Arterial Blood Gas Interpretation

A systematic approach for finals and the wards

Why a system helps

Arterial blood gases look intimidating because there are several numbers that interact. The trick is to interpret them in a fixed order every time, so you never miss a step. Work through oxygenation, then the acid-base picture, then compensation, and finally tie it back to the patient.

UK reference ranges: pH 7.35 to 7.45; PaCO2 4.7 to 6.0 kPa; PaO2 10 to 13 kPa on air; bicarbonate 22 to 26 mmol/L; base excess -2 to +2 mmol/L.

Step 1: Assess oxygenation

Look at the PaO2 in the context of the inspired oxygen. On room air it should be above about 10 kPa; a roughly healthy person should have a PaO2 about 10 kPa below their inspired oxygen percentage. A low PaO2 defines hypoxaemia and respiratory failure, which is type 1 when the PaCO2 is normal or low and type 2 when it is raised.

Step 2: Look at the pH

Decide whether the patient is acidotic (pH below 7.35) or alkalotic (pH above 7.45). Even if compensation has pulled the pH back into the normal range, note which side of 7.40 it sits, as that usually reveals the primary problem.

Step 3: Find the primary driver

Compare the PaCO2 and the bicarbonate against the pH:

  • Respiratory: the PaCO2 moves opposite to the pH. A high PaCO2 with a low pH is a respiratory acidosis; a low PaCO2 with a high pH is a respiratory alkalosis.
  • Metabolic: the bicarbonate moves in the same direction as the pH. A low bicarbonate with a low pH is a metabolic acidosis; a high bicarbonate with a high pH is a metabolic alkalosis.

Step 4: Assess compensation

The body tries to return the pH towards normal. The lungs compensate quickly by changing ventilation, while the kidneys compensate over hours to days by adjusting bicarbonate. If the secondary value has shifted to oppose the primary disturbance but the pH is still abnormal, it is partially compensated; if the pH has returned to normal, it is fully compensated. Remember that compensation never overcorrects the pH past 7.40.

Step 5: Use the anion gap for an acidosis

For a metabolic acidosis, calculate the anion gap to narrow the cause. A raised anion gap points to added acids: ketoacidosis, lactic acidosis, renal failure and certain toxins. A normal anion gap points to bicarbonate loss, as in diarrhoea or renal tubular acidosis.

Worked example: pH 7.25, PaCO2 3.9 kPa, bicarbonate 15 mmol/L, base excess -8. The low pH with a low bicarbonate is a metabolic acidosis, and the low PaCO2 shows respiratory compensation: a partially compensated metabolic acidosis, classic for diabetic ketoacidosis.

Always return to the patient

A blood gas is only meaningful alongside the clinical picture. A young person hyperventilating from anxiety and a patient with a salicylate overdose can both show a respiratory alkalosis, but the management is entirely different. Interpret the numbers, then ask what fits the person in front of you.