Extracorporeal membrane oxygenation and V/Q ratios: an ex vivo analysis of CO₂ clearance within the Maquet Quadrox-iD oxygenator

Abstract

While hypercapnia is typically well treated with modern membrane oxygenators, there are cases where respiratory acidosis persists despite maximal extracorporeal membrane oxygenation support. To better understand the physiology of gas exchange within the membrane oxygenator, CO2 clearance within an adult Maquet Quadrox-iD oxygenator was evaluated at varying blood CO2 tensions and V/Q ratios in an ex vivo extracorporeal membrane oxygenation circuit. A closed blood-primed circuit incorporating two Maquet Quadrox-iD oxygenators in series was attached to a Maquet PLS Rotaflow pump. A varying blend of CO2 and air was connected to the first oxygenator to provide different levels of pre-oxygenator blood CO2 levels (PvCO2) to the second oxygenator. Varying sweep gas flows of 100% O2 were connected to the second oxygenator to provide different V/Q ratios. Exhaust CO2 was directly measured, and then VCO2 and oxygenator dead space fraction (VD/VT) were calculated. VCO2 increased with increasing gas flow rates with plateauing at V/Q ratios greater than 4.0. Exhaust CO2 increased with PvCO2 in a linear fashion with the slope of the line decreasing at high V/Q ratios. Oxygenator dead space fraction varied with V/Q ratio—at lower ratios, dead space fraction was 0.3-0.4 and rose to 0.8-0.9 at ratios greater than 4.0. Within the Maquet Quadrox-iD oxygenator, CO2 clearance is limited at high V/Q ratios and correlated with elevated oxygenator dead space fraction. These findings have important implications for patients requiring high levels of extracorporeal membrane oxygenation support.

Impaired membrane lung CO2 elimination: is it dead space, V/Q ratio or acidosis?

“…With the assumption of zero V D and using the P v CO 2 and V/Q values reported by Zakhary et al, 1 This simulation confirms a substantial decrease in P e CO 2 and therefore VCO 2 at lower pH. The slopes of the P v CO 2 to P e CO 2 and V/Q to VCO 2 relationships are substantially altered with increasing acidosis.…”

“…We read with interest the study by Zakhary et al 1 reporting the relationship between V/Q ratio and CO 2 clearance. However, we would argue that the results reported by the authors have to be interpreted differently.…”

“…With the assumption of zero V D and using the P v CO 2 and V/Q values reported by Zakhary et al, 1 we simulated (MATLAB 2020a, MathWorks, Natick, MA, USA) the resulting VCO 2 for a pH of 7.2 and 6.7, according to a previously published approach 5…”

How I approach membrane lung dysfunction in patients receiving ECMO

“…ML dysfunction can also manifest as inadequate CO 2 clearance. Calculation of V′CO 2 is not typically performed as it varies in a nonlinear fashion with sweep gas flow rate and requires sampling ML exhaust CO 2 [10]. However, persistent P Post-ML CO 2 greater than 40 mmHg [6] and clearance of less than 10 mmHg PCO 2 between pre-and post-ML blood gases despite sweep gas flow rates of 10 L/min or greater is suggestive of ML dysfunction and ML exchange should be considered.…”

Response to “Impaired membrane lung CO2 elimination: is it dead space, V/Q ratio, or acidosis?”

“…Bachman and Berger for their thoughtful insight into CO 2 clearance within the membrane lung. 1,2 We particularly appreciate the opportunity to discuss mechanisms in addition to dead-space as a rationale for reduced CO 2 clearance.…”