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Acute Renal Failure

Definition

Renal failure depicts a group of diseases that may be associated with decreased GFR and manifested by retention of BUN and creatinine. Acute renal failure is defined as a rapidly (over a period of days) increasing creatinine level or decreasing urine output.

Causes

Causes of acute renal failure is caused by failure of the kidneys to perform their normal functions due to:

• Prerenal - Problems affecting the flow of blood before it reaches the kidneys
• Postrenal - Problems affecting the movement of urine out of the kidneys
• Renal - Problems with the kidney itself that prevent proper filtration of blood or production of urine

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Prerenal failure

This is by far the most common type of acute renal failure. Your kidneys do not receive enough blood to filter. Prerenal failure can be caused by the following conditions:

• Dehydration - From vomiting, diarrhea, water pills, or blood loss
• Disruption of blood flow to the kidneys - From a variety of causes
  • Drastic drop in blood pressure - From major surgery, severe injury or burns, or infection in the bloodstream (sepsis)
  • Blockage or narrowing of a blood vessel leading to the kidneys
  • Heart failure or heart attacks
  • Liver failure

There is no actual damage to the kidneys with prerenal failure. With appropriate treatment, it usually can be reversed. 

Postrenal failure

Postrenal renal failure is sometimes referred to as obstructive renal failure, since it is often caused by something blocking elimination of urine produced by the kidneys. This problem also can be reversed.

At the ureter level, this condition can be caused by the following:

• Kidney stone
• Cancer
• Medications

At the bladder level, the following conditions can cause obstruction:

• Kidney stone
• Enlarged prostate (the most common cause)
• Blood clot
• Bladder cancer
• Neurologic disorders of the bladder

Treatment consists of relieving the obstruction. Once the blockage is removed, the kidneys usually recover in 1-2 weeks if there is no infection or other problem.

Renal damage

Primary renal damage is the most complicated cause of renal failure. Renal causes of acute kidney failure can be subdivided into those affecting the filtering system of the kidney, those affecting the blood supply in the kidney, and those affecting kidney tissue.

Some of the kidney problems that can cause kidney failure include the following:

• Blood vessel diseases
• Blood clot in a vessel in the kidneys 
• Injury to kidney tissue and cells
• Glomerulonephritis
• Acute interstitial nephritis
• Acute tubular necrosis

Clinical manifestations

ARF passes through two phases 1. Destructive phase: oliguric and non-oliguric 2. Reconstructive phase: could be polyuric.

ARF due to pre-renal or renal causes (esp. ATN) usually presents as oliguric renal failure passing through the 3 phases of pathology:

1. Intitiation phase: due to the initiating insult on the kidney.

2. Maintenance phase: maintenance of oliguria (1-2 weeks).

3. Recovery phase: due to recovery of renal function (polyuria).

ARF due to other causes as partial obstruction may present as polyuria.

ARF due to postrenal obstruction may present as anuria.

Diagnosis

Renal failure is manifested by the rise of serum creatinine. The next step is to identify if chronic or acute renal failure. Then exclude if postrenal then identify if pre-renal or renal. If renal identify what type of intrisic renal disease – if no diagnosis is reached then by exclusion the diagnosis is ATN.

1. Prerenal azotemia

• Urine/plasma creatinine ratio > 40

• Urinary Na decreased < 20

• Fractional excretion of Na is <1%

• Urine osmolarity is > 500

2. Renal azotemia

• Urine/plasma creatinine ratio < 20

• Urinary Na decreased > 40

• Fractional excretion of Na is >1%

• Urine osmolarity is > 350

·        These findings can be altered by diuretic use and urine sample should be taken before institution of diuretic use.

3. Postrenal azotemia

Post-renal azotemia develops only if the obstruction is bilateral or affects a solitary functioning kidney. During the early stages of obstruction, continued glomerular filtration leads to increased intra-luminal pressure proximal to the site of obstruction. As a result, there is gradual distension of the proximal ureters, renal pelvis and calyces and fall in glomerular filtration which will be evident by renal ultrasound.

N.B.: the level of creatinine in serum has nothing to do with whether the ARF is oliguric, anuric or non-oliguric: it is an indication that tubular function is lost as creatinine is not filtered- it is secreted. K+ and H+ are retained even in nonoliguric renal failure due to the same principal (they are both secreted and not filtered).

Treatment  

Treatment of acute renal failure usually should be conservative and largely supportive. It requires careful and precise management. All patients will require close monitoring, many of them within intensive care settings.

Supportive care includes stabilizing the patient, monitoring input and output strictly, weighing daily, determining electrolyte values frequently, preventing sepsis via reducing the number of intravenous lines and removing an indwelling urinary.

Therapy for prerenal failure

Rapid volume replacement and treatment of the underlying condition that resulted in prerenal failure are the cornerstones of therapy. Initial fluid administration of isotonic saline (0.9%) or 5% albumin (10 to 20 mL/kg per dose) should be used to restore intravascular volume. This can be both a diagnostic and a therapeutic trial. Fluid administration also can convert oliguric to nonoliguric renal failure in its early stage.

Therapy for postrenal failure

Therapy for postrenal failure includes removal of obstruction by decompression or diversion of the urinary tract, stabilization of electrolyte abnormalities, management of postobstructive diuresis, and therapy for voiding dysfunction and for urinary tract infection. Surgical intervention will require urologic consultation. The site of the obstruction will determine the approach: placement of a Foley catheter, vesicostomy, ureteral catheters (stents), or nephrostomy tubes.

Therapy for established renal failure

Maintaining Balance of Fluid and Electrolytes

In a euvolemic state, fluid intake, including water generated from endogenous metabolism (insensible fluid gain), is balanced by fluid output.

Treating Hypertension

Kidney failure in any form can present as hypertension and hypertensive encephalopathy. It is essential to lower the blood pressure quickly and safely. The blood pressure should be reduced by at least 25% within 1 hour with an antihypertensive medicine whose onset of action is rapid. It is advisable to start with one antihypertensive medicine and increase the dose to its maximum recommended level. Therapy is individualized and needs titration. In most cases, hypertension is the result of sodium and fluid retention, but other factors, such as activation of the renin-aldosterone-angiotensin II and/or the alpha-adrenergic system, may have roles in kidney failure.

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