Influence of Stewart acid–base variables on plasma pH in critically ill patients
© BioMed Central Ltd 2008
Published: 13 March 2008
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.
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.