Abstract

HERNANDEZ JARAS, Julio et al. Does Stewart-Fencl approach improve the evaluation of acid-base status in stable patients on hemodiafiltration?. Nefrología (Madr.) [online]. 2010, vol.30, n.2, pp.214-219. ISSN 1989-2284.

Introduction: The traditional evaluation of acid-base status relies on the Henderson-Hasselbach equation. In 1983, an alternative approach, based on physical and chemical principles was proposed by P. Stewart. In this approach, plasma pH is determined by 3 independent variables: pCO₂, Strong Ion Difference (SIDm), which is the difference between the strong cations (Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺) and the strong anions (Cl^-, lactate) and total plasma concentration of nonvolatile weak acids (ATot), mainly inorganic phosphate and albumin. Bicarbonate is considered a dependent variable. The aim of this study was to evaluate the acid-base status using both perspectives, physical chemical and traditional approach. Material and methods: we studied 35 patients (24 male; 11 female) on hemodiafiltration, mean age was 67.2 ± 15.7, 8 ± 19.2 kg. We analyzed plasma chemistry including pH, pCO₂, HCO₃-, base excess and Na⁺, K⁺, Cl^-, Ca⁺⁺, Mg⁺⁺, lactate and SIDm. The SID estimated (SIDe) was calculated by Figge's formula (1,000 x 2.46^-11 x pCO₂/[10 - pH] + Album g/dl x [0.123 x pH -0.631] + P in mmol/l0 x [0.309 x pH -0.469]) and Gap of the SID as the difference SIDm-SIDe. Results: pH preHD was 7.36 ± 0.08 and pH post-HD 7.44 ± 0.08 (p <0.001). There was no significant differences between pCO₂ pre- and post-HD. HCO₃- and base excess increased during the session (p <0.001). SIDm decreased from 46.2 ± 2.9 pre-HD to 45 ± 2.3 mEq/l post-HD (p <0.05). On the opposite, SIDe increased from 38.5 ± 3.8 to 429 ± 3.1 mEq/l (p <0.001). The Gap Anion descended from 18.6 ± 3.8 pre-HD to 12.8 ± 2.8 mEq/l post-HD (p <0.001) and the Gap of the SID 7.6 ± 3 to 2.1 ± 2 (p <0.001). Anion Gap correlated with the Gap-SID so much pre-HDF as pos-HDF. ∆ Base excess correlated only with ∆ of the Gap SID. Conclusion: Stewart-Fencl's approach does not improve characterization of acid-base status in patients on chronic HDF. In presence of normocloremia the SIDm does not reflect the alkalinizing process of the session of hemodialysis. According this approach, hemodialysis therapy can be viewed as a withdrawal of inorganic anions, especially the sulphate. These anions are replaced by OH^- and secondarily for HCO₃^-. The approach only improves the evaluation of unmeasured anions by the Gap of the SID, without the effect of albumin and phosphate.

Keywords : Stewart-Fencl approach; Acidosis; Hemodiafiltration; Gap anion.

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