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)
MethodsA 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 rotameter 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 tonometered 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 gas mixtures, consisting of nitrogen, oxygen, and carbon dioxide, were analysed by the Lloyd-Haldane apparatus. The Pco2 electrodes were set up with mixtures of approximately 3% CO2 in 17 % 02 and 9 % CO2 in 17 % 02, the actual concentrations being determined by LloydHaldane analysis of the compressed gas mixtures. These reference gases were flushed through the cuvettes of the CO. electrodes at 25 ml./min. for at least 10 minutes, all gas connexions being by thick-wall rubber tubing. settings were made with zero gas flow for both Po2 and Pco2electrodes.The electrode solutions, membranes, and setting-up procedure were exactly as described by the manufacturers. The membranes were polypropylene for the Po2 electrodes and Teflon for the Pco2 electrodes. Before each reading of tonometered blood the Po2 electrodes were flushed with nitrogen, air, and finally tonometer gas, which immediately preceded the sample of tonometer blood. The Pco2 electrodes were flushed with tonometer gas before each blood sample. The electrode cuvettes were filled with 1: