A 60-year-old man with bipolar disorder on chronic lithium therapy undergoes an uncomplicated appendectomy. In the PACU he becomes delirious and agitated. His vital signs are within normal limits. He weighs 75 kg. Laboratory values are notable for sodium 148 mEq/L. He is maintained overnight on dextrose in half-normal saline at 125 mL/h. Urine output is approximately 300 mL/h overnight. In the morning serum sodium has increased to 155 mEq/L. His urine osmolality is 120 mOsm/kg, and urine sodium is 22 mEq/L. Arginine vasopressin is administered without a change in urine output.
What is the most appropriate change in IV fluids for treatment of his hypernatremia?
A. Increase the infusion rate to 400 mL/hCorrect Answer: C
Based on the medical history and laboratory results this patient likely has nephrogenic diabetes insipidus (DI) from long-term lithium therapy. Nephrogenic DI can be caused by several drugs, including lithium, demeclocycline, amphotericin B, and antiretroviral drugs such as tenofovir and indinavir. He developed severe hypernatremia from ongoing free water loss when he was unable to maintain oral intake. The goal of treatment is to correct his free water deficit by half over the 24 hours and then fully correct the sodium level within 3 days.
To determine the appropriate amount of fluid to administer, we must calculate the free water deficit. Assuming a total body water (TBW) of 60% lean body mass (this may be an overestimation in women or the elderly), this patient’s normal TBW is approximately 45 L (0.6 × 75 kg). Assuming normal sodium of 140 mEq/L, his current TBW = normal TBW × (normal sodium/current sodium) = 45 × (140/155) = 45 × 0.9 = 40.5 L. Thus, his free water deficit = 45 − 40 = 5 L. We would aim to replace half of this in 24 hours, or about 2.5 L. We also must account for urinary free water loss resulting from the elevated urine output 300 mL/h × 24 h = 7.2 L. Thus the total free water repletion for 24 hour should be 2.5 + 7.2 = 9.7 L. Averaged per hour, this is approximately 400 mL/h.
Thus, changing to D5 Water at 400 mL/h should appropriately correct his free water deficit. D5 half-normal saline at 300 mL/h would provide approximately one-fourth of the needed free water. And D5W at 200 mL/h is only half that required. Administering D5W at 300 mL/h would only provide three quarters of the needed free water. In addition he did not respond to arginine vasopressin, so response to the synthetic vasopressin analogue desmopressin would be unlikely.
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