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)*

## 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.

- The A-a gradient is high; ~78mmHg
- There is neither alkalaemia nor acidaemia
- The PaCO
_{2}is low, which is a move in the appropriate direction given the metabolic acidosis - The SBE is -8.6, suggesting a metabolic acidosis
- The respiratory compensation is excessive - the expected PaCO
_{2}(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 PaCO_{2 }= (40 - SBE) = 31.4mmHg.

So, in this case there is no Trans-Atlantic disagreement. - 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.
- 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?