Measurement of blood CO₂ concentration with a conventional PCO₂ analyzer

Objectives: CO₂ content can be determined from the PCO₂ in an acidified (forces all CO₂ into solution) and diluted blood sample. However, PCO₂ concentrations measured in conventional blood gas analyzers are only correct for samples with a significant buffer capacity (such as whole blood), so that mixing with the PCO₂ in the rinse solution and tubing walls does not significantly change the sample PCO₂. This study describes a calibration method and validation data for the Radiometer Medical ABL2 CO₂ electrode system to accurately measure unbuffered blood samples used in the determination of blood CO₂ content (or other aqueous fluids). Design: Prospective, criterion standard. Setting: Laboratory. Measurements and Main Results: Blood samples (0.4 mL) were acidified and diluted with 0.2 M lactic acid. After measuring PCO₂, CO₂ content was calculated using the CO₂ solubility coefficient and the dilution factor of 20. CO₂ content was determined in a series of sodium carbonate (Na₂CO₃) solutions spanning the physiologic range of CO₂ content. Regression of the measured vs. the actual CO₂ content data generated a straight line with a slope of 0.796 and y- intercept of 12.5 (r² = .99; n = 48). These coefficients were successfully used to correct CO₂ content determined in blood samples into which graduated amounts of sodium carbonate were added. Conclusions: This calibration procedure allows accurate measurement of PCO₂ in aqueous samples using the Radiometer ABL2 electrode system, and should be applicable to other blood gas analyzers. Necessary syringes and chemicals are readily available, the method is fast and simple, and the sample volume is small. In the practice of critical care medicine, accurate PCO₂ measurement in aqueous acidified and diluted blood provides direct determination of blood CO₂ content (useful in calculations of modified Fick cardiac output or tissue CO₂ production). Determinations of absolute CO₂ content in blood requiring complex methodology are not necessary. In addition, accurate measurement of aqueous gastric PCO₂ can help determine gastric pH, which is an important marker of tissue perfusion.