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Measurement of systemic carbon dioxide production during cardiopulmonary bypass: a comparison of Fick’s principle with oxygentor exhaust output

Department of Anaesthesia, Critical Care and Pain Medicine, University of Edinburgh and Department of Cardiothoracic Surgery, Royal Infirmary of Edinburgh, Edinburgh, Scotland, UK, p.alston{at}ed.ac.uk

Neil J Glassford

Department of Anaesthesia, Critical Care and Pain Medicine, University of Edinburgh and Department of Cardiothoracic Surgery, Royal Infirmary of Edinburgh, Edinburgh, Scotland, UK

Amanda Torrie

Department of Anaesthesia, Critical Care and Pain Medicine, University of Edinburgh and Department of Cardiothoracic Surgery, Royal Infirmary of Edinburgh, Edinburgh, Scotland, UK

Theoretically, systemic carbon dioxide (VCO₂) production should be an alternative means to systemic oxygen uptake (VO₂) for estimating the global efficacy of cardiopulmonary bypass (CPB). This study compared two methods of estimating VCO₂: Fick’s principle and oxygenator exhaust carbon dioxide (CO₂) output. Both of these estimates were then compared with VO₂. Fifty-one patients (39 male and 12 female) undergoing elective cardiac surgery requiring CPB were studied. Blood sampling was performed and measurements recorded during active cooling, environmental cooling/stable hypothermia and during rewarming. Blood samples were measured for CO₂ tension from which content was estimated. VCO₂ was calculated as the product of the arteriovenous difference in CO₂ content and pump flow rate (Fick’s principle), or the fresh gas flow rate and concentration of the oxygenator exhaust CO₂ (output technique). Over all measurements, method comparison analysis revealed a large mean bias of 41 (95% confidence intervals (CI) 32-50) mL/min with very wide limits of agreement (-23, 105 mL/min). Regression analysis found that the bias was also proportional to the size of measurement (ß=0.75 (95% CI 0.55, 0.95)). Although both methods of VCO₂ correlated significantly with VO₂ (p<0.01), regression analysis found that the coefficients (ß) of both techniques had wide CI (Fick’s principle: ß=1.37 (95% CI 1.20, 1.54); output technique: ß=0.58 (95%CI 0.44, 0.71)). In conclusion, both techniques of VCO₂ cannot be used interchangeably, and both are imprecisely related to VO₂ as estimated by Fick’s principle.

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