22-34: The Kidney & Aging

After the age of 40 years, GFR declines at an average rate of 0.8 mL/min/1.73 m²/yr (though some older patients show little or no change) and is accompanied by a decline in renal mass that spares medulla relative to cortex. In an attempt to maintain GFR, renal blood flow decreases, causing a resultant increase in arteriolar resistance and increased filtration fraction. Serum creatinine values may remain relatively constant if muscle mass decreases in parallel with the decrease in GFR; a stable creatinine in the face of significant weight loss suggests progression of kidney disease. GFR impairment is partially due to thickening of the GBM, leading to glomerulosclerosis.

Renal tubular changes include impaired sodium handling, decreased concentrating and diluting abilities, and impaired acidification. Thus, older patients are more prone to volume overload, dysnatremias, and acidosis. Decreased renin synthesis and 1alpha-hydroxylase activity are also observed. These abnormalities can result in hyperkalemia, hypocalcemia, and elevated PTH activity.

More adverse drug reactions occur in older patients. Three main pharmacokinetic changes occur: (1) altered volume of distribution, (2) altered drug half-life, and (3) altered elimination. The latter two are directly related to impaired renal clearance of drug. The SPRINT Trial showed that more intensive blood pressure control in nondiabetic patients 75 years of age and older with CKD decreases overall cardiovascular morbidity and mortality. However, patients in the intensive arm (less than 120 mm Hg) required 2.8 medications versus 1.8 medications in the standard control arm (systolic blood pressure less than 140 mm Hg).

Roughly half of patients starting dialysis are 65 years or older. Hemodialysis and peritoneal dialysis are both reasonable options for older adults with ESRD, though hemodialysis may be preferred among those with functional impairment who cannot independently manage their treatments or receive assistance from a responsible caretaker. Sudden fluid and electrolyte shifts can cause hypotension, ischemia, and arrhythmias. Renal transplantation is being offered to older individuals more often as it appears to offer survival benefit even those over 65 years. Over the age of 75, however, there is not a clear survival benefit. The most common transplant complications in this population are infection and CVD. A reduced corticosteroid requirement with the introduction of steroid-sparing agents, such as cyclosporine, has lowered infection rates.