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Dehydration occurs when the body loses more fluid than it takes in. Dehydration can be caused by illness, injury, infection, prolonged exposure to sun or high temperatures, inadequate water intake, or overuse of diuretics or other medications that increase urination.
Water is distributed throughout three compartments in the body: inside the cells (intracellular), in the tissue (interstitial), and in the bloodstream (intravascular). Each compartment contains differing amounts of electrolytes that must remain in balance in order for body organs and systems to function correctly. Dehydration upsets this delicate balance. Total body water also varies in relation to age, gender, and amount of body fat. Adult males have approximately 60% water content, adult females have 50%, infants have an estimated 77%, and the elderly have 46% to 52%. An increase in body fat causes a decrease in the percent fluid content because fat does not contain significant amounts of water.
Different types of dehydration have different causes. When managing patients with dehydration, the type of water loss must be determined to ensure appropriate treatment. In addition, water and sodium levels in the body are closely related; if one is abnormal, the other often is too.
Isotonic dehydration is an equal loss of water and sodium. Isotonic means that the number of particles contained on one side of a permeable membrane is the same as on the other side, thus there is no fluid shift in either direction. The amount of intracellular and extracellular water remains in balance. This can be caused by a complete fast, vomiting, and diarrhea.
Hypertonic dehydration occurs when water loss is greater than sodium loss. Blood sodium levels may be >145 mmol/l (normal range=135 to 145 mmol/l). Higher blood sodium levels combined with decreased water in the intravascular space increases the osmotic pressure in the bloodstream, which, in turn, pulls more fluid out of the cells. This type of dehydration is usually caused by extended fever with limited oral rehydration. Mortality is more likely to occur from hypertonic than from isotonic dehydration.
Hypotonic dehydration occurs when sodium loss is greater than water loss. Blood sodium levels may be less than 135 mmol/l; and the osmotic pressure is greater inside the cells, which pulls more fluid out of the intravascular space into the intracellular space. This type of dehydration occurs with overuse of diuretics, which causes excessive sodium and potassium loss. Potassium depletion affects respiration, increases nausea, and, if severe enough, may cause respiratory arrest or central nervous system (CNS) seizures. Potassium depletion may also cause arrhythmias (an alteration in the heartbeat). As a result, patients are told to take diuretics with orange juice or to eat a banana, both of which are high in potassium.
Strenuous activity, excessive sweating, prolonged time in the sun, and extended vomiting or diarrhea cause fluid loss. Elderly people who move to warm, dry climates frequently become dehydrated because of the climate change combined with a tendency to not drink enough water. Large amounts of fluid can also be lost from prolonged fever. Healthy people require about 1 milliliter of water for each calorie their body metabolizes;
Decreased oral intake of fluids is a common cause of dehydration and often occurs during times of appetite loss from illness or after oral surgery or injury. The elderly are at high risk for decreased intake because their thirst mechanism may no longer function or they may be physically unable to get a drink. Infants, another high-risk group, are more likely to develop dehydration than adults because they have a higher metabolic rate and their immature kidneys have difficulty concentrating urine. Children who do not wet their diapers for three hours or more are dehydrated. Dehydration is also associated with disorders of the adrenal glands, which regulate water-electrolyte balance; diabetes mellitus; eating disorders; renal disease; and chronic lung disease.
Symptoms of dehydration at any age may include some or all of the following: cracked lips, dry or sticky mucous membranes, sunken eyes, lethargy, and/or confusion. Urine output is minimal and the skin loses its elasticity (turgor) and is slow to return to its normal position after being raised off the back of the hand (tenting). The heart rate and respiratory rate may be elevated. A dehydrated infant may not shed tears when crying and may have a depressed fontanel (soft spot on their head), although recent studies have shown that a depressed fontanel is not an accurate indicator of dehydration.
Author Info: Abby Wojahn, R.N.,B.S.N.,C.C.R.N., The Gale Group Inc., Gale, Detroit, Gale Encyclopedia of Nursing and Allied Health, 2002
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