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      Circadian Rhythm of Hydration in Healthy Subjects and Uremic Patients Studied by Bioelectrical Impedance Analysis

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          Background: Healthy subjects and patients after successful kidney transplantation show a circadian rhythm for glomerular filtration rate and for the glomerular transport of macromolecules. We aimed to evaluate by bioelectrical impedance analysis (BIA) whether body hydration status also follows a circadian rhythm in patients with impaired renal function. Methods: The study was conducted on 28 subjects divided into 3 groups: 8 healthy volunteers, 8 patients affected by chronic kidney disease and 12 end-stage renal disease (ESRD) patients on hemodialysis. During 24 h, 9 BIA measurements were taken in every subject every 180 min. Results: BIA findings demonstrate that normal subjects have a circadian rhythm in hydration status that reaches maximum body water content at night, between 21.00 and 23.00 h. In patients with chronic kidney disease, this rhythm, with maximum at night, is maintained. The rhythm is also present in ESRD patients, if the residual diuresis is at least 500 ml/day, while there is no rhythm when residual diuresis is <300 ml/day. Conclusions: In normal subjects, body hydration status shows a circadian rhythm, which is weakened or lost in oligoanuric patients on dialysis, but partially maintained in subjects with preterminal uremia and in hemodialyzed patients with residual diuresis >500 ml/day.

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          The most essential nutrient: defining the adequate intake of water.

          Although water is quantitatively the most import nutrient, there are no recommended dietary allowances (RDA) or adequate intake (AI) values. Based on 718 assessments of 24-hour total water intake, urine volume, and urine osmolality, individual hydration status was characterized in 479 healthy boys and girls of the DONALD study aged 4.0 to 6.9 years and 7.0 to 10.9 years. Mean 24-hour total water intake ranged from 0.90 mL/kcal to 0.96 mL/kcal, and median 24-hour urine osmolality ranged from 683 mosm/kg to 854 mosm/kg. A maximum urine osmolality of 830 mosm/kg (mean - 2 SD) in healthy children with a typical affluent Western-type diet was the physiologic criterion of the upper limit of euhydration. "Water reserve" (24-hour urine volume - hypothetical urine volume to excrete 24 urine solutes at a concentration of 830 mosm/kg) was a quantitative measure of individual 24-hour hydration status and ensuring euhydration in 97% of the subjects in each group; AI values of total water in the 4 age and sex groups ranged from 1.01 mL/kcal to 1.05 mL/kcal. These procedures to quantify 24-hour hydration status may prove valuable in investigating the effects on health of different states of euhydration.
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            Dry weight in hemodialysis: volemic control.

            In dialysis patients the chronic fluid overload may represent a nonphysiologic condition which brings both arterial hypertension and hemodynamic instability. Volume expansion is significantly correlated to casual predialysis blood pressure and 24-hour arterial pressure. The normalization of the patient hydration status is not only followed by a reduction in pressure values but also by an improvement of the circadian blood pressure rhythm. On the other hand, hypovolemia and underhydration combined with an impaired cardiovascular regulatory response may generate the dialysis-related hypotension. Many techniques have been introduced to obtain an objective measurement of the hydration status: postdialysis radiological chest examination, plasma atrial natriuretic peptide (ANP), and plasma cyclic guanidine monophosphate (cGMP) levels, multifrequency electrical bioimpedance and the continuous plasma volume measurement. The latter, alone or in combination with provocative tests (stop and go of the ultrafiltration), may help in optimizing plasma volume contraction. The plasma volume monitoring avoids the risk of hypovolemic hypotension and facilitates the achievement of a correct dry body weight. The biofeedback system, exploiting the automatic control of the intradialytic variations, may represent an additional advantage in the formulation of an ideal postdialysis blood volume that overlaps the patient's dry weight.
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              Influence of the Hydration State on Blood Pressure Values in a Group of Patients on Regular Maintenance Hemodialysis

              The pathophysiology of hypertension in patients on renal replacement therapy is not yet clear, and the role of extracellular fluid overload is still a matter of debate. The main problem is the lack of techniques to determine the fluid state. Recently new noninvasive techniques have become available which make it possible to accurately determine the hydration state in these patients. We have studied the influence of the hydration state on interdialytic blood pressure in 45 patients: 21 (46.6%) using antihypertensive medication and 24 (53.4%) without antihypertensive medication. Total body water (TBW) was determined by bioelectrical impedance analysis performed just before a hemodialysis session. The TBW was then related to the fat-free mass calculated by the anthropometric method (aFFM) of Durnin. The hydration state was defined using the following formula: TBW/aFFM·100. Furthermore, for each patient the ideal TBW was calculated according to the Watson formula. The difference between TBW and ideal TBW was considered a further index of the hydration state. Ambulatory blood pressure monitoring was performed by using a Takeda 2420 recorder according to the Korotkoff method during the 24 h before the midweek hemodialysis session. Blood pressure monitoring showed a significant correlation with the hydration state of these patients. In conclusion, the hydration state seems to play a major role in interdialytic blood pressure control.

                Author and article information

                Nephron Physiol
                Nephron Physiology
                S. Karger AG
                July 2007
                15 June 2007
                : 106
                : 3
                : p39-p44
                aDepartment of Internal Medicine, University of Messina, Messina, and bDepartment of Clinical and Experimental Medicine, University of Ferrara, Ferrara, Italy
                103908 Nephron Physiol 2007;106:p39–p44
                © 2007 S. Karger AG, Basel

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                Figures: 3, Tables: 2, References: 15, Pages: 1
                Original Paper


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