Porth's Essentials of Pathophysiology, 4e

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Integrative Body Functions

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The corticosteroid hormones also increase sodium reabsorption by the kidneys; therefore, persons being treated with these medications and those with Cushing syndrome or disease often have problems with sodium retention (see Chapter 32). Isotonic fluid volume excess is manifested by an increase in interstitial and vascular fluids, and is char- acterized by weight gain over a short period of time. Mild fluid volume excess represents a 2% gain in weight; moderate fluid volume excess, a 5% gain in weight; and severe fluid volume excess, a gain of 8% or more in weight (see Table 8-3). 3 The presence of edema is characteristic of isotonic fluid excess. When the fluid excess accumulates gradually, as often hap- pens in debilitating diseases and starvation, edema may mask the loss of tissue mass. As vascular volume increases, central venous pressure increases, leading to distended neck veins, slow-emptying peripheral veins, a full and bounding pulse, and an increase in central venous pressure. There is often a dilutional decrease in hematocrit and BUN levels due to expansion of the plasma volume. When excess fluid accumulates in the lungs (i.e., pulmonary edema), there are complaints of shortness of breath and difficult breathing, respi- ratory crackles, and a productive cough. Ascites and pleural effusion may occur with severe fluid volume excess. The treatment of fluid volume excess focuses on providing a more favorable balance between sodium and water intake and output. A sodium-restricted diet is often prescribed as a means of decreasing extra- cellular sodium and water levels. Diuretic therapy is commonly used to increase sodium elimination. When there is a need for intravenous fluid administration or transfusion of blood components, the procedure requires careful monitoring to prevent circulatory overload. Hyponatremia Hyponatremia is usually defined as a serum sodium concentration of less than 135 mEq/L (135 mmol/L). 3 It is one of the most common electrolyte disorders seen in hospitalized patients and is also common in the outpatient population, particularly in the elderly. A number of age-related events make the elderly popu- lation more vulnerable to hyponatremia, including a decrease in renal function accompanied by limitations in sodium conservation. Although older people main- tain body fluid homeostasis under most circumstances, the ability to withstand environmental, drug-related, and disease-associated stresses often becomes progres- sively limited. Hyponatremia can present as a hypovolemic, euvolemic, or hypervolemic state. Hyponatremia can also present as a hypertonic hyponatremia resulting from an osmotic shift of water from the ICF to the ECF, such as occurs with hyperglycemia. In this situation, the sodium in the ECF becomes diluted as water moves out of body cells in response to the osmotic effects of the elevated blood glucose level. 21,22

Hypovolemic hypotonic hyponatremia is the most common type of hyponatremia. It occurs when water is used to replace the loss of iso-osmotic body fluids. Among the causes of hypovolemic hyponatremia is excessive sweating in hot weather, particularly during heavy exercise, which leads to loss of salt and water. Hyponatremia develops when water, rather than electrolyte-containing liquids, is used to replace fluids lost in sweating. Iso-osmotic fluid loss, such as occurs in vomiting or diarrhea, does not usually lower serum sodium levels unless these losses are replaced with dis- proportionate amounts of orally ingested or parenter- ally administered water. Gastrointestinal fluid loss and ingestion of excessively diluted formula are common causes of acute hyponatremia in infants and children. Hypovolemic hypotonic hyponatremia is also a com- mon complication of adrenal insufficiency, in which a lack of aldosterone increases renal losses of sodium and a cortisol deficiency leads to increased release of ADH with water retention. Euvolemic or normovolemic hypotonic hypona- tremia represents retention of water with dilution of sodium while maintaining the effective circulatory vol- ume within a normal range. It is usually the result of SIADH. The risk of normovolemic hyponatremia is increased during the postoperative period. During this time ADH levels are often high, producing an increase in water reabsorption by the kidney. The hyponatremia becomes exaggerated when electrolyte-free fluids (e.g., 5% glucose in water) are used for intravenous fluid replacement. Hypervolemic hypotonic hyponatremia occurs in edematous states such as decompensated heart failure, advanced liver disease, and renal disease. Although the total body sodium is increased in heart failure, the baroreceptors often sense the effective circulatory vol- ume as inadequate, resulting in fluid retention. Abuse of methylenedioxymethylamine (MDMA), also know as “ecstasy,” can lead to severe neurologic symptoms, including seizures, brain edema, and herniation due to severe hyponatremia. MDMA and its metabolites have been shown to produce enhanced release of ADH from the hypothalamus. 3,22 Manifestations. The manifestations of hyponatremia depend on the rapidity of onset and the severity of the sodium dilution. The signs and symptoms may be acute (refers to onset within 48 hours), as in severe water intoxi- cation, or more insidious in onset and less severe, as in chronic hyponatremia. Because of water movement, hypo- natremia produces an increase in intracellular water, which is responsible for many of the clinical manifestations of the disorder Muscle cramps, weakness, and fatigue reflect the effects of hyponatremia on skeletal muscle function and are often early signs of hyponatremia. These effects commonly are observed in persons with hyponatremia that occurs during heavy exercise in hot weather. Gastrointestinal manifestations such as nausea and vomiting, abdominal cramps, and diarrhea may also occur (Table 8-4). The cells of the brain and nervous system are the most seriously affected by increases in intracellular water. 22–25