The Hammer Effect: Variables that Promote Air Migration Across Semi-Permeable Membranes During Cardiopulmonary Bypass

Abstract Background: One of the most prominent concerns surrounding extracorporeal circulation (ECC) is gaseous emboli. Any air embolism, regardless of size, significantly increases the risk of adverse postoperative outcomes and sentinel events. Previous research by van Kaam and Stehouwer (2) has shown that the abrupt cessation of flow in a closed system can create a large pressure gradient, also known as the “water hammer” effect. In ECC, this phenomenon may drive air across the semi-permeable hollow-fiber membrane oxygenator when flow is suddenly terminated. Methods: This in-vitro experiment expanded on Van Kaam’s findings by manipulating flow speed, sweep rate, and post-oxygenator arterial line pressure (ALP). Air presence was defined as any visible macro air in the oxygenator following pump stoppage. Flow cessation was achieved by lifting the cover on a LivaNova S5 roller pump, stopping the pump through servo-regulation. An arterial line occluder was used to methodically constrict the tubing to achieve adequate ALPs. The sweep rate was set at a defined ratio according to the blood flow rate. The oxygenator tested was fully reprimed between each trial. Results: The data collected and analyzed was significant in various key findings. As the blood flow rate increased, the incidence of air migration increased (R2=.72). There was a significant association between air migration and ALP (p<0.001). When arterial line pressure is lower, the risk of air embolism following sudden pump stoppage is greater. Conclusion: With these findings, we aim to raise awareness of the hammer effect and its implications on patients undergoing ECC. A clinician’s attention to these details during the preoperative and intraoperative periods of care may prove beneficial in reducing the incidence of air embolism through proactive prevention rather than relying on reactive measures.