Prerenal azotemia

Alternative names:

azotemia, prerenal; renal underperfusion; uremia

Definition:

An abnormally high level of nitrogen-type wastes in the bloodstream, caused by conditions that reduce blood flow to the kidney.

Causes, incidence, and risk factors:

Prerenal azotemia is the most common form of acute renal failure. It is a result of conditions that impair blood flow to the kidney. The kidneys normally filter the blood as it is forced under pressure through the glomeruli. When the volume or pressure of the blood flow through the kidney decreases, glomerular filtration is reduced drastically and may not occur at all. Little or no urine is formed, and waste products remain in the bloodstream even though the internal structures of the kidney are intact and functional. The glomeruli and tubules continue to filter wastes, but the rate of filtration is so slow that many of these wastes are reabsorbed into the blood rather than being excreted in the urine.

The levels of urea rise faster than the levels of creatinine with prerenal azotemia. This is because of the difference in size of these molecules. Urea is small, and so more readily returns to the bloodstream when the rate of filtration in the glomeruli is slow. Creatinine is larger, so more of it is excreted, although its excretion is lower than normal.

Lab tests show accumulation of nitrogen-type wastes, such as creatinine and urea in the body (azotemia). Various waste products act as poisons when they accumulate in the body, 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 occurs in approximately 4 out of 1,000 people. Any condition that reduces blood flow to the kidney may cause it. Risks for prerenal azotemia include loss of blood volume such as may occur with dehydration, prolonged vomiting or diarrhea, bleeding, and burns and other conditions that allow escape of fluid from the 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.