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)

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) - (95  + 15) = 37, or 43.7 when calculated with potassium.
7. The delta ratio (assuming an albumin of 40) would therefore be (37 - 12) / (24 - 15) = 2.77 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?