# 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)

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

b)    Toxidrome – sympathomimetic agent.

## Discussion

a)

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.