Renal and Volume Regulation

Renal mass may decrease as much as 30% by the age of 80 (see Chapter 20 ).^[19] Loss of mass is most prominent in the renal cortex and correlates with a decreased number of functioning glomeruli. In addition, renal blood flow decreases about 10% per decade. A progressive fall in creatinine clearance occurs with age; yet with normal aging, serum creatinine remains relatively unchanged. This is due to the decrease in muscle mass with aging. Therefore, serum creatinine is a poor predictor of renal function in the elderly. This concept is important for proper dosage adjustment of medications that are excreted by the kidneys.

Functional changes in the kidneys with aging have been well described and include alterations in electrolyte handling and the ability to concentrate and dilute urine. Renal capacity to conserve sodium is decreased. Overall, an older patient has a tendency to lose sodium in the setting of inadequate salt intake. This tendency, paired with a decreased thirst response, may place an elderly patient at risk for dehydration and sodium depletion. An elderly patient also has a diminished ability to handle a salt load, as evidenced by increased sodium retention and expansion of the extracellular volume during the perioperative period. The ability of the kidney to concentrate and dilute urine is impaired with aging. This change assumes importance under conditions of limited fluid intake.