A   20   year   old   female   in   ICU following   a  diffuse   axonal   head   injury develops a severe exacerbation of intracranial hypertension on day 3. She is mechanically ventilated, paralysed and sedated. Investigations during a subsequent episode of marked polyuria are summarised below.

Test

Value

Normal Range

pH*

7.50

(7.36 –7.44)

PaCO2*

28 mm Hg

(36 – 44)

HCO3-*

21 mmol/L

(23 – 26)

Standard base excess

-1.5 mmol/L

(-2 .0 to +2.0)

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 mmol/L

(50 – 100)

Glucose

4.0 mmol/L

(3.0 – 6.0)

Measured plasma osmolality*

333 mosmol/kg

(280 –290)

Urine osmolality

410 mosmol/L

(50– 1200)

a)  What  is the most likely explanation  for the polyuria?  Give the reasoning behind your answer.

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College Answer

a)  What  is the most likely explanation  for the polyuria?  Give the reasoning behind your answer.

Mannitol therapy

There is increased measured plasma osmolality with an elevated osmolar gap. The gap is 44 mosmol/kg, if we use a calculated osmolality of 1.86 × ([Na] + [K]) + [urea]
+ [glucose]. If we use the simple formula of 2 × [Na] + [urea] + [glucose] for
calculated osmolality, the gap is 32 mosmol/kg. (There are also other formulae which are more difficult to remember). In the setting of treatment for an exacerbation of intracranial hypertension, the increased osmolar gap is likely to be due to mannitol administration. The high urinary osmolality rules out diabetes insipidus, and supports the diagnosis of mannitol induced polyuria

Discussion

This question is identical to Question 22.1 from the second paper of 2011, and Question 3 from the second paper of 2007 (which contains an answer with a more complete interpretation of this issue).