Educational Forum Hypernatremia due to Urea-Induced Osmotic Diuresis: Physiology at the Bedside Sonali Vadi, Kenneth Yim1 Private Practitioner, Mumbai, Maharashtra, India, 1Director of Inpatient Hemodialysis‑Davita Dialysis, University of Maryland Midtown Campus, Maryland, USA Abstract Hypernatremia secondary to urea‑induced solute diuresis is due to the renal excretion of electrolyte‑free water. This concept is explained here step‑wise physiologically with the help of a clinical vignette. Keywords: Electrolyte‑free water clearance, hypernatremia, hypertonicity, osmotic diuresis, urea BACKGROUND Her initial laboratory data are presented in Table 1 and electrolyte trends during the hospital stay are summarized in Equation between solutes and water determines serum sodium Table 2. levels. Increased urinary solute load in the form of urea nitrogen leads to urea‑induced osmotic diuresis with increased Acute issues at hand free water loss and ensued hypernatremia. Diagnostic 1. What is the cause of her renal dysfunction? CLINICAL VIGNETTE 2. Why is she hypernatremic? A 70‑year‑old woman was found unconscious at home. As Treatment per her family members, she complained of abdominal pain 1. Should she be urgently dialyzed? and diarrhea for 1 week’s duration. Her baseline mental 2. How should her hypernatremia be managed? status was noted to be alert and oriented three days prior to presentation. Her medical history was significant for DISCUSSION hypertension and osteoarthritis. There was no prior history of renal dysfunction. She has been a heavy alcohol drinker. This complex pathological situation has been de‑coded Her medications included ibuprofen and cyclobenzaprine step‑wise in a question format, with answers to each in two for osteoarthritis; lisinopril and felodipine for hypertension; parts. The first part deliberates on physiology. The second aspirin and alprazolam. On examination, the patient was one looks at the answer from a clinical perspective to help the actively vomiting in the emergency department. Her reader manage with the same real‑life situation. vitals included temperature 97.3°F, pulse 109 beats/min, What is the cause of her renal dysfunction? blood pressure (BP) 113/79 mmHg in supine posture, and Physiology respirations 18 breaths/min. Physical examination revealed Factors affecting the glomerular hydrostatic pressure are reduced skin turgor and dry mucous membranes, without any arterial BP, afferent arteriolar resistance, and efferent arteriolar evidence of jugular venous distention. Her lung fields were clear to auscultation, with normal heart sounds. Her abdomen was soft and nontender and did not reveal any organomegaly. Address for correspondence: Dr. Sonali Vadi, Neurologically, she was confused. Parel, Mumbai ‑ 400 012, Maharashtra, India. E‑mail: [email protected] This is an open access journal, and articles are distributed under the terms of the Creative Access this article online Commons Attribution‑NonCommercial‑ShareAlike 4.0 License, which allows others to Quick Response Code: remix, tweak, and build upon the work non‑commercially, as long as appropriate credit Website: is given and the new creations are licensed under the identical terms. www.ijccm.org For reprints contact: [email protected] DOI: How to cite this article: Vadi S, Yim K. Hypernatremia due to urea‑induced 10.4103/ijccm.IJCCM_266_18 osmotic diuresis: Physiology at the bedside. Indian J Crit Care Med 2018;22:664‑9. 664 © 2018 Indian Journal of Critical Care Medicine | Published by Wolters Kluwer ‑ Medknow Page no. 46 Vadi and Yim: Hypernatremia due to urea-induced osmotic diuresis resistance. Renal autoregulation maintains blood flow and distinguish the cause of acute renal failure, hypovolemic glomerular filtration rate (GFR) between mean arterial vs. post‑ischemic ATN. Reduced response to ADH in pressure (MAP) of 80 and 180 mmHg. This occurs at the level of postischemic ATN is suggested by a low Uosm.[1] afferent arterioles. Angiotensin II effects dilatation of the afferent At the bedside arterioles with a simultaneous increase in the resistance of the The enormity of azotemia cannot be used to determine its efferent arterioles and a resultant increase in the glomerular origin – prerenal, renal, or postrenal. hydrostatic pressure and GFR. Any reduction in the renal blood flow reduces glomerular hydrostatic pressure and GFR, the The cause of her renal dysfunction is prerenal: basis of prerenal azotemia, in turn stimulating the release of a. Autoregulation can be impaired in the elderly and in antidiuretic hormone (ADH). The result is an increase in the hypertensive patients. A BP of 113/79 mmHg with an MAP urine osmolality (Uosm). Ischemic acute tubular necrosis (ATN) of 90.2 mmHg may not have been sufficient to maintain can also occur in this state with additional hypotensive insult. adequate renal blood flow in this patient ATN can be associated with an impaired response to ADH. b. Review of her meds suggests that she was on both 1. Fractional excretion of sodium (FeNa) and Uosm ibuprofen and lisinopril. Ibuprofen can inhibit afferent in the setting of oliguria are useful tools in helping arteriolar dilatation with a consequential reduction in renal blood flow and hence the GFR[2] c. Lisinopril decreases resistance in efferent arterioles Table 1: Laboratory data on presentation which can further reduce the GFR.[3] The half‑life Investigations Patient values of lisinopril increases because of reduced GFR. Hb (g/dl)/HCT (%) 14.7/41.9 Moreover, volume depletion leads to increased WBC (cells per microliter) 7900 concentration of medications in the lumen of renal Platelet count (cells per microliter) 308,000 tubules for a longer period with consequential Sodium (mEq/L) 148 prolonged duration of action Potassium (mEq/L) 5.5 d. Further, She was hypovolemic secondary to gastro‑ Chloride (mEq/L) 99 intestinal losses with an FeNa < !% and Urine Na < Bicarbonate (mEq/L) 12 10% Urea (mg/dl) 297 e. The high Uosm in this patient makes the diagnosis of ATN Creatinine (mg/dl) 16.3 mg/dl less likely. Glucose (mg/dl) 195 mg/dl Albumin (mg/dl) 3.3 mg/dl What led to her catabolic state? Lipase (U/L) 1343 Physiology Urine analysis Albumin 25, occult blood 25, During illness and starvation, muscle proteins break down to moderate amorphic sediment Arterial blood gas 7.31/25/112/12 “R” air provide energy, generating urea nitrogen. Protein breakdown CT scan brain (without contrast) No acute intracranial generates ammonium ions. Ammonium ions subsequently are pathology identified converted to urea nitrogen and water in the liver urea cycle. HB: Hemoglobin, HCT: Hematocrit, WBC: White blood cell Resultant urea nitrogen is excreted through the kidneys. The Table 2: Electrolyte trends during the hospital stay Date BUN Serum Serum K+ Plasmaosmol Urine Urine Urine Urine Urine Urine Urine Creat Na+ osmol Creat Na+ K+ Cl− urea output August 7 13 0.8 143 3.6 ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ March 31 297 16.3 148 5.5 ‑ ‑ 546 <10 51 <15 531 2500 April 1 291 13.0 150 5.0 430 ‑ ‑ ‑ ‑ ‑ ‑ ‑ April 1 270 8.5 157 4.2 ‑ ‑ ‑ ‑ ‑ ‑ ‑ 2700 April 2 214 4.5 159 4.9 407 440 56 51 6.7 46 835 ‑ April 2 199 4.1 160 4.0 ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ April 2 174 3.0 159 3.8 ‑ ‑ ‑ ‑ ‑ ‑ ‑ 3800 April 3 127 1.8 159 3.9 374 ‑ ‑ 34 ‑ 37 929 ‑ April 3 91 1.5 163 4.0 ‑ ‑ ‑ ‑ ‑ ‑ ‑ 2300 April 4 72 1.4 159 3.9 ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ April 4 51 1.2 149 3.5 ‑ ‑ ‑ ‑ ‑ ‑ ‑ 1975 April 5 30 1.0 152 3.4 ‑ ‑ ‑ ‑ ‑ ‑ ‑ ‑ April 5 18 1.0 151 3.5 ‑ ‑ ‑ ‑ ‑ ‑ ‑ 2990 BUN: Blood urea nitrogen (mg/dl), Creat: Creatinine (mg/dl), Na+: Sodium (mmol/L), K+: Potassium (mmol/L), Plasma osmol: Plasma osmolality (mOsm/kg), + + Urine osmol: Urine osmolality (mOsm/kg H2O), Ur creat: Urine creatinine (mg/dl), Ur Na : Urine sodium (mmol/L), Ur K : Urine potassium (mmol/L), Ur Cl−: Urine chloride (mmol/L), Ur urea: Urine urea (mg/dl), U/o: Urine output (ml/24 h) Indian Journal of Critical Care Medicine ¦ Volume 22 ¦ Issue 9 ¦ September 2018 665 Page no. 47 Vadi and Yim: Hypernatremia due to urea-induced osmotic diuresis urea cycle accounts for 90% of nitrogen‑containing compounds What is the cause of hypernatremia in this patient? in the urine. Physiology At the bedside The mechanisms of hypernatremia include: Stress of acute illness (diarrhea) increased her protein a. Hypovolemic hypernatremia: Reduction in total body catabolism. Bacterial sepsis increases urinary nitrogen water >> reduction of total body sodium (fluid loss) excretion. Combined with these, her renal dysfunction b. Euvolemic hypernatremia: Reduction in total body water; would have contributed to high plasma urea levels. Side total body sodium is normal (fluid loss) effects of negative protein balance are not tolerated by c. Hypervolemic hypernatremia: Total body water is normal; total body sodium is increased (sodium gain). those with already reduced lean body mass reserves. Endogenous protein wasting contributed to this patient’s At the bedside osmotic diuresis. Hypovolemia resulting from vomiting, diarrhea, and solute What is the clinical significance of osmotic diuresis? diuresis (hyperosmolar urine rich in urea but poor in sodium and potassium led to loss of free water) contributed to her Physiology hypernatremia. Furthermore, the patient’s impaired thirst Excessive glucose or urea nitrogen can lead to osmotic diuresis. response in addition to her altered mental status further Consequent polyuria reduces extracellular fluid (ECF) volume. precluded her water ingestion. Sodium lost in the urine is relatively less than the amount of water diuresis, leading to hypernatremia. How can sodium trends (rise, followed by downward trend) be explained? Calculating urinary osmoles and their nature helps in the diagnosis of solute diuresis.[4] Physiology Water follows solute electrolytes (i.e., sodium). However, Formula a: water excreted without solute electrolytes = free water Calculated Uosm(=×2/urine sodium [mmolL] + clearance (FWC). (urine urea nitrogen[mg /]dl ) FWC indicates how body handles the water load.[5] urinepotassium[mmool / L) ++ 28.