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The diagnosis of fluid and electrolyte disorders is complicated by the often-nonspecific symptoms and physical examination findings, which patients suffering from these disorders manifest. A thorough understanding of the homeostatic mechanisms that control fluid and electrolyte balance is essential to the accurate assessment and treatment of the wide variety of pathologies the emergency physician may encounter.

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Approximately 60% of an adult male's body weight is composed of water (less in women and the elderly), of which 2/3 is intracellular and 1/3 is extracellular. Of the extracellular volume, 3/4 is in the interstitial compartment and 1/4 within the vasculature. Osmolality is maintained within a narrow range (285–295 mOsm) and is the same across all fluid compartments; it may be estimated using serum sodium, glucose, and blood urea nitrogen (BUN) values (see Appendix).

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Shifts in volume, osmolality, and electrolyte concentrations can occur independently of each other and typically manifest in profoundly different ways. The severity of symptoms is usually contingent on how rapidly the shifts occur. Changes in volume are most readily apparent on the cardiovascular physical examination. Signs and symptoms attributable to changes in osmolality are primarily neurological, resulting from brain dehydration (in hyperosmolar states) or brain edema (in hypoosmolar states). Electrolyte changes are more variable and exert their effect primarily on cell membrane potential with resultant cardiac, neurologic, and musculoskeletal dysfunction. See Table 44–1 for a list of serum electrolytes and normal values.

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Table 44–1. Normal Serum Electrolyte Concentrations.
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Disorders of Serum Sodium Concentration

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Hyponatremia

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Essentials of Diagnosis
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  • Hyponatremia reflects a relative inability to excrete free water
  • Symptoms relate to the rate of change
  • Measure urine osmolality and sodium
  • Review medications and assess thyroid and adrenal function
  • Initial treatment is guided by the severity of symptoms and the patient's volume status

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General Considerations
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Hyponatremia is defined as a serum sodium below 136 mEq/L and is associated with a number of different drugs and disease processes. Hyponatremia may be hyper-, hypo-, or isotonic. Isotonic hyponatremia is the result of laboratory artifact due to a decreased water component of plasma such as may be seen in hyperlipidemia and hyperproteinemia. Newer ion-specific methods of laboratory analysis have essentially eliminated this as a cause of hyponatremia. Hypertonic hyponatremia results from the presence of solutes that do not freely cross cell membranes, such as mannitol and glucose (in the absence of insulin), and represents a hyperosmolar ...

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