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Essentials of Diagnosis

  • Hyponatremia is commonly defined as a serum sodium concentration less than 135 mEq/L (or 135 mmol/L).

  • Hypernatremia is commonly defined as a serum sodium greater than 148 mEq/L (or 148 mmol/L).

  • Hypokalemia is typically defined as a serum potassium concentration of less than 3.5 mEq/L.

  • Hyperkalemia is typically defined as a serum potassium concentration greater than 5.0 mEq/L.

  • Nocturnal polyuria is present when urine production during 8 hours of sleep is >33% of 24-hour urine production, nighttime urine production rate is >0.9 mL/min or 7 PM to 7 AM urine volume is >50% of total 24-hour volume.

General Principles in Older Adults

Fluid and electrolyte abnormalities are common among older adults as a consequence of age-related functional changes in the kidney, multiple comorbidities, and polypharmacy. This chapter discusses concepts of sodium disorders, potassium disorders, and nocturnal polyuria as they relate to older adults.

Hyponatremia

General Principles in Older Adults

Older adults are more vulnerable to developing sodium disorders as a result of age-related changes in water and sodium metabolism. Older adults may have an impaired ability to excrete water and to dilute urine because of reductions in the number of functioning nephrons and decreased renal blood flow with age, predisposing them to water overload and possible hyponatremia. Geriatric patients also tend to be on multiple medications that are associated with sodium disorders, such as diuretics and psychotropic medications (Table 37–1). Reviewing all medications is an integral part of evaluating patients with sodium disorders.

Table 37–1.Medications associated with hyponatremia.

Hyponatremia is commonly defined as a serum sodium concentration less than 135 mEq/L (or 135 mmol/L). This occurs in 7% to 11% of older patients and up to 50% of older hospitalized patients.

Pathogenesis

Hypervolemic Hyponatremia

In older adults, the most common etiology of hyponatremia is increased intake and subsequent retention of water. This type of hyponatremia is commonly described as dilutional or hypervolemic hyponatremia. These patients typically will exhibit edematous states, resulting from conditions such as congestive heart failure, cirrhosis, or the nephrotic syndrome. These conditions decrease effective circulating blood volume, leading to increased antidiuretic hormone (ADH) secretion, which results in water retention. Dilutional hyponatremia can also be iatrogenic, as a result of administration ...

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