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Part II Nutrition Assessment, Consequences, and Implications

is extracellular (extracellular fluid, or ECF). Normally, about 25% of the ECF is in the intravascular compart- ment; the other 75% is interstitial fluid. Extracellular fluid travels easily between the two spaces, regulated by blood pressure and colloid osmotic pressure. In response to sodium concentration, water can rapidly cross cell membranes to maintain equal osmolality between the intracellular and extracellular spaces. When sodium concentration increases, so does extracel- lular fluid volume. When sodium concentration decreases, extracellular fluid volume contracts (26). Normally, daily water intake equals daily water output. Daily fluid intake can come from food, bever- ages, liquids, medical nutritional products, intravenous fluids, and water produced when nutrients are metabolized.

When fluid intake is insufficient, typically the kidneys reduce urine output to as little as 500 mL/d, but the lungs and skin continue to lose up to 1,000 mL of water. When a negative fluid balance occurs, blood osmolality rises (26).

“Because maintaining normal blood osmolality (280 mOsm/kg H2

O) is vital, the body has a number of

mechanisms to ensure water balance. A rise in blood osmolality as small as 1% to 2% causes the hypothala- mus to secrete vasopressin.” This stimulates thirst and a powerful urge to drink. At the same time, the pituitary gland secretes antidiuretic hormone, which causes the kidneys to concentrate urine and minimize urine volume. Because of the relationship between sodium and fluid volume, body sodium content is an important factor in controlling body water. A number of mecha- nisms regulate body sodium. The kidneys filter about 25,000 mEq of sodium daily. Most of this sodium is reabsorbed. In the face of extracellular fluid depletion, the adrenal cortex secretes aldosterone, causing the kidneys to reabsorb even more sodium. Conversely, extracellular fluid expansion stimulates secretion of atrial natriuretic factor, which increases glomerular fil- tration rate and reduces sodium reabsorption by the kidneys (26). Box 8.2 and Box 8.3 provide information about risk factors for dehydration and fluid/electrolyte imbalance.

THE EFFECTS OF AGING ON HYDRATION STATUS

Older adults are less resistant than younger adults to negative fluid balance. Physiological, medical, envi- ronmental, and situational factors increase older adults’ vulnerability to dehydration.

Physiological Factors

Aging is associated with reduced thirst perception. Older adults do not feel as thirsty, and they do not

BOX 8.2 Conditions and Factors That May Increase Risk for Dehydration or Fluid/Electrolyte Imbalance

Critical Conditions ●

●

● diarrhea ● vomiting

● dependence on staff for eating and drinking ●

● draining wounds or pressure ulcers excessive sweating rapid breathing

● ● ● GI bleeding

● previous episodes of dehydration ● difficult or painful swallowing ● depression

● ●

small amount of dark or concentrated urine excessive urination

● nothing-by-mouth or fluid-restriction orders ●

chronic comorbidities (eg, stroke, diabetes, congestive heart failure)

● infection ● dizziness

Environmental Factors ●

tube feeding

● use of specialty beds ●

● ● ● ● ●

lack of social or family support inadequate staffing language barriers isolation restraints

facility-specific factors that may expose patients to excessive heat (eg, malfunctioning air conditioners)

Source: Reprinted with permission from American Medical Directors Association. Dehydration and Fluid Maintenance. Clinical Practice Guideline. Columbia, MD: American Medical Directors Association; 2001, with permission from American Medical Directors Association. Copyright ©2009, American Medical Directors Association. All rights reserved.

drink as much in response to volume depletion as do younger people. Consequently, they do not respond to fluid deprivation and blood hyperosmolality by volun- tarily increasing fluid intake (28,29). Additionally, kidney function declines with aging and aged kidneys

use of medications that can cause dehydration (eg, diuretics, phenytoin, lithium, laxatives)

dementia or cognitive impairment fever (including low-grade fever)