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.
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
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).