Porth's Essentials of Pathophysiology, 4e

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Integrative Body Functions

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pulled into the ECF preventing a change in volume; as a hypovolemic state, in which water loss is greater than sodium loss; or as a hypervolemic state if there is an addition of a hypertonic solution containing both sodium and water. 3,26–28 All forms of hypernatremia represent a hyper- tonic state with an increase in intracellular osmolal- ity that causes activation of the thirst mechanism and an increased ability of the kidneys to conserve water by producing concentrated urine. Thirst is highly effective in preventing hypernatremia. Therefore, hypernatremia is more likely to occur in infants and in persons who do not experience or cannot express their thirst or obtain water to drink. 28 Hypodipsia is particularly prevalent among the elderly. A defect in thirst or inability to obtain or drink water can inter- fere with water intake. An increase in the intracellular osmolality normally leads to an increase in ADH levels with increased reabsorption of water by the kidneys. Hypernatremia develops when there is impaired ability of the kidneys to conserve water by producing concen- trated urine, most commonly due to acute or chronic renal failure. Net water loss can occur through the urine, gastroin- testinal tract, lungs, or skin. It can result from increased losses from the respiratory tract during fever or strenu- ous exercise, or from the gastrointestinal tract due to watery diarrhea or when highly osmotic tube feedings are given with inadequate amounts of water. With pure water loss, both the ICF and ECF compartments lose an equal percentage of their volume. Because ICF con- tains a greater percentage of water than the ECF, more actual water volume is lost from the ICF than the ECF compartment. The therapeutic administration of excess amounts of sodium-containing solutions may also cause hypernatremia. Manifestations. The clinical manifestations of hyper- natremia caused by water loss are largely those of ECF loss and cellular dehydration (see Table 8-4). The severity of signs and symptoms is greatest when the increase in serum sodium is large and occurs rapidly. Body weight is decreased in proportion to the amount of water that has been lost. Because blood plasma is roughly 90% to 93% water, the concentrations of blood cells and other blood components increase as ECF water decreases. Thirst is an early symptom of water deficit, occur- ring when water losses are equal to 0.5% of body water. Urine output is decreased and urine osmolality increased because of renal water-conserving mechanisms. Body temperature frequently is elevated, and the skin becomes warm and flushed. Hypernatremia produces an increase in serum osmolality and results in water being pulled out of body cells. 3 As a result, the skin and mucous mem- branes become dry, and salivation and lacrimation are decreased. The mouth becomes dry and sticky, and the tongue becomes rough and fissured. Swallowing is dif- ficult. The subcutaneous tissues assume a firm, rubbery texture. Most significantly, movement of water out of the CNS causes decreased reflexes, agitation, headache,

and restlessness. Coma and seizures may develop as hypernatremia progresses. Treatment. Treatment of hypernatremia includes mea- sures to treat the underlying cause of the disorder and fluid replacement therapy to treat the accompanying dehydration. Replacement fluids can be given orally or intravenously. Oral glucose–electrolyte replacement solutions are widely available in grocery stores and pharmacies for use in the treatment of acute hyperna- tremia due to diarrhea and other dehydrating disorders in infants and young children (see Chapter 29). 29 Oral replacement therapy is less expensive than intravenous therapy and has a lower complication rate. Intravenous therapy may be required for children and adults with severe dehydration. One of the serious aspects of sustained hyperna- tremia is dehydration of brain and nerve cells. The treatment of sustained hypernatremia requires con- trolled gradual correction of sodium and water lev- els to avoid serious neurologic complications. 26–28 As with severe hyponatremia, brain cells protect against changes in cell volume by changing their concentra- tion of organic osmolytes, increasing their concentra- tion in hypernatremia to prevent water from being pulled into the ECF. If hypernatremia is corrected too rapidly—before the osmolytes have had a chance to dissipate—the plasma may become relatively hypo- tonic in relation to brain cell osmolality. When this occurs, water moves into the brain cells, causing cerebral edema and potentially severe neurologic impairment. ■■ The volume and distribution of body fluids between the intracellular fluid (ICF) and extracellular fluid (ECF) compartments depend on the concentration of water, which provides approximately 90% to 93% of its fluid volume, and sodium salts, which provide approximately 90% to 95% of the ECF solutes. ■■ The main determinant of water and sodium balance is the effective circulating blood volume, which is monitored by stretch receptors in the vascular system that exert their effects through thirst, which controls water intake, and the antidiuretic hormone (ADH), which controls urine concentration.The sympathetic nervous system and the renin-angiotensin-aldosterone system contribute to fluid balance through the regulation of sodium balance. ■■ Isotonic fluid disorders result from contraction or expansion of ECF volume brought about by SUMMARY CONCEPTS