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

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. 
    It is not clear where the college gets their answer from, as typically they both omit potassium and use normal values for the anion gap which are probably anachronistic in this era of ion-sensing electrodes. Even with new values and with poassium included, it is impossible to generate a delta ratio in the 0.4-0.8 range.

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.