A 5 year-old male is anesthetized for craniopharyngioma resection. Near the end of the procedure he is note to have polyuria, with urine output of 18 mL/kg for the past hour. Of the following, which would be LEAST likely to be found on assessment of this patient?
a) Serum sodium of 156 mEq/L
b) CVP of 1 mmHg
c) Serum osmolality of 325 mOsm/kg
d) Urine osmolality of 850 mOsm/kg
e) Persistently high urine output for the next 3 hours
Answer d. Based on the history of craniopharyngioma resection and the presence of polyuria, there is concern for the development of diabetes insipidus in this patient. As shown above in Table 1, these patients normally have a high serum sodium and osmolality, low central venous pressure and urine osmolality, and high urine output without another explanation. Initiation of vasopressin infusion is therapeutic for this condition; some patients may also require urine replacement therapy to maintain intravascular volume pending efficacy of vasopressin. A high urine osmolality would be very unusual in DI and may warrant investigation of other causes of polyuria.