Quantitative Accuracy of the Simplified Strong Ion Equation to Predict Serum pH in Dogs

Background

Electrochemical approach to the assessment of acid‐base states should provide a better mechanistic explanation of the metabolic component than methods that consider only pH and carbon dioxide.

Hypothesis/Objectives

Simplified strong ion equation (SSIE), using published dog‐specific values, would predict the measured serum pH of diseased dogs.

Animals

Ten dogs, hospitalized for various reasons.

Methods

Prospective study of a convenience sample of a consecutive series of dogs admitted to the Massey University Veterinary Teaching Hospital (MUVTH), from which serum biochemistry and blood gas analyses were performed at the same time. Serum pH was calculated (Hcal+) using the SSIE, and published values for the concentration and dissociation constant for the nonvolatile weak acids (A_tot and K_a), and subsequently Hcal+ was compared with the dog''s actual pH(Hmeasured+). To determine the source of discordance between Hcal+ and Hmeasured+, the calculations were repeated using a series of substituted values for A_tot and K_a.

Results

The Hcal+ did not approximate the Hmeasured+ for any dog (P = 0.499, r² = 0.068), and was consistently more basic. Substituted values A_tot and K_a did not significantly improve the accuracy (r² = 0.169 to <0.001). Substituting the effective SID(Atot−[HCO3−]) produced a strong association between Hcal+ and Hmeasured+ (r² = 0.977).

Conclusions and clinical importance

Using the simplified strong ion equation and the published values for A_tot and K_a does not appear to provide a quantitative explanation for the acid‐base status of dogs. Efficacy of substituting the effective SID in the simplified strong ion equation suggests the error lies in calculating the SID.