Evaluation of severinghaus' equation and its modification for 2, 3-dpg.

Severinghaus' equation can safely be used for the indirect estimation of oxygen half saturation pressure (P50) on the basis of blood gas parameters in normal subjects. However, 139 blood samples from 65 patients with severe injuries, the estimated P50 values differed significantly from directly measured values. The difference is highly correlated to the molar ratio of 2, 3-diphosphoglycerate to hemoglobin tetramer (2, 3-DPG ratio, [DPG]). Using this correlation, a modified Severinghaus' equation, including 2, 3-DPG molar ratio, was derived: i.e.,deltalog Po2 equals +0.48 (7.4-pH)+0.024 (T-37), +0.0013BE+0.135[DPG]-0.116 where deltalog Po2 is the shift from the standard oxygen dissociation curve, pH is plasma pH, and T and BE refer to temperature and base excess of blood, respectively. The modified equation enables one to indirectly estimate Po2 and oxygen saturation with an accuracy of plus or minus 2.5 mmHg and plus or minus 5 percent, respectively, based on blood gas parameters and 2, 3-DPG molar ratio in most clinical cases. The limitations of the equation was discussed.