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Introduction Fractional excretion of urea (FEU) is a useful marker in order to evaluate renal perfusion state in people with normal renal function. In a well hydrated patient, FEU is around 50%, while during a low renal perfusion state, such as dehydration, cirrhosis and cardiac failure, this marker reaches values lower than 35%. Conversely, during over-hydrated states, such as syndrome of inappropriate secretion of antidiuretic hormone (SIADH), FEU reaches values higher than 65% (1). However, in a previous study, we found that healthy old and very old people, under normal conditions, had a FEU around 60%. This means that they reached higher values respect to an adult under the same normal conditions: FEU 50% (2). We present the following case report because it is a good opportunity to evaluate FEU behavior in a severely dehydrated old patient. Case Report A 73 years old female, living in a nursing-home and suffering from severe dementia and immobility syndrome, was admitted due to a respiratory infection, hypotension, acute renal insufficiency, severe dehydration and hypernatremia. Initially, she was treated with intravenous antibiotic, and intravenous normal saline until she improved her blood pressure and renal insufficiency, then she received tap water in her tube feeding. Progressively, the patient and her blood exams were getting better until, she solved her clinical problems (sepsis, dehydration, hypernatremia, and renal failure) and she returned to the nursing-home. Results The following table shows the evolution of main laboratory values (plasma sodium, urea, creatinine and FEU) along the admission days.
Discussion Physiologically urea suffers, along nephrons, all the following processes: filtration, secretion, reabsorption and excretion, and finally, it reaches a fractional excretion of about 50% in healthy adult people. Moreover, during renal low perfusion states, urea reabsorption is higher, specially in the more distal part of the collecting tubules, where antidiuretic hormone stimulates urea reabsorption and consequently reduces its fractional excretion to less than 35%. During the opposite situation, such as the over-hydration state in a SIADH, urea is secreted in the more distal part of the proximal tubule (S3), reaching FEU values higher than 65 % (3). In elderly people, we documented higher values of FEU, and we speculated that this could be secondary to the medullary hypotonicity that they have, and the sort of antidiuretic hormone resistance that this hypotonicity produces (4). In this report we can appreciate that an old person, who belongs to a population that usually has higher levels of FEU, was able to reach a low value of FEU during a severe volume contraction, this means as an adult would do it. However, while she was still dehydrated but less severely, her FEU started to be higher than the expected in an adult under that condition. Then we thought that her kidney was able to reach an urea handling similar to a younger kidney, but solely under the influence of an extreme stimulus, such as severe dehydration. For instance: when the patient was severely dehydrated her FEU was 20%, but later when she was moderately dehydrated and still hypernatremic (plasma sodium 161 mmol/l), her FEU was 65%, while in an adult under this condition it would be lower than 35%. Conclusion We found that in spite of the patient was old, she was able to obtain a low FEU during a severe dehydration state, however FEU values were higher, respect to the one expected in an adult, when she was moderately dehydrated. References 1) Bazerque F. Sindromes hipoosmolares e hiperosmolares. In Pacin J (Ed). Terapia Intensiva. Buenos Aires. Panamericana 1995:389-376. 2) Musso C, Fainstein I, Kaplan R, Macías Núñez J. Función tubular en el muy anciano. Revista Española de Geriatría y Gerontología. 2003 (en prensa). 3) Lise Bankir.Urea and the kidney. In Brenner B (Ed)The Kidney. Philadelphia. W.B.Saunders. 1996: 571-598. 4) Musso C. Geriatric nephrology and the " nephrogeriatric giants". International Urology and Nephrology. 34: 255-256, 2002. |
