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This chapter addresses the following Geriatric Fellowship Curriculum Milestone: #9

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LEARNING OBJECTIVES

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Learning Objectives

  • Understand the effects of normal aging on body composition, and how these changes may contribute to the development of fluid and electrolyte imbalance.

  • Describe the clinical features—including symptoms, signs, and results of diagnostic tests—that accompany common fluid and electrolyte disorders.

  • Describe unique clinical situations that place older individuals at risk for fluid and electrolyte disturbances.

  • Understand current treatment of common fluid and electrolyte abnormalities tailored to the geriatric population.

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Key Clinical Points

  1. Aging is associated with extensive changes in body composition that alter water and electrolyte distribution and predispose older adults to the development of fluid and electrolyte abnormalities.

  2. The aging kidney and chronic kidney disease are not synonymous.

  3. The causes of fluid and electrolyte disturbances in older adults may differ from those in younger individuals.

  4. Medications and comorbidities contribute substantially to the greater frequency of fluid and electrolyte abnormalities in older adults.

  5. Fluid and electrolyte management follows the same principles in older and younger patients, but in older patients must be tailored to account for diminished kidney function, a smaller volume of distribution of water and electrolytes, and altered hormonal function.

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INTRODUCTION

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The kidney is the primary organ responsible for salt, water, and mineral homeostasis through selective reabsorption and secretion of these substances. Under physiologic conditions, normal kidney function makes it possible for a person to ingest a wide variety and quantity of nutrients and fluids yet remain in water and electrolyte balance. This remarkable capacity of the kidneys to maintain fluid and electrolyte homeostasis in the face of widely divergent intake is generally taken for granted, until kidney failure develops or some other process prevents the kidney from functioning optimally. Abnormalities of fluid and electrolyte balance are generally recognized through physical examination and routine laboratory work. This chapter will review changes in kidney function or in the regulators of kidney function that occur with aging that have an impact on fluid and electrolyte handling followed by a more in-depth discussion of specific disorders of fluid and electrolyte balance in older adults.

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Effect of Aging on Body Composition

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An examination of the changes that occur in fluid and electrolyte balance in older adults has to take into account how the composition of the human body changes with aging (Table 89-1 and Chapter 34). The percentage of body weight occupied by “water” progressively decreases. Whereas the water weight of a young person with a relatively normal body habitus is approximately 60% of the total body weight, this percent declines to 50% in older adults as muscle mass decreases and fat mass increases. The implications of this dramatic change are tremendous. Loss or gain of mathematically the same amount of body water will have a much greater effect on blood pressure, cardiovascular status, and osmolality in the older ...

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