Acid-base disorders are commonly encountered in clinical practice and can have a substantial impact on a patient’s prognosis and outcome. The elderly are more prone to develop acid-base disturbances than the young. With age, the kidney undergoes structural and functional changes that limit the adaptive mechanisms responsible for maintaining acid-base homeostasis in response to dietary and environmental changes. In addition to a decrease in glomerular filtration rate (GFR), the capacity of the kidneys to handle electrolyte alterations and excrete an acid load is diminished with advancing age. Therefore, an improved recognition of these disorders and underlying causes will improve our ability to better manage elderly patients. In this review we have summarized age-related changes in acid-base homeostasis that may impact clinical outcomes.
INTRINSIC RENAL CHANGES WITH AGING
Several changes occur in glomerular number, function, and the GFR with advancing age. By the seventh decade, there is a 30-50% reduction in the number of glomeruli and a reduction in proximal renal tubule volume and glomerular surface area. There is also a progressive fall in total renal plasma flow and the cortical component of blood flow. Commensurate with the decline in the number of glomeruli is an average decline in GFR of about 1% per year or 10% per decade after age 40. Despite the fall in GFR, serum creatinine levels remain unchanged in most instances due to a proportional reduction in muscle mass and endogenous creatinine production. The common assumption of normal kidney function on the basis of normal serum creatinine level predisposes older patients to many iatrogenic complications, including but not limited to acute renal failure, drug toxicities, and a variety of acid-base and electrolyte abnormalities. canada drugs pharmacy
Table 1. Commonly Used Formula for Estimating Glomerular Filtration Rate
|A. Cockcroft and Goult
Estimated GFR (ml/min) = (140 – aae in years) x lean body mass in Ka *(x 0.85 if female)
72 x Serum Creatinine (mg/dl)
|B. Abbreviated Modification of Diet in Renal Disease (MDRD) Study Formula
Estimated GFR (ml/min) = 186.3 * (sCr)1154 * age”0203 * (0.742 if female) * (1.21 if African-American)
The Cockcroft and Gault formula has been used for many years to estimate GFR with adjustment for body weight and age. Using this formula, a serum creatinine of 1.2 mg/dl in a 20-year-old, 80-kg male reflects a GFR of 111 ml/min., while a serum creatiine of 1.2 mg/dl in a 70-year-old, 60-kg female reflects a GFR of 41 ml/min., a nearly three-fold difference. Thus, the importance of a more accurate assessment of GFR in an older patient cannot be overestimated. A new formula derived from the Modification of Diet in Renal Disease (MDRD) study has been reported to be even more accurate than Cockcroft-Gault formula in predicting GFR as measured by 1251-iothalamate clearance (see Table 1 for GFR estimation formulas; easy-to-use programs are available on websites and PDAs that need only serum creatinine level, and patient age, gender, and race).