Pressure drop, shear stress, and activation of leukocytes during cardiopulmonary bypass: a comparison between hollow fiber and flat sheet membrane oxygenators

The membrane oxygenator is known to be superior to the bubble oxygenator, but little information is available about the difference between the hollow fiber and flat sheet membrane oxygenators with regard to pressure drop, shear stress, and leukocyte activation. In this study, we compared these 2 types of membrane oxygenators in patients undergoing cardiopulmonary bypass (CPB) surgery with special focus on leukocyte activation and pressure drop across the oxygenators. Plasma concentration of elastase, a marker indicating leukocyte activation, increased to 593+/-68% in the flat sheet oxygenator group versus 197+/-42% in the hollow fiber oxygenator group (p<0.01) at the end of CPB compared to their respective baseline concentrations before CPB. Pressure drop across the oxygenator was significantly higher in the flat sheet group than in the hollow fiber group throughout the entire period of CPB (p<0.01). High pressure drop across the oxygenator as well as the calculated shear stress was positively correlated with the release of elastase at the end of CPB (r = 0.760, p<0.01, r = 0.692, p<0.01). However, this positive correlation existed in the flat sheet oxygenator but not in the hollow fiber oxygenator. Clinically, both membrane oxygenators have satisfactory performance in O2 and CO2 transfer. These results suggest that a higher pressure drop across the flat sheet oxygenator is associated with more pronounced activation of leukocytes in patients undergoing cardiopulmonary bypass.