Problems with the gas-calibrated PCO2 electrode

The PCO2 of tonometered human blood was measured with three different Radiometer PCO2 electrodes mounted in three different blood gas analysis systems. When the PCO2 electrode was calibrated with humidified gases it measured the PCO2 of gases accurately but showed a consistent error measuring the PCO2 of human blood. This error was reduced but not abolished by calibrating the electrodes over a wide range of PCO2 and by measuring each sample repeatedly. The PCO2 of human blood was measured accurately by electrodes calibrated with tonometered human blood or buffer. The electrode responded differently to gas and human blood. This could not be explained by an electrode memory effect or by exchange of CO2 between the sample and the electrode. When human, rat, dog and goat blood were tonometered and measured under identical conditions, the difference between gas and blood PCO2 varied. The error of measurement in all species was greatest at high PCO2.