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Influence of Stewart acid–base variables on plasma pH in critically ill patients


The study objective was to determine and quantify the influence of strong-ion approach variables on the plasma pH in critically ill patients.


A retrospective analysis of clinical records for 284 consecutive patients admitted to the medical adult ICU of a university hospital. Analysis was made of plasma acid–base data for 5,172 blood samples collected at admission and throughout hospitalization (one sample per patient). By substituting bicarbonate with the apparent strong-ion difference (SID), the weak acid anionic component (A-) and unmeasured ions (UI) in the Henderson–Hasselbalch equation, and after selecting samples having at least one variable within the normal range, we determined possible linear relationships between the study variables. We then compared our results with those calculated using derivatives of the simplified strong-ion equation.


In samples with normal UI and PaCO2, the SID had a strong correlation with plasma pH (r2 = 0.84), yielding a δpH/δSID ratio of +0.013 (strong-ion acidosis/alkalosis) (Figure 1). In samples with normal SID and PaCO2, UI also correlated strongly with pH (r2 = 0.69), yielding a δpH/δUI ratio of -0.014 (uncompensated metabolic acidosis) (Figure 2). Hypoalbuminemia caused a compensatory reduction in SID of about 3 mEq/l per g/dl, thereby also influencing the pH (δpH/δalbumin ratio of -0.040). In samples with normal SID and UI, PaCO2 correlated strongly with pH (r2 = 0.85), yielding a δpH/δPaCO2 ratio of -0.01 (pure acute respiratory acidosis/alkalosis) (Figure 3).

Figure 1
figure 1

Correlation between the SID and pH (patients with normal UI and PaCO2).

Figure 2
figure 2

Correlation between UI and pH (patients with normal SID and PaCO2).

Figure 3
figure 3

Correlation between the PaCO2 and pH (patients with normal UI and SID).


The SID correlates strongly with changes in pH, thus identifying a strong-ion acidosis or alkalosis. The changes in pH related to the SID, total nonvolatile weak anions and PaCO2 in critically ill patients almost match those calculated using the Stewart's simplified strong-ion equation.


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Marcelli, A., Bucci, L., Pochini, A. et al. Influence of Stewart acid–base variables on plasma pH in critically ill patients. Crit Care 12 (Suppl 2), P170 (2008).

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