Role of minor electrolytes when applied to Stewart's acid-base approach in an acidotic rabbit model

Current clinical practice regarding acid-base balance is derived from the traditional Henderson-Hasselbalch equation. An alternate theory, developed by Stewart, relates hydrogen ion concentration ([H+]) changes to three independent variables: the strong ion difference, weak acids, and PCO2. The insight into acid-base balance gained by this approach has potentially important clinical applications. To test the theory in vivo, electrolyte and blood gas values were measured in rabbits subjected to a period of hypoxia sufficient to produce a severe metabolic acidosis, followed by a period of reoxygenation. These variables were used to calculate [H+] by Stewart's original formula, which was then compared to the measured value. A high correlation was found between measured and calculated values for a pH range of 7.3-7.6; however, progressive deterioration in correlation was observed outside this range. Perhaps the presence of an unidentified anion, thought to be phosphate, causes a breakdown in the accuracy of Stewart's formula at pH extremes. The addition of phosphate values to the calculation, along with calcium and magnesium, restored the agreement throughout the physiologic range. To make the formula clinically applicable, modification to incorporate phosphate levels is required to achieve the necessary accuracy in biologic solutions at more acidemic pH ranges.