Accuracy of oxygen and carbon dioxide electrodes.

Direct determinations of the oxygen tension (Po2) and carbon dioxide tension (Pco2) of arterial blood are now widely used in the diagnosis and management of diseases of the heart and lungs. Commercial electrode systems, based on the Clark oxygen cell (Clark, 1956), and the Severinghaus Pco2 electrode (Severinghaus and Bradley, 1958) are expensive items, and it is important to know the accuracy of these commercial instruments. This report describes the performance of two commercial electrode systems, the Radiometer Co. Po2 electrode type E 5044 and Pco2 electrode type E 5030, and the Instrumenta-tion Laboratories Inc. combined Po2, Pco2, and pH electrode system type 113-S1, when used to measure the Po2 of gas mixtures and blood samples and the Pco2 of blood samples. Both these oxygen electrodes are of the modern microcathode type, in which the blood samples are not stirred in the cuvettes. The performance of the instruments was studied over the range 0-200 mm. Hg for Po2 and 20-100 mm. Hg for Pco2, as this represented the range most commonly encountered in clinical work. Methods A new tonometer system, based on the rotating disc principle of the Melrose oxygenator (Flenley and Millar, 1967) was used to prepare blood samples of known Po2 and Pco , by mixing blood with gas mixtures at 370 C. The tonometer was assessed for efficiency of oxygen transport by calculation of its diffusing capacity, the rate of equilibration, the amount of haemolysis produced, and for the effect of bacterial contamination on the results. The tonometer uses a continually flowing stream of gas, the mixtures being prepared in Douglas bags by the rota-meter mixing device of Flenley (1964). Samples of gas for Lloyd-Haldane analyses (duplicate analyses within 0.03 %) were taken from the gas stream as it left the tonometer. Freshly drawn heparinized human blood was used in the tonometer, samples being transferred to the electrode cuvettes in siliconed glass syringes, after equilibration with the gas mixtures had occurred. The Po2 electrodes were set up with " white spot " nitrogen as the low reference point, and humidified air, or blood tono-metered with air, as the high reference point. Air and nitrogen were introduced by 50-ml. syringes and the meter settings made when the readings were steady, between three and five minutes later. When the oxygen electrodes were used to measure the Po2 of gas mixtures the samples were also introduced from 50-ml. syringes. These …