A SIMPLIFICATION OF THE STEWART EQUATION TO DETERMINE ACID-BASE STATUS IN EXERCISE

The aim of the present study was to simplify the Stewart equation and to test the validity of the proposed form. Twenty-four men performed a constant load exercise test for 30 min on a treadmill. Capillary blood samples were taken at rest, and again 10, 20 and 30 min into the test. Acid-base variables were measured using a blood-gas analyser and lactate levels were measured enzymatically. The [H(+)] was calculated using the Stewart equation: A[H(+)](4)+B[H(+)](3)+C[H(+)](2)+D[H(+)]+ E=0, and using a proposed, simplified version of this equation: A[H(+)](2)+B[H(+)]+C=0. The difference in the mean [H(+)] results obtained with the two equations was 0.004 nmol.L(-1). However, the difference between the means of the equation-derived results and the measured values was highly significant at > 8 nmol.L(-1) (p < 0.001). The proposed equation can be used to estimate [H(+)] instead of the full Stewart equation, although the values obtained are significantly different to those actually measured.