Working toward Best Practice: Microbubble Filtration and Patient Safety During Extracorporeal Circulation.

I have read with great interest the article from Potger et al. (1), and commend the authors for the contribution they make in this investigation. While congratulating them on their work, I also raise the following points to continue what I believe is much needed discussion on the very important topic of gas-bubble transmission during extracorporeal circulation (ECC). To build an appropriate context for the discussion as it relates to the present study, we should first consider what happens to a bolus of air traveling down the venous line in an ECC circuit. As a bolus of air enters the venous reservoir, it passes through a venous filter usually consisting of a large micron-pore screen (>60 mm). Although a portion of the air volume entering may become trapped inside the reservoir, the relatively small numbers of large bubbles that make up the original air bolus are transformed into a much larger number of smaller bubbles as they traverse the venous filter. The large cluster of smaller microbubbles now pass through the membrane oxygenator where they are either absorbed into the oxygenator’s gas phase, or undergo another transformation in size as they are forced through the tightly wound fiber-bundle and then expelled into the oxygenator’s outlet. Next, the well-refined cluster makes its way to the arterial-line filter where bubbles are either purged out, or escape and pass through the filter toward the patient. The main point here being the stepwise reduction in bubble size that an air bolus passing through the venous reservoir undergoes before reaching the arterial-line filter. It follows then that to maximize the benefit of arterial-line filtration, the rational for the rated pore size used should consider the size range of bubbles being emitted from the oxygenator system. Although Potger’s group suggests that the integrated system of the Fusion oxygenator is more efficient at capturing microbubbles than a 38-mm stand-alone Affinity filter with NT oxygenator, an alternate perspective given in a review of the data presented from their original work may help expand on this interpretation. As shown in Tables 1 and 2, data taken from the Potger et al. article (1) are used to show the percent increase in median bubble volume and count for each system between the 3-L/min and 5-L/min tests. Although both systems exhibit an increase in transmitted bubble volume and count at higher flow rate settings, the Fusion oxygenator displayed a larger percent increase over the Affinity NT system with 2.4 times the measured volume and nearly a twofold increase in bubble count at 5 L/min as compared to 3 L/min. This suggests that air-handling performance of the Fusion oxygenator is affected more by increasing flow rate, which could have a significant clinical impact. And while blood flow rates of 5 L/min may be common during adult ECC procedures, flow rates above 5 L/min are not uncommon. Given the implications, this finding warrants further investigation to better determine air-handling capabilities of the Fusion oxygenator at higher flow rate settings. To continue, results from the Potger et al. paper (1) also presented in Table 3 show the percent increase in median bubble size measured above the rated pore size of each system in the first and third minute of testing at each flow setting in the study. Although it is not surprising that bigger bubbles were shown to pass through the larger pore diameter of the 38-mm Affinity filter, the Fusion oxygenator again showed a higher percent increase in emitted bubble size over its rated pore size at both periods and at both flow rate settings. When considering the differences in volume between a 25-mm bubble (.007 nL) and a 38-mm bubble (.028 nL), results derived from the cumulative volume measured at the outlet of each system might be more predictable then they prove to be remarkable. But what is remarkable in my opinion is the degree of stability in performance that the stand-alone filter displayed over the integrated system of the Fusion oxygenator under increasing levels of stress. Even with a larger pore diameter, the 38-mm Affinity filter with NT oxygenator still showed more consistent behavior in filtration performance over the Fusion oxygenator as flow rate increased. Another point for discussion is the noted increase in bubble volume measured at the inlet of the affinity NT system while conducting the tests at 3 L/min. The authors