The Effect of High Pressure on Microporous Membrane Oxygenator Failure

Abstract: A retrospective study to determine the relationship between early microporous membrane oxygenator (MMO) failure and blood pressure at the MMO outlet ( P mo ) conducted using data collected with 19 dogs (22 ± 1 kg, mean ± SEM) undergoing routine normothermic cardiopulmonary bypass. Because gas flow was maintained at a high level, it could not be used to control C0 2 exchange. Instead, blood Pco 2 was controlled by adding C0 2 to the sweep gas. Blood Po 2 was controlled as suggested by the manufacturer, by adjusting the %O 2 in the gas phase (g). Blood flow was 2575 ± 54 ml/min; P mo ranged from 173 to 790 mm Hg; and hematocrit was 33 ± 1%. O 2 exchange was calculated from blood gas parameters. Changes in the diffusion potential of O 2 (APo 2 ) and CO 2 (APco 2 ) and MMO performance ( P , taken as oxygen exchange normalized to a diffusion potential of 100 mm Hg) indicated MMO failure. Initial values, taken within 60 min of bypass initiation, were compared to final values taken at 226 ± 9 min of bypass. Despite higher final APo 2 (411 ± 9 vs. 538 ± 19 mm Hg, p < 0.0001 paired t–test) and APco 2 (18.6 ± 2.4 vs. 30.5 ± 4.7 mm Hg, p < 0.0017), arterial blood Po 2 decreased (159 ± 15 to 89 ± 6 mm Hg, p < 0.0005) and Pco 2 increased (36.4 ± 1.5 to 46.1 ± 3.0 mm Hg, p < 0.0039), and the performance decreased [24.5 ± 1.1 to 20.1 ± 0.7 (ml/min)/(100 mm Hg), p < 0.0001]. Regression analysis of variance between the difference in initial ( I ) and final ( F ) values and MMO outlet pressure showed that APco 2 (F – I ) = 0.051 p mo –3.2 ( r = 0.75) and δPo 2 = 0.378 P mo – 11.0 (r = 0.93). Final performance as a percentage of initial performance was related to P mo by P (%) = 98 – 0.0541 P mo (r = 0.87). MMO failure was observed within 3 h and was usually associated with ultrafiltrate expelled from the MMO gas exhaust port. Failure was most likely due to plasma breakthrough and was not prevented by high sweep gas flow. MMO failure was not correlated with time on cardiopulmonary bypass, probably due to the large number of variables in the study. Key Words: Oxygenator–Microporous membrane failure–Plasma breakthrough–Pressure.