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KEY CONCEPTS

KEY CONCEPTS

  • image Blood volume and serum osmolality are essential for normal cellular function and are tightly regulated in the human body. Simply put, water balance determines serum sodium concentration, and sodium balance determines volume status.

  • image Total body water (TBW) ranges from 45% to 60% of body weight depending on sex and age and is distributed primarily into two compartments: the intracellular compartment or intracellular fluid (ICF; two-thirds [67%] of TBW) and the extracellular compartment or extracellular fluid (ECF; one-third [33%] of TBW).

  • image Arginine vasopressin (AVP), also known as antidiuretic hormone (ADH), is synthesized in the hypothalamus and secreted by the posterior pituitary in response to both osmotic and nonosmotic regulators.

  • image Hyponatremia, defined as a serum sodium concentration less than 135 mEq/L (mmol/L), is the most common electrolyte abnormality encountered in clinical practice in both adults and children affecting 3 to 6 million persons and 1 million hospitalized patients yearly.

  • image Hyponatremia is predominantly the result of an excess of extracellular water relative to sodium because of impaired water excretion.

  • image Hypovolemic hypotonic hyponatremia is common in patients taking thiazide diuretics.

  • image Euvolemic (isovolemic) hypotonic hyponatremia is associated with a normal or slightly decreased ECF sodium content and increased TBW and ECF volume. Euvolemic hyponatremia is most often caused by the syndrome of inappropriate ADH secretion (SIADH).

  • image Hyponatremia with ECF volume expansion (hypervolemia) occurs in conditions in which sodium and water excretion is impaired. Patients with HF, cirrhosis, or nephrotic syndrome have an expanded ECF volume and edema but a decreased effective arterial blood volume.

  • image The brain's adaptation to chronic serum hypoosmolality or hyperosmolality leads to neurologic symptoms when either hyponatremia (hypoosmolality) or hypernatremia (hyperosmolality) is corrected too rapidly.

  • image Hypernatremia, defined as a serum sodium concentration greater than 145 mEq/L (mmol/L), is always associated with hypertonicity and intracellular dehydration, resulting from a water deficit relative to ECF sodium content.

  • image Edema, defined as a clinically detectable increase in interstitial fluid volume, is usually due to heart, kidney, or liver failure, or a combination of these conditions, although it can develop with a rapid decrease in serum albumin concentration along with excess fluid intake such as seen in the setting of burns or trauma.

PATIENT CARE PROCESS

Patient Care Process for the Management of Disorders of Sodium and Water Homeostasis

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Collect

  • Patient characteristics (eg, age, sex, pregnant)

  • Patient history (past medical, diet, recent GI losses, see Tables 66-2 and 66-7)

  • Current medications (eg, diuretics, IV fluids, Na-containing treatments; see Tables 66-3 and 66-8)

  • Objective data

    • Body weight (current and historical) and recent intake/output

    • ECF volume status (eg, BP, mucous membranes, skin turgor, cardiopulmonary examination, and level of consciousness; see Figs. 66-1 and 66-3)

    • Labs (eg, serum Osm, electrolytes, glucose, protein, lipids; urine Osm, Na, and K; and thyroid function tests)

Assess

  • Chronicity and severity of the disorder

  • Potential causative underlying disorders (eg, diarrhea, CNS disorders, pulmonary disease, cirrhosis, nephrotic syndrome, polydipsia, diabetes insipidus; see Tables 66-2 and ...

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