Question 3.2

For each set of the following biochemical and arterial blood gas parameters:

  • Describe the abnormalities.
  • Give one example of an associated clinical scenario.

Any reasonable scenario accepted that was both biochemically correct AND clinically likely.



Normal Adult Range


145 mmol/L

135 – 145


4.0 mmol/L

3.2 – 4.5


91 mmol/L

100 – 110


30 mmol/L

24 – 32



7.35 – 7.45


30 mmHg (3.9 kPa)

35 – 45 (4.6 – 5.9)

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College Answer

Increased anion gap, metabolic alkalosis and respiratory alkalosis.

Clinical scenario – salicylate overdose.


Let us dissect these results systematically.

  1. The A-a gradient cannot be calculated - the information is missing
  2. There is alkalaemia
  3. The PaCO2 is not compensatory - its contributing to the alkalosis
  4. The SBE is not supplied, but the bicarbonate is 30, suggesting that there is a metabolic alkalosis
  5. The respiratory compensation is inadequate - the expected PaCO2(0.7 × 30) + 20 = 41mmHg, and thus there is also a respiratory alkalosis
  6. The anion gap is raised:
    (145) - (91  + 30) = 24, or 28 when calculated with potassium
    The delta ratio is irrelevant here.
  7. The urinary electrolytes and pH would not be helpful.

This is a mixed respiratory and metabolic alkalosis with a raised anion gap. There is only one beast which presents in this way: the salicylate overdose. To be precise, metabolic acidosis and respiratory alkalosis are characteristic of salicylate toxicity, and the metabolic alkalosis is some sort of unrelated sideshow. It is either due to torrential vomiting (which frequently accompanies salicylate intoxication) or due to the forced alkaline diuresis (which is the favoured method of enhancing salicylate clearance). Either way, all three acid-base disturbances are frequently seen together in the suicidal aspirin eater.

This level of discussion is probably sufficient for a question which was probably worth no more than 2 marks in a ten-mark question. These derangements of physiology are discussed in greater detail in the chapter on salicylate overdose from the "Acid-Base Disturbances" section.


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