Abnormalities of water homeostasis in aging

Phenotyping nocturnal polyuria: circadian and age-related variations in diuresis rate, free water clearance and sodium clearance

BACKGROUND

this study compares diuresis rate, sodium clearance and free water clearance (FWC) by age and time of day (nighttime vs. daytime) in subjects with and without nocturnal polyuria (NP) to determine whether these variables affect the phenotype of NP.

METHODS

post hoc analysis of two prospective observational studies. Eight urine samples collected at 3-h intervals and a single blood sample were used to calculate daytime (10a/1p/4p/7p/10p) and nighttime (1a/4a/7a) diuresis rates, sodium clearance and FWC. Three mixed linear models were constructed for diuresis rate, sodium clearance and FWC using four predictor variables: NP status (present [nocturnal urine production >90 ml/h] vs. absent [≤90 ml/h]), time of day, age and study identification.

RESULTS

subjects with NP experienced higher nighttime versus daytime diuresis rates, sodium clearance and FWC. Regardless of NP status, increased age was accompanied by an increase in the ratio of nighttime/daytime diuresis rate, nighttime sodium clearance and daytime sodium clearance. FWC showed a complex age effect, which was independent of time of day or NP status.

CONCLUSIONS

age-related increases in nighttime/daytime diuresis rate, 24-h sodium clearance and 24-h FWC are not specific to subjects with NP. The age-related surge in either nocturnal sodium clearance or nocturnal FWC may represent the relevant substrate for behavioural or pharmacologic interventions targeting sodium diuresis or free water diuresis, respectively. Increases in FWC in older age groups may reflect impaired circadian rhythmicity of endogenous AVP or changes in responsiveness of the aged nephron to water clearance.

Fluid balance concepts in medicine: Principles and practice

The regulation of body fluid balance is a key concern in health and disease and comprises three concepts. The first concept pertains to the relationship between total body water (TBW) and total effective solute and is expressed in terms of the tonicity of the body fluids. Disturbances in tonicity are the main factor responsible for changes in cell volume, which can critically affect brain cell function and survival. Solutes distributed almost exclusively in the extracellular compartment (mainly sodium salts) and in the intracellular compartment (mainly potassium salts) contribute to tonicity, while solutes distributed in TBW have no effect on tonicity. The second body fluid balance concept relates to the regulation and measurement of abnormalities of sodium salt balance and extracellular volume. Estimation of extracellular volume is more complex and error prone than measurement of TBW. A key function of extracellular volume, which is defined as the effective arterial blood volume (EABV), is to ensure adequate perfusion of cells and organs. Other factors, including cardiac output, total and regional capacity of both arteries and veins, Starling forces in the capillaries, and gravity also affect the EABV. Collectively, these factors interact closely with extracellular volume and some of them undergo substantial changes in certain acute and chronic severe illnesses. Their changes result not only in extracellular volume expansion, but in the need for a larger extracellular volume compared with that of healthy individuals. Assessing extracellular volume in severe illness is challenging because the estimates of this volume by commonly used methods are prone to large errors in many illnesses. In addition, the optimal extracellular volume may vary from illness to illness, is only partially based on volume measurements by traditional methods, and has not been determined for each illness. Further research is needed to determine optimal extracellular volume levels in several illnesses. For these reasons, extracellular volume in severe illness merits a separate third concept of body fluid balance.

Review on Mechanisms, Importance of Homeostasis and Fluid Imbalances in the Elderly

Water is considered an essential nutrient, but very often is overlooked in favour of nutrition. Due to age related changes, older people are more vulnerable to both, dehydration and overhydration. This article describes the principles of water homeostasis and provides a description of common age-related physiological changes related to water balance. Fluid homeostasis largely depends on osmo- and baro-receptors that respectively react to changes in osmolality and blood pressure, water-regulating hormone release, the subsequent response of kidneys to the hormonal stimulation and activation of thirst. These mechanisms diminish with age and put elderly at risk of dehydration. All involved in a care for the elderly should be informed and become vigilant to prevent dehydration from occurring.

Individuality of the plasma sodium concentration

Older literature has suggested that the plasma sodium concentration is not individual, that it is neither intrinsic to an individual nor reproducible, longitudinally. We recently observed that the plasma sodium concentration is heritable. Because demonstrable heritability requires individuality of the relevant phenotype, we hypothesized that the plasma sodium concentration was substantially individual. In two large health plan-based cohorts, we demonstrated individuality of the plasma sodium concentration over a 10-yr interval; the intraclass correlation coefficient (ICC) averaged 0.4-0.5. The individuality of plasma sodium increased significantly with age. Plasma sodium individuality was equal to or only slightly less than that for plasma glucose but was less than the individuality for creatinine. The individuality of plasma sodium was further confirmed by comparing the Pearson correlation coefficient for within-individual versus between-individual pairs of sodium determinations and via application of the agreement index. Furthermore, the distribution of all sodium determinations for all participants within a population was similar to the distribution for the mean sodium concentration for individuals within that population. Therefore, the near-normal distribution of plasma sodium measurements within a population is likely not attributable to assay-specific factors but rather to genuine and durable biological variability in the osmotic set point. In aggregate, these data strongly support the individuality of the plasma sodium concentration. They further indicate that serial plasma sodium values for any given individual tend to cluster around a patient-specific set point and that these set points vary among individuals.

Age-associated abnormalities of water homeostasis

Findley first proposed the presence of age-related dysfunction of the hypothalamic-neurohypophyseal-renal axis more than 60 years ago. More sophisticated studies have since corroborated his findings. As a result, it is now clear that multiple abnormalities in water homeostasis occur commonly with aging, and that the elderly are uniquely susceptible to disorders of body volume and osmolality. This article summarizes the distinct points along the hypothalamic-neurohypophyseal-renal axis where these changes have been characterized, as well as the clinical significance of these changes, with special attention to effects on cognition, gait instability, osteoporosis, fractures, and morbidity and mortality.

Heritability of serum sodium concentration: evidence for sex- and ethnic-specific effects

Serum sodium concentration is the clinical index of systemic water balance. Although disordered water balance is common and morbid, little is known about genetic effects on serum sodium concentration at the population level. Prior studies addressed only participants of European descent and either failed to demonstrate significant heritability or showed only modest effect. We investigated heritability of serum sodium concentration in large cohorts reflecting a range of races/ethnicities, including the Framingham Heart Study (FHS, non-Hispanic Caucasian), the Heredity and Phenotype Intervention Heart Study (HAPI, Amish Caucasian), the Jackson Heart Study (JHS, African American), the Strong Heart Family Study (SHFS, American Indian), and the Genetics of Kidney Disease in Zuni Indians Study (GKDZI, American Indian). Serum sodium was transformed for the osmotic effect of glucose, and participants with markedly elevated glucose or reduced estimated glomerular filtration rate (eGFR) were excluded. Using a standard variance components method, incorporating covariates of age, glucose, and eGFR, we found heritability to be high in African American and American Indian populations and much more modest in non-Hispanic Caucasian populations. Estimates among females increased after stratification on sex and were suggestive among female participants in FHS (0.18 ± 0.12, P = 0.057) and male participants in JHS (0.24 ± 0.16, P = 0.067) and statistically significant among female participants in JHS (0.44 ± 0.09, P = 1 × 10 ⁻⁷), SHFS (0.59 ± 0.05, P = 9.4 × 10⁻⁴⁶), and GKDZI (0.46 ± 0.15, P = 1.7 × 10⁻⁴), and male participants in HAPI (0.18 ± 0.12, P = 0.03) and SHFS (0.67 ± 0.07, P = 5.4 × 10⁻²⁶). Exclusion of diuretic users increased heritability among females and was significant in all cohorts where data were available. In aggregate, these data strongly support the heritability of systemic water balance and underscore sex and ethnicity-specific effects.

Understanding clinical dehydration and its treatment

Dehydration in clinical practice, as opposed to a physiological definition, refers to the loss of body water, with or without salt, at a rate greater than the body can replace it. We argue that the clinical definition for dehydration, ie, loss of total body water, addresses the medical needs of the patient most effectively. There are 2 types of dehydration, namely water loss dehydration (hyperosmolar, due either to increased sodium or glucose) and salt and water loss dehydration (hyponatremia). The diagnosis requires an appraisal of the patient and laboratory testing, clinical assessment, and knowledge of the patient's history. Long-term care facilities are reluctant to have practitioners make a diagnosis, in part because dehydration is a sentinel event thought to reflect poor care. Facilities should have an interdisciplinary educational focus on the prevention of dehydration in view of the poor outcomes associated with its development. We also argue that dehydration is rarely due to neglect from formal or informal caregivers, but rather results from a combination of physiological and disease processes. With the availability of recombinant hyaluronidase, subcutaneous infusion of fluids (hypodermoclysis) provides a better opportunity to treat mild to moderate dehydration in the nursing home and at home.

Disorders of body water homeostasis in critical illness

Disorders of sodium and water homeostasis are among the most commonly encountered disturbances in the critical care setting, because many disease states cause defects in the complex mechanisms that control the intake and output of water and solute. Because body water is the primary determinant of extracellular fluid osmolality, disorders of body water balance can be categorized into hypoosmolar and hyperosmolar disorders depending on the presence of an excess or a deficiency of body water relative to body solute. Because the main constituent of plasma osmolality is sodium, hypoosmolar and hyperosmolar disease states are generally characterized hy hyponatremia and hypernatremia, respectively. After a brief review of normal water metabolism, this article focuses on the diagnosis and treatment of hyponatremia and hypernatremia in the critical care setting.