Acid-base regulation: a comparison of quantitative methods.

The [H+] and [HCO3-] of biological solutions is determined by the PCO2, the concentration of strong ions (mainly Na+, K+, Ca2+, Cl-, lactate-), and the concentration of weak acids (mainly proteins, phosphates). Two mathematical models are available that use a quantitative approach to describe the acid-base behaviour of plasma, but which differ in their treatment of the weak acid component: Stewart model (using PCO2, strong ion difference (SID = [Na+ + K+ + Ca2+] - [Cl- + lactate-]) and [protein]TOT); Fencl model (using PCO2, SID, [albumin], and [Pi]TOT). The present study compared measured and estimated [H+] and [HCO3-] in whole-blood samples collected from eight subjects during two double-ramp exercise protocols to the limit of tolerance to assess the accuracy with which each of the quantitative models predicts measured values. Arterialized-venous blood was analyzed for [H+], PCO2, [protein]TOT, [albumin], [Pi]TOT, and SID (= [Na+ + K+ + Ca2+] - [Cl- + lactate-]), and these independent variables were then substituted into the appropriate mathematical model to estimate [H+] and [HCO3-]. Analysis showed that the [H+] and [HCO3-] estimated using either model provided a good estimate of the [H+] (Stewart model, r = 0.81; Fencl model, r = 0.81) and [HCO3-] (Stewart model, r = 0.93; Fencl model, r = 0.93) measured in plasma; linear regression analysis demonstrated that the slopes and intercepts for each of the relationships were not different (p > 0.05) from the line of identity. Differences between estimated and measured values were small, averaging < 3 nmol.L-1 for [H+] and < 2 mmol.L-1 for [HCO3-].(ABSTRACT TRUNCATED AT 250 WORDS)