A 20-year-old female mechanically ventilated, paralysed and sedated in ICU following a diffuse axonal head injury, develops a severe exacerbation of intracranial hypertension on day 3. Investigations taken during a subsequent episode of polyuria are as follows:

Parameter

Patient Value

Normal Range

pH

7.5*

7.35 – 7.45

PaCO2

28* mmHg (3.7 kPa)

35 – 45 (4.6 – 6.0)

HCO3

21* mmol/l

22 – 27

Standard base excess

-1.5 mmol/l

-2 – +2

Sodium

147 mmol/l

135 – 145

Potassium

3.2 mmol/l

3.2 – 4.5

Chloride

110 mmol/l

100 – 110

Urea

3.0 mmol/l

3.0 – 8.0

Creatinine

65 µmol/l

50 – 100

Glucose

4.0 mmol/l

3.0 – 6.0

Measured plasma osmolality

333* mOsm/kg

275 – 295 mOsm/kg

Urine osmolality

410 mmol/l

300 – 1300 mOsm/kg


a) What is the most likely explanation for the polyuria?

b) Give your reasoning.

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College Answer


a)  What is the most likely explanation for the polyuria?

Mannitol therapy

b)  Give your reasoning.

Increased measured plasma osmolality with an elevated osmolar gap - 32 mOsm/kg with formula (2xNa + glucose + urea) or 44 mOsm/kg with 1.86 x (Na+K) + urea + glucose. High urinary osmolality rules out diabetes insipidus. History supports osmotherapy to treat episode of raised ICP

Discussion

This question is identical to Question 3.1 from the second paper of 2010, and Question 3 from the second paper of 2007 (which contains an answer with a more complete interpretation of this issue).