Measured and derived quantities with modern pH and blood gas equipment: Calculation algorithms with 54 equations

Modern pH and blood gas equipment, combining electrochemical and optical methods, measures pH, Pco2, po2, hemoglobin concentration, oxygen saturation, carboxy-hemoglobin, and methemoglobin. Measurements in arterial and mixed venous blood and in expired air (including measurement of expiratory ventilation) allow calculation of a series of derived quantities: concentrations of bicarbonate, total carbon dioxide, base excess, standard bicarbonate, total oxygen, 2,3-diphosphoglycerate; p50, arteriovenous CO2 and O2 differences, pulmonary shunting, ventilation-perfusion ratio, dead space ventilation, CO2 production, O2 consumption, respiratory quotient, energy metabolism, and cardiac output. Accurate measurement of carboxyhemoglobin even gives an opportunity of measuring blood volume.Equations are reviewed for these calculations including the Henderson-Hasselbalch equation for plasma and whole blood, the Van Slyke equation, the ODC-equation (hyperbolic tangent function), alveolar air equation, Bohr equation, and others. Some of the algorithms have not been previously published. The precision of the calculated quantities is evaluated on the basis of the precision of the measured quantities.