ROLE OF MINOR ELECTROLYTES WHEN APPLIED TO STEWARTS 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, d eveloped by Stewart, relates hydrogen ion concentration ([H+]) changes to three independent variables: the strong ion difference, weak acids , and Pco(2). The insight into acid-base balance gained by this approa ch has potentially important clinical applications. To test the theory in vivo, electrolyte and blood gas values were measured in rabbits su bjected to a period of hypoxia sufficient to produce a severe metaboli c acidosis, followed by a period of reoxygenation. These variables wer e used to calculate [H+] by Stewart's original formula, which was tl;e n compared td the measured value. A high correlation was found between measured and calculated values for a pH range of 7.3-7.6; however, pr ogressive deterioration in correlation was observed outside this range . perhaps the presence of an unidentified anion, thought to be phospha te, causes a breakdown in the accuracy of Stewart's formula at pH extr emes. The addition of phosphate values to the calculation, along with calcium and magnesium, restored the agreement throughout the physiolog ic range. To make the formula clinically applicable, modification to i ncorporate phosphate levels is required to achieve the necessary accur acy in biologic solutions at more acidemic pH ranges.