Question 23.2

A 26-year-old patient was found collapsed in the street. On arrival in the Emergency Department, they were unresponsive and hypotensive with a temperature of 42ºC. The following is the arterial blood gas result following intubation:

Parameter

Patient Value

Normal Adult Range

FiO2

1.0

pH

7.21*

7.35-7.45

PCO2

54 mmHg (7.1 kPa)*        

35-45 (4.6-6.0)

Bicarbonate                     

21 mmol/L

21-28 (10-13)

Base Excess

-6 mmol/L*

-2-+2

Sodium

143 mmol/L

135-145

Potassium

4.9 mmol/L*

3.5-4.5

Chloride

112 mmol/L*

95-110

Calcium ionised

1.09 mmol/L*

1.12-1.32

Glucose

9.6 mmol/L*

3.0-5.4

Lactate

2.3 mmol/L*

< 1.3

Creatinine

219 µmol/L*

60-110

Haemoglobin

139 g/L

135-180

23.1.1    Explain the acid-base abnormality.    (2 marks)
23.1.2    List the likely underlying cause for this clinical picture.    (1.5 marks)
 

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College Answer

Syllabus topic/section:
2.1.14 Environmental Injuries and Toxicology in ICU – L1.
2.1.21 Applied Pharmacology in Intensive Care.
Aim:
To explore the understanding of data interpretation, toxidromes and management of dysnatraemias.
Discussion:
Overall, this question scored highly but the answers were not as well structured as the other data question 19. Some candidates missed parts of the question, which was really the only way to 'fail' this repeat data interpretation. The management of hyponatraemia was frequently muddled by candidates with many stressing that the correction must be slow but then giving both a fluid restriction and iv hypertonic saline or normal saline. These candidates appeared to have remembered parts of the management but not fully applied it correctly.

Discussion

Apart from being asked to "explain" the abnormalities in the now-gender-neutral patient instead of "describe" them, this question is otherwise identical to Question 13.1 from the second paper of 2016, and so is the discussion section reproduced below: 

Let us dissect these results systematically.

  1. The A-a gradient is raised; at 100% FiO2 the PaO2 should be 645mmHg
  2. There is acidaemia
  3. The PaCO2 is contributing
  4. The SBE is -6, suggesting a mild metabolic acidosis
  5. The respiratory compensation is inadequate - the expected PaCO2 (21 × 1.5) + 8 = 39.5mmHg, and so there is also a respiratory acidosis according to the Boston rules.
  6. The anion gap is essentially normal. 
    (143) - (112 + 21) = 10, or 14.9 when calculated with potassium. With an anion gap of 10, assuming the albumin is normal, a delta ratio calculation should not be possible. If you instisted on subtracting the ideal anion gap, which is 12, from the calculated gap, which is 10, you would  get a delta ratio of  (10-12)/(24-21) = -0.66. This would make absolutely no sense, as there is no such thing as a negative delta ratio. Instead, if we used the anion gap according to its original purpose (as a screening tool to classify acid-base disorders), we would come to the conclusion that this patient has a normal anion gap metabolic acidosis, and that a delta ratio calculation is not necessary. Which still leaves us with the question: how did the college examiners arrive at their answer? 

The college only wanted us to comment on the acid-base abnormalities, but other features are also interesting. In summary:

  • Decreased level of consciousness
  • Extreme hyperthermia
  • Hypotension
  • Raised creatitinine, which could be
    • due to acute renal failure in the context of shock
    • due to rhabdomyolysis
  • Slightly low calcium (consistent with rhabdomyolysis, if that's what is happening)

b)

Informed by the above features, the differentials must include:

  • Heat stroke
  • Neuroleptic-malignant syndrome
  • Serotonin syndrome
  • Seizures
  • Amphetamine toxicity
  • Sepsis

If he didn't come from the street, malignant hyperthermia would also have to be mentioned.

References