Monitoring of CO2 exchange during cardiopulmonary bypass: the effect of oxygenator design on the applicability of capnometry

Abstract

The correlation between pCO₂ values in blood and in exhaust gas from the oxygenators was examined during cardiopulmonary bypass (CPB) using one bubble oxygenator and three membrane oxygenators. Forty-seven CPBs were performed, 17 with Compactflow® (Dideco, ltaly), 10 with Maxima® (Medtronic Inc., USA), 10 with Cobe CML®(Cobe Laboratories, USA) membrane oxygenators and 10 with Hi-Flex® (Dideco, Italy) bubble oxygenators. Blood samples were taken both from arterial and venous lines of the oxygenator. A capnometer was connected to the oxygenator gas exhaust port and CO₂ fraction was measured at the time of drawing blood samples. CO₂ pressure in the gas phase was calculated from the product of the CO₂ fraction and water vapour- corrected barometric pressure. Blood gases were measured at 37°C and the pCO₂ value was corrected to the temperature of the arterial line. The correlation between blood and exhaust gas pCO₂ was good in all the oxygenators examined, ranging from 0.921 to 0.976. The standard error of estimate (SEE) was in the range of about ± 2 mmHg for all the oxygenators. The systematic error (slope and intercept of the correlation line) varied depending on the construction of the oxygenator, with countercurrent design having the best overall correspondence. Based on the results of this study it can be concluded that arterial or venous CO ₂ pressure can be monitored with a capnometry device coupled to the oxygenator gas outlet port. The use of a 'target FCO₂ line' or a calculator program is proposed in order to aid the perfusionist in adjusting the oxygenator gas flow to attain normocarbia during CPB.