Fractional excretion of K, Na and Cl following furosemide infusion in healthy, young and very old people.

UNLABELLED: Furosemide test is a simple and useful test of renal physiology used to evaluate the capability of the collecting ducts to secrete potassium under the effect of serum aldosterone. Its behaviour pattern has been established in children and young adults but not described in very old healthy people, which we explored in this study. MATERIAL AND METHODS: Twenty-six healthy volunteers on a standard Western diet (50 mmol of K/day) were studied: 20 of them were young (between 17 and 40 years old) and the rest were very old (between 75 and 85 years old). They suffered from no diseases and were not on any medication. Before, during the test and 180 min after a single dose of intravenous furosemide (1 mg/kg), urine and blood samples were obtained for creatinine and electrolytes levels. From these data we calculated fractional excretion (FE) of electrolytes; serum aldosterone was measured pre and post furosemide infusion. Statistical analysis was performed by applying Student's t-test. RESULTS: There was no significant difference regarding pre-furosemide (basal) FE of potassium between the very old and young group. Post-furosemide average FE of potassium was significantly lower in the very old group (27.4 +/- 2%) compared with the young group (35.4 +/- 9%) (P = 0.04). Even though there was no significant difference in post-furosemide peak FE of potassium value, it was reached later in the very old (120 min) compared with the young (30 min). Serum aldosterone levels were significantly higher post furosemide in both groups: 18.3 +/- 12.2 ng/dl (pre) versus 32.5 +/- 18.6 ng/dl (post) in the young (P = 0.007) and 69.8 +/- 13.7 ng/dl (pre) versus 113.3 +/- 54.8 ng/dl (post) in the very old (P = 0.04). Furthermore, all serum aldosterone values (pre and post furosemide) were significantly higher in very old people compared with young people (P < 0.001). Basal fractional excretion of sodium and chloride were slightly higher in the very old group compared with the young group (P = 0.05). Average post-furosemide FE of sodium and chloride were slightly and significantly lower in the very old (P = 0.05 and P = 0.03), respectively. However, there was no significant difference in peak post-furosemide FE of sodium and chloride values, which were reached later in the very old (120 min) compared with the young (30 min). CONCLUSION: Furosemide test showed a significantly lower average post-furosemide FE of potassium value, delayed post-furosemide peak FE of Na, K and Cl and a hormonal pattern of aldosterone resistance in very old people.

Creatinine reabsorption by the aged kidney.

AIM: The handling of renal creatinine in human beings has classically been described as the result of two particular physiological processes: glomerular filtration and proximal tubular secretion. However, there are particular physiological situations in which tubular creatinine reabsorption has been documented, such as in the case of healthy newborns and premature babies. We performed a prospective study in order to evaluate if there is tubular creatinine reabsorption in healthy elderly people. PATIENTS AND METHOD: We studied prospectively nine healthy volunteers, four of them young (20-33 years old) and the remaining five, old (65-73 years old). Since creatinine is secreted in the proximal tubules, and its secretion can be completely blocked by cimetidine administration, a creatinine clearance with cimetidine reliably represents the glomerular filtration rate. Therefore, if the ratio creatinine clearance (Ccr)/creatinine clearance with cimetidine (CcrWC) is higher than one, this would indicate net creatinine secretion, whereas a ratio lower than one would indicate a net renal creatinine tubular reabsorption; a ratio equal to one indicates creatinine filtration. Finally, the Ccr, CcrWC, and Ccr/CcrWC ratios were compared between the young and old group. STATISTICAL TESTS: Mann-Whitney and Wilcoxon tests were used. RESULTS: As expected, creatinine clearance in the elderly was significantly lower than in the young [Ccr: 74.4 ml/min (47.9-100.9) (old) vs. 153.8 ml/min (108.3-199.2) (young), p = 0.014]. Similarly, the creatinine clearance with cimetidine (CcrWC) was significantly lower in the elderly compared to the young [CcrWC: 81.8 ml/min (69.2-94.5) (old) vs. 122.5 ml/min (82.6-162.4) (young), p = 0.028]. The ratio of Ccr/CcrWC was 0.9 in the elderly vs. 1.26 in the young (p = 0.014), indicating net creatinine reabsorption in the elderly and net creatinine secretion in the young. CONCLUSION: Our findings indicate that there seems to be a net reabsorption of creatinine in the renal tubules of healthy old persons.