Stewart SID Acidosis — FRCA Primary MCQ

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HardPhysiology - RespiratoryStewart SID AcidosisFRCA Primary

The strong ion difference (SID) approach to acid-base physiology (Stewart approach) defines acidosis as:

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Correct answer: D — Decreased SID (difference between strong cations and strong anions)

Stewart's approach: pH is determined by three independent variables: PaCO2 / SID (sum of strong cations minus strong anions / normally ~40 mEq/L) / and total weak acid concentration (albumin / phosphate). Decreased SID (e.g. hyperchloraemia) causes acidosis. Increased SID causes alkalosis. This approach explains saline-induced hyperchloraemic acidosis.

Reference: Power Kam 2018 Principles of Physiology for the Anaesthetist