Critical Care Medicine-Renal, Electrolyte and Acid Base Disorders>>>>>Oliguria and Polyuria
Question 4#

A 52-year-old man with history of bipolar disorder and headaches is admitted for elective pituitary macroadenoma resection in the early morning. Following the otherwise uncomplicated neurosurgical procedure, he is admitted to the ICU for postoperative observation. You receive a call near midnight reporting that urine output has increased to 1000 mL in the past 3 hours while serum sodium increased from 142 to 146 mmol/L.

What are the next steps you should take to prevent worsening of hypernatremia? 

A. Start maintenance fluids with dextrose 5% in water until sodium level returns to baseline
B. Ask for the value of urine specific gravity and if <1.005, administer a dose of desmopressin while asking patient to drink to thirst
C. Increase rate of maintenance IV fluids with 0.9% normal saline
D. Institute fluid restriction to 1.5 L/d and recheck sodium in the morning before rounds
E. Call neurosurgery and ask if 3% hypertonic saline was given intraoperatively

Correct Answer is B

Comment:

Correct Answer: B

Low urine specific gravity in the context of polyuria and a rise in serum sodium are sufficient to make the diagnosis of DI. This postoperative neurosurgical patient is most likely experiencing central DI. Here, there is a decrease in antidiuretic hormone (ADH) which leads to polyuria (urine output >30 mL/kg body weight or >200 mL/h for 2 hours), a hallmark of DI. Additional tests that help confirm the diagnosis are measurement of urine specific gravity and serum sodium. In DI, urine will be dilute, evidenced by low urine specific gravity <1.005. With loss of dilute urine, serum sodium is expected to rise. A patient with adequate mental status may demonstrate polydipsia due to significant thirst.

Desmopressin (DDAVP) is a synthetic vasopressin analog, which acts specifically on the V2 receptor. Administration of desmopressin in central DI to replace the lack of ADH reverses the effects of central DI and thereby prevents rise in serum sodium (Answer B).

Answer A is incorrect because the relative hyponatremia is not causing clinical signs or symptoms, at this time. It is also preferable to avoid hypotonic fluids such as D5W in immediate postoperative neurosurgical or other patients with potential for cerebral edema. Answer C is not correct because increasing the rate of maintenance IV fluids (normal saline) with hypotonic fluid loss in urine (seen in DI) may further increase serum sodium. Fluid restriction (Answer D) would cause hypovolemia and may raise serum sodium.

Reference:

1. Schreckinger M, Szerlip N, Mittal S. Diabetes Insipidus following resection of pituitary tumors. Clin Neurol Neurosurg. 2013;115(2):121- 126. doi:10.1016/j.clineuro.2012.08.009.