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

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.