MEASUREMENT OF BLOOD CO2 CONCENTRATION WITH A CONVENTIONAL PCO(2) ANALYZER

Objectives: CO2 content can be determined from the PCO2 in an acidifie d (forces all CO2 into solution) and diluted blood sample. However, PC O2 concentrations measured in conventional blood gas analyzers are onl y correct for samples with a significant buffer capacity (such as whol e blood), so that mixing with the PCO2 in the rinse solution and tubin g walls does not significantly change the sample PCO2. This study desc ribes a calibration method and validation data for the Radiometer Medi cal ABL2 CO2 electrode system to accurately measure unbuffered blood s amples used in the determination of blood CO2 content (or other aqueou s fluids). Design: Prospective, criterion standard. Setting: Laborator y. Measurements and Main Results: Blood samples (0.4 mt) were acidifie d and diluted with 0.2 M lactic acid. After measuring PCO2, CO2 conten t was calculated using the CO2 solubility coefficient and the dilution factor of 20. CO2 content was determined in a series of sodium carbon ate (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 (r(2) = . 99; n = 48). These coefficients were successfully used to correct CO2 content determined in blood samples into which graduated amounts of so dium carbonate were added. Conclusions: This calibration procedure all ows accurate measurement of PCO2 in aqueous samples using the Radiomet er ABL2 electrode system, and should be applicable to other blood gas analyzers. Necessary syringes and chemicals are readily avail able, th e method is fast and simple, and the sample volume is small. In the pr actice of critical care medicine, accurate PCO2 measurement in aqueous acidified and diluted blood provides direct determination of blood CO 2 content (useful in calculations of modified Pick cardiac output or t issue CO2 production). Determinations of absolute CO2 content in blood requiring complex methodology are not necessary. In addition, accurat e measurement of aqueous gastric PCO2 can help determine gastric pH, w hich is an important marker of tissue perfusion.