Terri G. Monk, M.D.
Professor of Anesthesiology,
University of Florida
College of Medicine
Gainesville, FL 32610
monk@anest2.anest.ufl.edu

Aging results in both structural and functional changes in the kidney that effect drug metabolism and kinetics as well as predisposing the patient to fluid and electrolyte abnormalities.

Between the ages of 40 and 80, the kidney loses approximately 20 percent of its mass, primarily from the cortex. Microscopically there is a reduction in the number of functional glomeruli, but the size and capacity of the remaining nephrons increase to partially compensate for this loss. Vascular changes also occur in the aging kidney, and after the age of 30 years renal blood flow (RBF) declines progressively at a rate of 10 percent per decade. Most of the decline in RBF occurs in the cortex with a relative increase in blood flow to the juxtamedullary region. The glomerular filtration rate (GFR) decreases by approximately 1 ml/min/year beginning by age 40. However, this decline in GFR is accompanied by a gradual loss of muscle mass and is rarely associated with an increase in serum creatinine. Thus, serum creatinine is a poor indicator of GFR in the elderly patient. Dosing intervals for drugs that are excreted by the kidney, such as aminoglycoside antibiotics, digoxin and pancuronium need to be adjusted and drug levels closely monitored.

Under normal circumstances, age has no effect on electrolyte concentrations or the ability of the individual to maintain normal extracellular fluid volume. However, the adaptive mechanisms responsible for regulating fluid balance are impaired in the elderly and the aging kidney has a decreased ability to dilute and concentrate urine. This problem is compounded by the fact that older individuals have a decreased thirst perception and fail to increase water intake when dehydrated. Age also interferes with the kidneyâs ability to conserve sodium. The geriatric patient excretes a sodium load more slowly and has a decreased ability to conserve sodium if dietary sodium is restricted, possibly predisposing the elderly patient to hemodynamic instability. Thus, fluid and electrolyte status should be carefully monitored in the elderly patient.

Bibliography:

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A review of the clinical significance of aging on the kidney accompanied by case reports