You are asked to review a 44-year-old male known epileptic following a prolonged generalised tonic-clonic convulsion. He is intubated and ventilated.
The arterial blood gas analysis is as follows:

Parameter Patient value Normal adult range
FiO2 0.5  
pH 7.15* 7.35–7.45
pCO2 35 mmHg (4.6 kPa) 35-45 (4.6-6)
pO2 105 mmHg (14 kPa) 75-98 (10-13)
HCO3- 10.3 mmol/L 22.0-26.0

List the abnormalities on the blood gas and give the most likely cause of each abnormality.
(30% marks)

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College Answer

Metabolic acidosis – lactic acidosis secondary to prolonged seizures
Respiratory acidosis (or inadequate compensation) – central hypoventilation or Inadequate mechanical ventilation
Increased A-a gradient - aspiration pneumonia

Discussion

Let us dissect these results systematically.

  1. The A-a gradient is high:
    The alveolar oxygen tension is (0.5 × 713) - (35 × 1.25) = 312 mmHg;
    thus the A-a gradient is around 208 mmHg.
  2. There is acidaemia
  3. The PaCO2 is low-normal, which is a move in the appropriate direction given that there is acidaemia.
  4. The SBE is not offered, but there is a low bicarbonate which suggests a metabolic acidosis.
  5. The respiratory compensation is inadequate - the expected PaCO2 (10.5 × 1.5) + 8 = 23.8 mmHg, and so there is also a respiratory acidosis according to the Boston rules. There is no base excess measurement, and thus the Copenhagen rules cannot be used.
  6. The anion gap or delta ratio cannot be calculated.

Thus, there is:

  • Hypoxia (widened A-a gradient)
  • Respiratory acidosis
  • Metabolic acidosis

The college expected their candidates to make an intuitive leap, and to assume that the history of prolonged seizure is enough to explain the acidosis (because lactate) and the hypoxia (because aspiration).