Acute Renal Failure

The clinical conditions that give rise to rapid, steadily increasing azotemia, with or without oliguria (< 500 mL/day).

Acute renal failure (ARF) is more common in the elderly than in younger persons. The prognosis is nearly as favorable; therefore, treatment need not be denied because of age. ARF may be prerenal, renal, or postrenal.

Prerenal ARF results from poor perfusion of the kidneys. Most cases are due to loss of fluids (dehydration, volume depletion), internal redistribution (hypoproteinemia), decreased cardiac output, or certain drugs (diuretics, angiotensin-converting enzyme inhibitors, nonsteroidal anti-inflammatory drugs [NSAIDs]), often in combination. The most important contributor to the high rate of prerenal ARF in the elderly is the loss of thirst regulation compounded by a relative inability to retain salt and a loss of urinary concentrating ability.

Renal ARF may result from a number of renal disorders, including acute tubular necrosis (ATN), acute glomerulonephritis, and acute interstitial nephritis. ATN may be ischemic, nephrotoxic, or pigment-induced. Ischemic ATN is caused by disorders similar to those described for prerenal ARF. Surgical interventions (most notably cardiac and aortic surgery) and infections (sepsis), especially when associated with prolonged hypotension, are the most common causes of ischemic ATN. Nephrotoxic ATN is commonly caused by aminoglycoside antibiotics, although most antibiotics used to treat serious infections have been associated with this entity. Age is a risk factor for aminoglycoside nephrotoxicity; preexisting renal dysfunction and hypovolemia also may contribute. Pigment-induced ATN may be caused by hemoglobin or myoglobin being deposited in the urinary tubules, the latter resulting from rhabdomyolysis. Pigment-induced ATN occurs in all age groups.

Postrenal ARF results from urinary tract obstruction, which is one of the most common causes of ARF in the elderly. Early diagnosis is necessary because the condition is often reversible if the obstruction is relieved early.

Symptoms and Signs

Clinical features in patients with ARF vary depending on the cause, the severity of renal injury, and the speed with which ARF develops. Patients may present with an absent or decreased urine flow and evidence of fluid retention with an unexpected elevation in the blood urea nitrogen and serum creatinine levels. Patients may also present with the symptoms of the underlying cause or with the clinical or biochemical complications of uremia. Nonoliguric renal failure is increasingly being recognized, because use of biochemical monitoring of the severely ill patient and use of therapeutic agents that precipitate renal injury are increasing.

Diagnosis

Laboratory findings distinguishing between prerenal ARF and ATN are listed in Table 98-2. These findings indicate intact tubular function in a hypoperfused kidney. Another indicator of the prerenal state is the response to treatment with volume repletion (salt and water); elderly patients may have a delayed response to volume expansion.

When oliguria or anuria is present, urinary obstruction should be excluded rapidly, particularly in men with known prostatic hypertrophy or cancer and in women with gynecologic malignancies. The kidney in elderly persons can recover from acute ischemic and toxic insults, although not as rapidly as in younger persons.

Complications and Treatment

Volume overload precipitating acute pulmonary edema, hypertensive crisis, hyperkalemia, and infection are the major complications and causes of death occurring during ARF.

The principles of treatment are the same for elderly as for younger patients. All complications except infection can be successfully managed by supervision of fluid, electrolyte, and nutritional replacement and by early initiation and frequent use of dialysis. Urinary tract infection secondary to bladder catheterization is common. Little is gained from placing a urinary catheter in an oliguric patient; volume status and blood urea nitrogen, creatinine, and potassium levels are better guides to treatment than is urinary output. Infections from IV lines also are common; IV lines should be monitored scrupulously and discontinued as soon as possible.

Fluid and electrolyte replacement and nutritional support: A patient with ARF can lose up to 1 pound of body mass daily from tissue catabolism, even with adequate nutritional support. Because catabolized protein contributes to the serum urea nitrogen increase, calories should be given primarily as carbohydrate. Attempts to keep body weight constant result in gradual expansion of extracellular fluid volume and a consequent increase in blood pressure, with the risk of precipitating heart failure. Overzealous fluid restriction impairs the patient's general condition and central nervous system function and may delay the recovery of renal function. However, when an obstruction causing postrenal ARF is relieved, the patient may experience a remarkable diuresis that requires substantial fluid and electrolyte replacement.

Hyperkalemia becomes a problem primarily when oliguria or anuria is present, when there is excessive tissue catabolism (eg, rhabdomyolysis or inadequate nutritional support), or when there is an exogenous or endogenous source of potassium (eg, gastrointestinal [GI] bleeding, which is often precipitated or aggravated by azotemia, because potassium lysed from red cells is absorbed from the gut). Treatment of hyperkalemia is discussed elsewhere.

Acidosis progresses with the duration and degree of renal failure and can potentiate hyperkalemia by shifting potassium out of cells as hydrogen moves in. Infusion of sodium bicarbonate or dialysis can be used to maintain circulating bicarbonate levels > 18 mEq/L.

Other treatment measures: Oral phosphate binding agents can be used to prevent the serum phosphate elevation. The increase in serum phosphate results in precipitation of calcium phosphate, depression of serum calcium concentrations, and an increase in parathyroid hormone secretion (secondary hyperparathyroidism). The physician should increase the dosing interval of drugs excreted partially or totally by the kidney, while recognizing the enhanced sensitivity of elderly uremic patients to psychoactive drugs (eg, hypnotics, antipsychotics).

Dialysis: Hemodialysis, peritoneal dialysis, and ultrafiltration are effective in maintaining homeostasis; complications seem to result more from concurrent cardiovascular disease than from age. Dialysis is best initiated early in patients with ARF. Ultrafiltration therapy (removing excessive water by increasing hydrostatic pressure on the intravascular side of the dialysis membrane and/or decreasing hydrostatic pressure on the dialysate side) can be used to treat pulmonary edema and volume overload unresponsive to diuretics.