Electrolyte-free water clearance: a clinical utility in managing osmotic urea diuresis-associated polyuria in tube feeding syndrome

Abstract

Hypernatremia is a common problem in critically ill patients. Part of the reason for the inadequate treatment of hypernatremia may be that we tend to replace only the free water deficit (FWD) and neglect replacing ongoing losses of free water. In most cases, ongoing losses of free water occur in the urine, the quantity of which can be easily calculated using the electrolyte free water clearance (EFWC). We report a case of man presenting with tube feeding syndrome (TFS) and hyperglycemic hyperosmolar nonketotic syndrome (HHNS). He developed life-threatening hypernatremia from an osmotic urea diuresis that was appropriately treated by replacing both the FWD as well as ongoing free water losses calculated from the EFWC. A 37 year-old man in a persistent vegetative state from a left thalamic hemorrhage status post ventriculostomy and PEG tube placement was admitted with sepsis and HHNS. He was receiving large amounts of Jevity 1.2. He had acute kidney injury due to severe volume depletion from an osmotic urea diuresis. BUN was 200 mg/dL, sodium was 165 mEq/L, glucose was 635 mg/dL, and serum osmolality was 449 mOsm/kg. To prevent the development of cerebral edema from overly rapid correction of hyperosmolality, he was started on an insulin drip and hypernatremia was allowed to persist until the hyperglycemia was fully normalized. This was followed by correcting the hypernatremia over 3 days. He was urinating up to about 4 L/day of mostly electrolyte free water. Using the FWD to arrive at the rate of free water administration, he would only have received 2 L of free water per day and hypernatremia would have persisted. Instead, he required about 4.5 L/day of free water as calculated by the FWD and EFWC together, and his hypernatremia resolved as expected. Our patient presented with a triad of TFS including hypernatremia, dehydration, and azotemia. Rapidly increased fluid loss from sepsis and HHNS initially caused severe hypernatremia. Only by using the combination of FWD and EFWC calculations could we have arrived at the appropriate rate of free water administration.