Integral Assessment of Gas Exchange During Veno-Arterial ECMO - Accuracy and Precision of a Modified Fick Principle in a Porcine Model

Assessment of native cardiac output during extracorporeal circulation is challenging. We assessed a modified Fick principle under conditions such as deadspace and shunt in 13 anesthetized swine undergoing centrally canulated veno-arterial extracorporeal membrane oxygenation (V-A ECMO, 308 measurement periods) therapy. We assumed that the ratio of carbon dioxide elimination (V̇CO2) or oxygen uptake (V̇O2) between the membrane and native lung corresponds to the ratio of respective blood flows. Unequal ventilation/perfusion (V̇/Q̇) ratios were corrected towards unity. Pulmonary blood flow was calculated and compared to an ultrasonic flow probe on the pulmonary artery with a bias of 99 mL/min (limits of agreement -542 to 741 mL/min) with blood content VO2 and no-shunt, no-deadspace conditions, which showed good trending ability (least significant change from 82 to 129 mL). Shunt conditions led to underestimation of native pulmonary blood flow (bias -395, limits of agreement -1290 to 500 mL/min). Bias and trending further depended on the gas (O2, CO2), and measurement approach (blood content vs. gas phase). Measurements in the gas phase increased the bias (253 [LoA -1357 to 1863 mL/min] for expired V̇O2 bias 482 [LoA -760 to 1724 mL/min] for expired V̇CO2) and could be improved by correction of V̇/Q̇ inequalities. Our results show that common assumptions of the Fick principle in two competing circulations give results with adequate accuracy and may offer a clinically applicable tool. Precision depends on specific conditions. This highlights the complexity of gas exchange in membrane lungs and may further deepen the understanding of V-A ECMO.