Question 19.1

College Answer

Syllabus topic/section:
2.1.14 Environmental Injuries and Toxicology in ICU – L1.
2.1.18 Peri-operative Issues in Intensive Care – L1.
Aim:
To explore the level of knowledge of common investigations and synthesis of information.
Discussion:
Some candidates lost time by adding in detail not asked for e.g. A-a gradient. The albumin correction in 2.1 was not done by any candidate. Some candidates missed part of the question which was essentially the only way a candidate achieved less than 5. Overall, it is commendable that the standard of ABG interpretation is high.

Discussion

Let us dissect these results systematically.

1. The A-a gradient was not asked for,  nor can it be calculated without a PaO₂, which makes all the more puzzling that some candidates tried to calculate it, but here we are.
2. There is acidaemia.
3. The PCO₂ is a compensatory response.
4. The SBE is negative, suggesting that there is a metabolic acidosis.
5. The respiratory compensation is adequate:
   The expected CO₂ is (5 × 1.5) + 8 = 15.5mmHg
   Alternatively, subtracting the SBE from 40, you get an expected CO₂ of 16 mmHg.
6. The anion gap is raised:
   (135) - (100 + 5) = 30, where the expected AG would be 11 (considering that it would decrease by 1 for every 4-5g/L albumin decrement from 40g/L)

So, this is a high anion gap metabolic acidosis with appropriate respiratory compensation.

What of the rest of the bloods? The abnormalities are:

• Hypocalcemia
• Hyperglycaemia
• Ketosis
• Modestly impaired renal function
• Trivially elevated LFTs

The osmolality is offered. When such a thing is offered by the examiners, one grabs it and shakes it. In this scenario, the calculated osmolality is  295.8 mOsm/kg, which gives an osmolar gap of 34.2 mOsm/kg.  As "found collapsed" is the only history we get, and the results exclude both ketons and lactate, we are left with mostly toxicological causes of HAGMA with a high osmolar gap:

• Methanol intoxication (the anion is formic acid)
• Ethylene glycol intoxication (the anions are glycolic acid and oxalic acid)
• Diethylene glycol intoxication (the anion is 2-hydroxyethoxyacetic acid, HEAA)
• Propylene glycol intoxication (the anions are pyruvate, lactate and acetate)
• Salicylate intoxication (less likely, as the anions are salicylate and lactate)

The effective therapies which the college would have been expecting therefore would have to include the standard approach to toxic alcohol ingestion:

• Haemodialysis
• Alcohol (IV or NG)
• Fomepizole

Ketoacidosis would be another differential.  Though the glucose is accounted for in the usual osmolality calculation, the presence of ketones (specifically of acetone) can raise the osmolar gap significantly.  Davidson (1992) reported gaps that were on average 20 and 11, respectively, on admission and 12 hours later in the course of a "proper" DKA (first bicarbonate measurement ranged from 3 to 8 mmol/L). Anion gaps as high as 40 are reported. The question therefore is not whether the DKA would have been a plausible differential, but whether the college examiners would have had DKA on their list of pointscoring elements in the answer marking rubric. Judging by the syllabus item attributed to this SAQ ("2.1.14 Environmental Injuries and Toxicology in ICU") we can assume that toxic alcohols probably had to be in the list of differentials.