Question 19.1

A 44-year-old patient was found collapsed. The following results were obtained from arterial blood gas analysis and venous biochemistry.

Parameter

Patient value

Adult normal range

FiO2

0.28

pH

7.05 *

7.35-7.45

PCO2

15 mmHg (1.99 kPa)*     

35.0-40.0 (4.6-6.0)

SaO2

100%

Bicarbonate

5 mmol/L*

22.0-26.0

Base excess

-24 mmol/L*

-2.0-+2.0

Lactate

3.0 mmol/L*

0.5-1.6

Parameter

Patient value

Adult normal range

Sodium

135 mmol/L

135-145

Potassium

5 mmol/L

3.5-5.0

Chloride

100 mmol/L

95-105

Ionised Calcium

0.9 mmol/L*

1.10-1.35

Calcium corrected

2.1 mmol/L*

2.12-2.62

Glucose

18 mmol/L*

3.5-6.0

Ketones

4.0 mmol/L*

< 1

Urea

7.8 mmol/L

3.0-8.0

Creatinine

118 µmol/L*

45-90

Measured osmolality

330 mosmol/Kg*

285-295

Albumin

35 g/L

35-50

Bilirubin

18 µmol/L

< 26

Aspartate transaminase (AST)      

230 U/L*

<35

Alanine transaminase (ALT)

139 U/L*

<35

Alkaline phosphatase (ALP)

53 U/L

30-110

19.1.1 Explain the laboratory results. Show your calculations where appropriate and list two differential diagnoses consistent with these abnormalities. (3 marks)
19.1.2 List two effective therapies. (2 marks)

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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 + 15) = 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

References

Erstad, Brian L. "Osmolality and osmolarity: narrowing the terminology gap."Pharmacotherapy: The Journal of Human Pharmacology and Drug Therapy23.9 (2003): 1085-1086.

Gennari, F. John. "Current concepts. Serum osmolality. Uses and limitations."The New England journal of medicine 310.2 (1984): 102-105.

Hoffman, Robert S., et al. "Osmol gaps revisited: normal values and limitations."Clinical Toxicology 31.1 (1993): 81-93.

Dorwart, William V., and Leslie Chalmers. "Comparison of methods for calculating serum osmolality from chemical concentrations, and the prognostic value of such calculations." Clinical chemistry 21.2 (1975): 190-194.

Kraut, Jeffrey A., and Shelly Xiaolei Xing. "Approach to the evaluation of a patient with an increased serum osmolal gap and high-anion-gap metabolic acidosis." American Journal of Kidney Diseases 58.3 (2011): 480-484.

Rasouli, Mehdi. "Basic concepts and practical equations on osmolality: Biochemical approach." Clinical biochemistry 49.12 (2016): 936-941.