The Stewart approach for analysis of acid-base disturbances in patients with chronic respiratory failure.

We read with great interest the article by Guérin and colleagues, in the November 2010 issue of RESPIRATORY CARE,1 describing analysis of acid-base disorders with the Stewart method versus the conventional approach in patients with chronic respiratory failure in stable or unstable respiratory condition. Unfortunately, we find several aspects of the analysis to be rather misleading. First, according to the Stewart concept2 and as Guérin et al clearly explained, the blood pH depends on 3 independent variables: PCO2, the strong ion difference (which represents the net balance between strong positive ions and strong negative ions), and the total concentration of nonvolatile weak acids (Atot). As we know,3 weak acids, as opposed to strong ions, can exist at physiologic pH as dissociated (A) or associated with a proton (AH):