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Mnemonic

Arterial Blood Gas Reading Order

A memory aid for the order in which to read an arterial blood gas.

Investigations Respiratory Blood Gases and Acid Base Respiratory Physiology and Function Tests
Expansion

Oxygenation first, then pH, then carbon dioxide, then bicarbonate

Mnemonic

ROME, for matching the pH to the primary disturbance:

  • R - Respiratory Opposite: in a respiratory problem, pH and PaCO2 move in opposite directions
  • M - Metabolic Equal: in a metabolic problem, pH and bicarbonate move in the same direction

So a low pH with a high carbon dioxide is a respiratory acidosis; a low pH with a low bicarbonate is a metabolic acidosis.

Then read in a fixed order: oxygenation with the FiO2, pH, PaCO2, bicarbonate, anion gap, lactate.

Compensation never fully corrects the pH, so a normal pH with grossly abnormal carbon dioxide and bicarbonate indicates a mixed disorder rather than perfect compensation.

Expansion

  1. Is the patient hypoxic? Look at the PaO2 together with the FiO2. A rough rule is that the PaO2 in kPa should be about 10 kPa below the inspired oxygen percentage, so 60 per cent oxygen should give around 50 kPa
  2. Type 1 or type 2 respiratory failure: type 1 is a low PaO2 with a normal or low PaCO2; type 2 is a low PaO2 with a raised PaCO2, meaning ventilatory failure
  3. pH: acidaemic below 7.35, alkalaemic above 7.45
  4. PaCO2: raised in a respiratory acidosis, low in a respiratory alkalosis. Ask whether it moves with the pH (respiratory problem) or against it (compensation)
  5. Bicarbonate and base excess: low in a metabolic acidosis, high in a metabolic alkalosis
  6. Anion gap if there is a metabolic acidosis
  7. Everything else on the gas: lactate, glucose, potassium, sodium, haemoglobin, carboxyhaemoglobin and methaemoglobin

Which is the primary disorder: the one that would produce the observed pH. If the pH is acidaemic and both the carbon dioxide and bicarbonate are low, the low bicarbonate is primary and the low carbon dioxide is compensation.

Compensation is never complete, so the pH does not return fully to normal. A normal pH with grossly abnormal carbon dioxide and bicarbonate means a mixed disorder, not perfect compensation.

Alveolar-arterial gradient, when a normal or low carbon dioxide accompanies hypoxia:

A-a gradient = (FiO2 x 94.8) - (PaCO2 / 0.8) - PaO2 in kPa on air

A raised gradient indicates a gas exchange problem, being shunt, ventilation-perfusion mismatch or diffusion impairment; a normal gradient with hypoxia indicates hypoventilation or low inspired oxygen.

A venous gas is adequate for pH, bicarbonate, lactate, potassium and haemoglobin, but not for oxygenation, and only approximates the carbon dioxide.