The radiometer ABL300 blood gas analyzer.

A modern blood gas analyzer such as the ABL300 directly accepts samples from syringes or capillary tubing. The ABL300 measures pH, carbon dioxide tension, and oxygen tension on 85-microliters samples by using specific electrodes, and estimates hemoglobin concentration from the optical density of a nonhemolyzed sample. From these it calculates bicarbonate, standard bicarbonate, total carbon dioxide, base excess, standard base excess, and oxygen saturation and content. The electrodes are automatically calibrated by pumping bicarbonate/phosphate buffer solutions, gas-equilibrated within the thermostatic chamber, into the measuring cuvettes. Samples are preheated in metal tubing before being pumped automatically into the measuring chamber. An open liquid junction to a half-saturated potassium chloride solution is renewed with each sample to complete the pH circuit. The electrodes are housed in a 37 degrees C circulated-air thermostat, which also contains the equilibrators for the calibration solutions. Two known carbon dioxide mixtures are made from pure carbon dioxide by dilution with room air, which is compressed continuously, avoiding the use of premixed calibration gases. The computer determines the derived variables and can correct pH, carbon dioxide tension, and oxygen tension values to a user-defined body temperature. Corrections are made for predicted discrepancy between the measured and true value caused by the small sample size. The corrections depend on the difference in gas tension between sample and rinse solution.