The following data are from the arterial blood gas analysis of a 71-year-old male with necrotising fasciitis:

Parameter Patient Value Normal Adult Range
Barometric pressure 760 mmHg (100 kPa) .
FiO2 0.3 .
pH 7.43 7.35 – 7.45
PCO2 23 mmHg (3.1 kPa)* 35 – 45 (4.6 – 5.9)
PO2 107 mmHg (14.3 kPa) .
Bicarbonate 15 mmol/L* 22 – 26
Standard Base Excess -8.6 mmol/L* -2.0 – +2.0
Lactate 23.0 mmol/L* 0.2 – 2.5
Sodium 147 mmol/L* 137 – 145
Potassium 6.7 mmol/L* 3.2 – 4.5
Chloride 95 mmol/L* 100 – 110

List the acid-base abnormalities. (30% marks)

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College Answer

Lactic acidosis
Anion gap elevation (37 mEq/L)
Metabolic alkalosis
Respiratory alkalosis

Discussion

This is a triple disorder.

Let us dissect these results systematically.

  1. The A-a gradient is high; ~78mmHg
  2. There is neither alkalaemia nor acidaemia
  3. The PaCO2 is low, which is a move in the appropriate direction given the metabolic acidosis
  4. The SBE is -8.6, suggesting a metabolic acidosis
  5. The respiratory compensation is excessive - the expected PaCO2 (15 × 1.5) + 8 = 30.5mmHg, and so there is also a respiratory alkalosis according to the Boston rules.
    According to the Copenhagen rules, the the expected PaCO2 = (40 - SBE) = 31.4mmHg.
    So, in this case there is no Trans-Atlantic disagreement.
  6. The anion gap is (147 + 6.7) - (95 + 15) = 43.7.  This value is exactly 6.7 mEq higher than the dodgy college calculation, because this time the examiners decided not to use potassium.
  7. The delta ratio (assuming an albumin of 40) =  31.7 / 9 = 3.5; thus there is a high anion gap metabolic acidosis which co-exists with a metabolic alkalosis.
    Note that without taking this step, the candidates would still have guessed that there is an underlying metabolic alkalosis. How else would you have a normal pH with a lactate of 23?