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INTRODUCTION

ESSENTIALS OF DIAGNOSIS

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

  • Hypernatremia is commonly defined as a serum sodium >145 mEq/L (145 mmol/L).

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

  • Hyperkalemia is typically defined as a serum potassium concentration >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

Fluid and electrolyte abnormalities are common among older adults as a consequence of age-related functional changes in the kidney in addition to 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

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 due to 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 take multiple medications that are associated with sodium disorders, such as diuretics and psychotropic medications (Table 48–1). Reviewing all medications is an integral part of evaluating patients with sodium disorders.

Table 48–1.Medications associated with hyponatremia.

Hyponatremia is commonly defined as a serum sodium concentration <135 mEq/L (135 mmol/L). In the community setting, hyponatremia occurs in 7% to 11% of older patients, and it occurs in up to 50% of older hospitalized patients.

Pathogenesis

A. Hypervolemic Hyponatremia

In older adults with impaired cardiac, renal, or hepatic function, a common etiology of hyponatremia is excessive water retention. This type of hyponatremia is commonly described as dilutional or hypervolemic hyponatremia. These patients typically exhibit edematous states, resulting from conditions such as congestive heart failure, cirrhosis, or 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 of excess hypotonic intravenous (IV) fluids, especially ...

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