Prerenal azotemia

Definition

Prerenal azotemia is an abnormally high level of nitrogen-type wastes in the bloodstream. It is caused by conditions that reduce blood flow to the kidneys.

Alternative Names

Azotemia - prerenal; Uremia; Renal underperfusion

Causes

Prerenal azotemia is relatively common, especially in hospitalized patients.

The kidneys normally filter the blood. When the volume or pressure of blood flow through the kidney drops, blood filtration also drops drastically, and may not occur at all. Waste products remain in the bloodstream and little or no urine is formed, even though the internal structures of the kidney are intact and functional.

Lab tests show that nitrogen-type wastes, such as creatinine and urea, are accumulating in the body (azotemia). These waste products act as poisons when they accumulate, damaging tissues and reducing the ability of organs to function. The build-up of nitrogen waste products and accumulation of excess fluid in the body are responsible for most of the symptoms of prerenal azotemia and acute renal failure.

Prerenal azotemia is the most common form of kidney failure seen in hospitalized patients. Any condition that reduces blood flow to the kidney may cause it -- including loss of blood volume, which may occur with dehydration, prolonged vomiting or diarrhea, bleeding, burns, and other conditions that allow fluid to escape from circulation.

Conditions where the volume is not lost, but where the heart cannot pump enough blood, or the blood is pumped at low volume, also increase risk for prerenal azotemia. These conditions include shock (such as septic shock), heart failure, and conditions where the blood flow to the kidney is interrupted, such as trauma to the kidney, surgery of various types, renal artery embolism, and other types of renal artery occlusion.

Symptoms

• Decreased or no urine produced
• Fatigue
• Decreased alertness
• Confusion
• Pale skin color
• Rapid pulse
• Dry mouth
• Thirst
• Swelling

Additional symptoms may include:

• Excessive urination at night
• Abdominal pain

Exams and Tests

An examination may show signs of low heart function or signs of hypovolemia. Blood pressure may be low or may drop when the person stands up. The pulse pressure (difference between systolic blood pressure and diastolic blood pressure) may be reduced. The heart rate may be rapid.

Skin turgor may be poor, and mucous membranes may be dry. The neck veins may be collapsed. There may be little or no urine in the bladder even when drained by a catheter. If the condition is prolonged, other signs of acute kidney failure may be present.

A urinalysis may show decreased kidney function preserving the ability of the tubules. Nitrogen wastes and electrolytes continue to be excreted, but at abnormally low rates.

• Urine sodium may be low, with fractional excretion at less than 1%.
• Urine creatinine to serum creatinine ratio is high.
• Urine urea to serum urea (BUN) ratio is high.
• Fractional excretion of urea is low.
• Osmolality and specific gravity show concentrated urine.

Blood lab tests show a rapid accumulation of nitrogen wastes:

• Increased BUN
• Increased creatine
• Increased BUN/creatinine ratio

Treatment

The main goal of treatment is to rapidly correct the cause of the prerenal azotemia before damage occurs to the internal kidney structures. Hospitalization is often required, and may involve treatment in an intensive care unit. Treatment may include hemodialysis or peritoneal dialysis.

Intravenous fluids, including blood or blood products, may be used to increase blood volume. After blood volume has been restored, medications may be used to increase blood pressure and cardiac output. These may include dopamine, dobutamine, and similar cardiac medications. The cause of the decreased blood volume or blood pressure should be identified and treated as appropriate.

If other symptoms of acute renal failure are present, treatment for it should continue, including medications, dietary restrictions, or dialysis.

Outlook (Prognosis)

Prerenal azotemia is reversible if the cause can be identified and corrected within 24 hours. However, if the cause is not corrected quickly, damage may occur to the internal structures of the kidney (acute tubular necrosis).

Possible Complications

• Acute kidney failure
• Acute tubular necrosis (tissue death)

When to Contact a Medical Professional

Go to the emergency room or call the local emergency number (such as 911), if symptoms indicating prerenal azotemia may be present.

Prevention

Prompt treatment of any condition that reduces the volume or force of blood flow through the kidneys may help to prevent prerenal azotemia from developing.