Cooling Gradients and Formation of Gaseous Microemboli With Cardiopulmonary Bypass: An Echocardiographic Study

Background. Previous studies demonstrated gas emboli formation during rewarming from hypothermia on cardiopulmonary bypass when the temperature gradient exceeded a critical threshold. It also has been suggested that formation of arterial gas emboli may occur during cooling on cardiopulmonary bypass when cooled oxygenated blood exiting the heat exchanger is warmed on mixture with the patient’s blood. The purpose of this study was to determine under what circumstances gas emboli formation would occur during cooling on cardiopulmonary bypass. Methods. Eight anesthetized mongrel dogs were placed on cardiopulmonary bypass using a roller pump, membrane oxygenator, and arterial line filter. For emboli detection, we positioned a transesophageal echocardiographic probe at the aortic arch distal to the aortic cannula and Doppler probes at the common carotid artery and the arterial line. Cooling gradients between normothermic blood and cooled arterial perfusate of 5°, 10°, 15°, 20°, and 0°C (isothermal controls) were investigated. In addition to preestablished temperature gradients, we investigated the effect of rapid cooling (maximal flow through the heat exchanger at a water bath temperature of 4°C) after the initiation of normothermic cardiopulmonary bypass. Results. Minimal gas emboli were detected at the aortic arch at gradients of 10°C or greater. The incidence of emboli was related directly to the magnitude of the temperature gradient (p < 0.01). No emboli were detected at the carotid artery. During rapid cooling, no emboli were observed either at the aorta or at the carotid artery. Conclusions. Cooling gradients of 10°C or greater may be associated with gas emboli formation, but they may be of limited clinical significance because no emboli were detected distal to the aortic arch. During the application of rapid cooling, no emboli formation was observed.