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Blood gases, acid–base and electrolytes in anaesthesia

An arterial blood gas in theatre is read in a fixed order — pH, the primary problem, compensation, and the anion gap — and electrolytes in anaesthesia matter mainly because of the heart and muscles: high potassium after succinylcholine, low calcium after massive transfusion, and high magnesium that potentiates muscle relaxants.

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Why it matters in the exam

The exam asks about reading a blood gas, calculating the anion gap and hyperchloraemic acidosis after large volumes of saline, the expected compensation in metabolic and respiratory acidosis, alpha-stat and pH-stat in cooling, the rise in potassium from succinylcholine and the states in which it is dangerous, treating high potassium, low calcium in massive transfusion and after thyroid surgery, and magnesium and muscle relaxants.

The reading: a pH below 7.35 is acidaemia and above 7.45 alkalaemia; then check whether the change in PaCO2 or in HCO3 explains it. In metabolic acidosis the expected PaCO2 is 1.5 times HCO3 plus 8, plus or minus 2 (Winter's formula). The anion gap is sodium minus (chloride plus bicarbonate), and is normal up to about 12. A high-gap acidosis comes from lactate, ketones, renal failure or toxins; a normal-gap acidosis appears after large amounts of saline (high chloride) or from diarrhoea. In acute respiratory acidosis HCO3 rises about 1 for every rise of 10 in PaCO2, and in chronic 3.5–4. In cardiac surgery with cooling, the alpha-stat method does not correct the gas to temperature, and the pH-stat method adds carbon dioxide to keep a normal pH at the low temperature.

Succinylcholine raises potassium by about 0.5 mmol/L, but in burns, crush, denervation (spinal cord injury, stroke), prolonged immobility and some muscle diseases, receptors outside the junction amplify the rise and may cause cardiac arrest; the danger starts about 24–48 hours after the injury. Treatment of high potassium: calcium to protect the heart, insulin with glucose and salbutamol to move potassium into cells, bicarbonate in acidosis, and removal by dialysis. Hyperventilation lowers potassium. Low calcium appears in massive transfusion (citrate binds calcium) and after thyroid or parathyroid surgery: spasms, laryngospasm, QT prolongation and low pressure; calcium chloride 10% contains 3 times more calcium than calcium gluconate. High magnesium, for example in treating pre-eclampsia, potentiates muscle relaxants and abolishes tendon reflexes first; the treatment is calcium. In correcting chronic low sodium, the rise should not exceed 8–10 mmol/L in 24 hours.

Key points

• Blood gas: pH, primary problem, compensation, anion gap.
• Much saline: hyperchloraemic acidosis.
• Succinylcholine raises potassium by about 0.5, and much more in burns and denervation.
• Massive transfusion: low calcium.
• High magnesium potentiates muscle relaxants; the treatment is calcium.

Summary table

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Sources

• Miller's Anesthesia, 9th edition (2020) — Acid-base balance and blood gas analysis; Fluids and electrolytes
• Morgan & Mikhail's Clinical Anesthesiology, 7th edition (2022) — Acid-base management; Management of patients with fluid and electrolyte disturbances

Sources are given for further reading; the text is written afresh and not copied from them.

Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.