Acid-Base & Blood Gas

Calculate the alveolar-arterial oxygen gradient to distinguish hypoxaemia from V/Q mismatch or shunt from hypoxaemia caused by hypoventilation alone.

Correct the anion gap for hypoalbuminaemia, which lowers the measured gap and can hide a clinically significant high-anion-gap acidosis.

Calculate alveolar pO₂ from FiO₂, PaCO₂, atmospheric pressure and respiratory quotient — the reference value the A-a gradient is built on.

Calculate the serum anion gap from sodium, chloride and bicarbonate — with the interval that belongs to this formula, not the one the textbooks printed before ion-selective electrodes.

Calculate CaO₂ from haemoglobin, saturation and pO₂ — and see why anaemia costs far more oxygen content than a modest desaturation.

Estimate the bicarbonate needed to reach a target level, and see why bicarbonate replacement remains a genuinely controversial therapy.

Classify a high-anion-gap acidosis as pure or mixed by comparing the rise in the anion gap against the fall in bicarbonate.

Calculate pH from bicarbonate and pCO₂ using the equation blood gas analysers themselves rely on.

Convert blood lactate between mg/dL, mmol/L, µmol/L and mg/L, with the 2 and 4 mmol/L thresholds, the sampling artefacts that invent a high result, and why clearance beats any single value.

Calculate DO₂ from oxygen content and cardiac output, and see why chasing a supranormal target does not help.

Calculate the P/F ratio, the oxygenation criterion in the Berlin definition of ARDS severity.

Convert arterial pCO₂ from mmHg to kPa, using the same factor as pO₂, with the type 2 respiratory failure threshold.

Convert arterial pO₂ from mmHg to kPa — the commonest source of confusion when reading a blood gas from another country.

Check whether the measured pCO₂ shows appropriate respiratory compensation for a metabolic acidosis, or a second acid-base disorder.