventions by the perfusionist account for a large proportion of previously unexplained cerebral microemboli during CPB. These emboli likely represent air bubbles that are not eliminated by the arterial line filter. Although further studies of additional types of CPB circuits are required, we believe that air in the venous reservoir should be avoided whenever possible to minimize the risk of neurologic injury.