Acid-Base Disorders

The answer first

Acid-base interpretation is a process, not a diagnosis. The diagnosis falls out of a reproducible, six-step algorithm applied to every blood gas — arterial or venous. The registrar who can recite the algorithm under viva pressure and reach a complete answer (primary disorder, compensation, mixed disorders, and aetiology) will pass the acid-base station; the one who eyeballs the gas and guesses will miss mixed disorders and fail. ^[1][14]

The mandate is simple: ^[1]

1. Read the pH first — acidaemia or alkalaemia.
2. Name the primary disorder — respiratory or metabolic, using PaCO2 and bicarbonate.
3. Assess compensation — is it appropriate? If not, a second primary disorder is present (a mixed disorder).
4. Calculate the anion gap (AG) — every metabolic acidosis is high-AG or normal-AG.
5. Apply the delta-delta — does the AG explain the bicarbonate? This unmasks a hidden metabolic alkalosis or a second acidosis.
6. Check the osmolar gap — when a toxic alcohol is possible. ^[1][8]

The single most important principle for the exam: never stop at the primary disorder. A septic patient with a lactic acidosis and an apparent "appropriate" PaCO2 may still have a hidden metabolic alkalosis from vomiting — only the delta-delta reveals it. Mixed disorders are the discriminator that separates a pass from a distinction ^[1].

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References15ShowHide

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