Biomed Pap Med Fac Univ Palacky Olomouc Czech Repub. 2005, 149(1):69-73 | DOI: 10.5507/bp.2005.007

RELATION BETWEEN pH AND THE STRONG ION DIFFERENCE (SID) IN BODY FLUIDS

Otto Schück^a, Karel Matoušovic^b

^a Institute for Clinical and Experimental Medicine, Prague,

^b 2nd School of Medicine of Charles University Prague and the University Hospital Motol, Prague

Acid-base balance evaluation according to the Henderson-Hasselbalch equation enable us to assess the contribution of respiratory (pCO₂) and/or non-respiratory (metabolic, HCO₃^-) components to the acid-base balance status. A new approach to acid-base balance evaluation according to Stewart-Fencl, which is based on a detailed physical-chemical analysis of body fluids shows that metabolic acid-base balance disorders are characterized not only by [HCO₃^-]. According to this concept independent variables must be taken into an account. The abnormality of concentration of one or more of the independent variable(s) determines the pH of a solution. The independent variables are: 1. strong ion difference (SID); 2. total concentration of nonvolatile weak acids [A_tot]; 3. in agreement with the Henderson-Hasselbalch concept also pCO₂. Traditional evaluation of acid-base balance disorders is based on the pH of body fluids (though pH may be within normal range if several acid-base balance disturbances are present). In order to maintain this view and simultaneously to respect the Stewart-Fencl principle, we invented a new equation, which uses only the independent variables to define the pH of body fluids. This analysis shows that for a given value of pCO₂, the pH of body fluids is determined by a difference between SID and [A_tot^-]. pH = 6.1 + log((SID - [A_tot^-])/(0,03*pCO₂)) or in itemized form: pH = 6.1 + log((([Na⁺] + [K⁺] + [Ca²⁺] + [Mg²⁺] - [Cl^-] - [UA^-]) - (k₁[Alb] + k₂[P_i]))/(0,03*pCO₂)) Evaluation of the individual components of this equation enables us to detect, which of the independent variable (or a combination of independent variables) deviates from the normal range and therefore which one or ones is a cause of the acid-base balance disorder. At the end of this paper we give examples of a practical application of this equation.

Keywords: Acid-base balance, Henderson-Hasselbalch equation, Stewart-Fencl principle

Received: April 20, 2005; Accepted: June 5, 2005; Published: July 1, 2005  Show citation