Question 13.1

A 26-year-old male found was collapsed in the street. On arrival in the Emergency Department, he was unresponsive and hypotensive with a temperature of 42°C. The following is his arterial blood gas result following intubation:

Parameter	Patient Value	Normal Adult Range
Fi02	1.0
pH	7.21*	7.35 - 7.45
PC02	54 mmHg (7.1 kPa)*	35 - 45 (4.6 - 6.0)
P02	500 mmHg (65.8 kPa)
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
Haemoqlobin	139 q/L	135 - 180

a) Describe the acid-base abnormality. (20% marks)

b) Give the likely underlying cause for this clinical picture. (15% marks)

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College answer

a) Mixed respiratory, high anion gap and normal anion gap metabolic acidosis.

b) Toxidrome – sympathomimetic agent.

Discussion

Let us dissect these results systematically.

The A-a gradient is raised; at 100% FiO₂ the PaO₂ should be 645mmHg
There is acidaemia
The PaCO₂ is contributing
The SBE is -6, suggesting a mild metabolic acidosis
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.
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)

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.

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