The relationship between oxygenator exhaust PCO2 and arterial PCO2 during hypothermic cardiopulmonary bypass

During cardiopulmonary bypass the partial pressure of carbon dioxide in oxygenator arterial blood (PaCO2) can be estimated from the partial pressure of gas exhausting from the oxygenator (PECO2). Our hypothesis is that P(E)CO2 may be used to estimate PaCO2 with limits of agreement within 7 mmHg above and below the bias. (This is the reported relationship between arterial and end-tidal carbon dioxide during positive pressure ventilation in supine patients.) During hypothermic (28-32 degrees C) cardiopulmonary bypass using a Terumo Capiox SX membrane oxygenator, 80 oxygenator arterial blood samples were collected from 32 patients during cooling, stable hypothermia, and rewarming as per our usual clinical care. The PaCO2 of oxygenator arterial blood at actual patient blood temperature was estimated by temperature correction of the oxygenator arterial blood sample measured in the laboratory at 37 degrees C. PECO2 was measured by connecting a capnograph end-to-side to the oxygenator exhaust outlet. We used an alpha-stat approach to cardiopulmonary bypass management. The mean difference between PECO2 and PaCO2 was 0.6 mmHg, with limits of agreement (+/-2 SD) between -5 to +6 mmHg. PECO2 tended to underestimate PaCO2 at low arterial temperatures, and overestimate at high arterial temperatures. We have demonstrated that PECO2 can be used to estimate PaCO2 during hypothermic cardiopulmonary bypass using a Teruno Capiox SX oxygenator with a degree of accuracy similar to that associated with the use of end-tidal carbon dioxide measurement during positive pressure ventilation in anaesthetized, supine patients.