Prediction of Arterial Blood pCO2 by Measuring the Ventilating Gas Exhaust pCO2 from a Bubble Oxygenator

Abstract

Ventilating gas exhaust pCO2 (PexCO2) was continuously monitored by a mass spectrometer during hypothermic, total heart lung bypass employing a bubble oxygenator. The hypothesis that the arterial blood pCO2, temperature corrected to the arterial blood temperature (pCO2, TBo) is in equilibrium with the PexCO2 was tested. While the PexCO2 trends well with the pCO2, TBo during rapid cooling, it assumes a lower value compared to the arterial pCO2 corrected to the nasopharyngeal temperature (pCO2, TNP) because the blood temperature is much less than nasopharyngeal. During warming, PexCO2 reads consistently lower than pCO2, TBo apparently due to increased blood CO2 removal (V̇CO2) and an inability of the PexCO2 to come to equilibrium with pCO2, TBo in the oxygenator. During low V̇CO2 and steady state conditions, PexCO2 predicts pCO2, TBo well (r = .803, p <.01 ). The percent disagreement of PexCO2 and pCO2, TBo is a linear function of the V̇CO2 (r = −.606, p <.01).