ACID-BASE REGULATION - A COMPARISON OF QUANTITATIVE METHODS

The [H+] and [HCO3-] of biological solutions is determined by the Pco(2), 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 Pco(2) strong ion difference (SID = [Na+ + K+ + Ca2+] - [Cl- + lactate(-)]) and [protein](TOT)); Fend model (using Pco(2), SID [albumin], and [P-i](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+], Pco(2), [protein](TOT), [albumin], [P-i](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; Fend 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-]. However, in the case of plasma [H+], the difference between estimated and measured values became skewed (i.e., [H+](M) < [H+](Est)) above [H+](M) approximate to 55 nmol.L(-1), or at [SID] less than or equal to 35 mequiv.L(-1). Reasons for the difference between measured and estimated values are discussed, with attention given to the [SID] and weak acid components.