ICU Case Presentation Juan Carlos Ayus, MD, FACP, FASN Director of Clinical Research Renal Consultants of Houston
Case A 75 year-old woman with a history of a recent cerebrovascular accident currently receiving enteral nutrition by nasoduodenal tube at a nursing home, is transferred to the hospital with altered mental status. According to her caregiver, her urine output had increased significantly three days prior to transfer.
Laboratory data on arrival to the hospital reveals a serum sodium 162 mmol/L, BUN 55 mmol/L, creatinine 1.5 mg/dL. BUN and serum sodium were normal 3 weeks prior to this admission. Additional laboratory data included a serum osmolality 330 mOsm/Kg and urine osmolality 620 mOsm/Kg, urine sodium 8 mmol/L, urine potassium 32 mmol/L.
If the patient has an appropriate urine osmolality response to changes in serum sodium why is his serum sodium elevated with an increased urine output, and what would be the appropriate test that will assess the renal response to hypernatremia in this patient? 1. Nephrogenic diabetes insipidus; measurement of ADH 2. Central diabetes insipidus; measurement of ADH 3. Lithium toxicity; serum lithium level 4. Increased solute load in enteral nutrition; measurement of electrolyte-free water reabsorption
If the patient has an appropriate urine osmolality response to changes in serum sodium why is his serum sodium elevated with an increased urine output, and what would be the appropriate test that will assess the renal response to hypernatremia in this patient? 1. Nephrogenic diabetes insipidus; measurement of ADH 2. Central diabetes insipidus; measurement of ADH 3. Lithium toxicity; serum lithium level 4. Increased solute load in enteral nutrition; measurement of electrolyte-free water reabsorption
RATIONALE: Case presents an increased solute load in enteral nutrition. Patient’s history reveals polyuria, as the result of the osmotic effect of urea secondary to the large protein intake that this patient received with the tube feedings. Using the plasma and urine osmolality as an index of water conservation in this case is misleading, since large amounts of urea in the urine account for increased urine osmolality. Since urea is freely permeable to all the cells in the body, urea should not affect fluid translocation and serum sodium. Thus, urine osmolality, must not be used in calculating water excretion or retention by the kidney, but instead the electrolyte- free water reabsorption test must be used. This test uses the following formula: U Na + U K T e CH 2 O = V • 1 P Na
Means of Quantitating Renal Water Excretion 1. Urine flow is divisible into two components 2. One component is the urine volume needed to excrete solutes. This isotonic component has been termed osmolar clearance (Cosm). 3. The other component is called free-water clearance (CH 2 0) Electrolyte Adaptation
MEANS OF QUANTITATING RENAL WATER EXCRETION Electrolyte-free water clearance Serum osmolality is replaced by serum sodium Urine osmolality is replaced by U Na + U K CH 2 O(e) = V (1 – U Na + U K ) P Na U Na + U K < P Na , CH 2 O (e) positive will raise P Na U Na + U K > P Na , CH 2 O (e) negative will lower P Na
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