Intraoperative Stewart's strong ion difference (SID) approach to acid-base: a patient receives 3 litres of 0.9% NaCl. Which change in SID correctly explains the resultant acidosis?
- A SID rises because excess sodium raises total strong cation concentration
- B Acidosis is due to dilution of bicarbonate buffer, independent of SID
- C SID falls because chloride concentration rises relative to sodium, reducing the electrochemical gradient that drives proton exclusion ✓
- D SID is unchanged; acidosis is due to lactate accumulation from saline infusion
Explanation
In Stewart's physicochemical model, pH is determined by three independent variables: SID, PCO2, and total weak acid concentration (Atot). Normal saline has a SID of 0 (equal Na and Cl concentrations). When large volumes are infused, plasma chloride rises disproportionately to sodium (haemodilution of the small Na-Cl gap widens), decreasing the plasma SID from the physiological ~42 mEq/L toward lower values; this electroneutrality-driven fall in SID is electrically satisfied by a rise in [H+] (acidosis). This is the mechanistic basis of dilutional/hyperchloraemic acidosis distinct from the traditional bicarbonate buffer explanation.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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