The Relationship between Oxygenator Exhaust Pco 2 and Arterial Pco 2 During Hypothermic Cardiopulmonary Bypass

Abstract

During cardiopulmonary bypass the partial pressure of carbon dioxide in oxygenator arterial blood (P_aco₂) can be estimated from the partial pressure of gas exhausting from the oxygenator (P_Eco₂). Our hypothesis is that P_Eco₂ may be used to estimate P_aco₂ 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°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 P_aco₂ 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°C. P_Eco₂ 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 P_Eco₂ and P_aco₂ was 0.6 mmHg, with limits of agreement (±2 SD) between -5 to +6 mmHg. P_Eco₂ tended to underestimate P_aco₂ at low arterial temperatures, and overestimate at high arterial temperatures.

We have demonstrated that P_Eco₂ can be used to estimate P_aco₂ during hypothermic cardiopulmonary bypass using a Terumo 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.