Failure classified according to urine flow rates is known as oliguric, nonoliguric, or polyuric failure. Oliguria is defined as a urinary flow rate less than 0.5 mL/kg/hr in a patient subjected to acute stress. This rate is higher than that seen in unstressed patients because acutely stressed patients cannot maximally concentrate urine. Polyuric failure is associated with elevations of blood urea, nitrogen, and serum creatinine levels and is characterized by urine flow rates that exceed 2.5 L/day.
Conditions that lead to prerenal oliguria include acute reductions in glomerular filtration rate (GFR), excessive reabsorption of salt or water, or both. Increases in circulating levels of catecholamines, antidiuretic hormone (ADH), or aldosterone are physiologic factors that can decrease urinary output. Hypotension may or may not be present in the initiation of acute renal failure. If not reversed, prerenal oliguria may progress to parenchymal damage and tubular necrosis.
Acute tubular necrosis may be produced by a variety of factors that interfere with glomerular filtration or tubular reabsorption. The pathogenesis of acute tubular necrosis may be divided into an initiation period, a maintenance period, and a recovery period. Renal hypoperfusion or a nephrotoxic insult may initiate renal failure. Surgical patients with external and internal fluid losses or sepsis may have renal hypoperfusion. The initiating insult culminates in the development of one or more maintenance factors, such as decreased tubular function, tubular obstruction, and sustained reductions in renal blood flow and glomerular filtration. Urine flow and solute excretion are reduced. When the maintenance period has begun, pharmacologic interventions to improve renal blood flow do not reverse the failure.
Prerenal oliguria is associated with physiologic mechanisms that conserve salt and water. In this case, urine has low sodium levels and high osmolality. Urine sodium levels are high, and osmolality is low. Renal damage is also associated with a progressive rise in serum urea, creatinine, uric acid, and polypeptide levels. Serum potassium levels may increase by 0.3 to 3 mEq/L/day, and a decrease occurs in the serum levels of sodium, calcium, and proteins such as albumin. The creatinine clearance remains the single most helpful test in defining renal status and predicting the prognosis in cases of severe renal dysfunction.
A number of conditions may place patients at high risk for acute renal failure and are listed in the following section. Renal reserve decreases progressively with age. For each year after 50 years of age, creatinine clearance decreases by 1.5 mL and renal plasma flow by 8 mL. Older patients are less able to cope with fluid and electrolyte imbalance and are more prone to renal damage. Overall mortality rates associated with acute renal failure increase from 50% for those younger than age 40 years to 80% for those older than age 60 years.