Chapter 68: Disorders of Sodium and Water Homeostasis

KEY CONCEPTS

• Maintenance of normal blood volume and serum osmolality is essential for cellular function and is tightly regulated in the human body. Simply put, water balance determines serum sodium concentration, and sodium balance determines volume status.

• Total body water (TBW) ranges from 45% to 80% of body weight depending on sex, age, gestational age, and disease states 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).

• 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 (serum sodium greater than 135 mEq/L [mmol/L]) and non-osmotic regulators to maintain water balance.

• 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 and is associated with significant morbidity and mortality.

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

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

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

• Hyponatremia with ECF volume expansion (hypervolemia) occurs in conditions in which sodium and water excretion is impaired such as heart failure, cirrhosis, or nephrotic syndrome.

• 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.

• 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.

• 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* for the Management of Disorders of Sodium and Water Homeostasis

Collect

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

• Patient history (eg, medical, surgical, diet, recent gastrointestinal [GI] losses; see Tables 68-2 and 68-7)

• Current medications (eg, diuretics, intravenous [IV] fluids, sodium-containing therapies; see Tables 68-3 and 68-8)

• Objective data

  • Body weight (current and historical)

  • Recent intake/output

  • ECF volume status (eg, blood pressure [BP], mucous membranes, skin turgor, cardiopulmonary examination, level of consciousness; see Figs. 68-1 and 68-3)

  • Labs (eg, serum osmolality, electrolytes, glucose, protein, lipids; urine osmolality, ...