Azotemia
Encyclopedia
Azotemia is a medical condition characterized by abnormally high levels of nitrogen
-containing compounds, such as urea
, creatinine
, various body waste compounds, and other nitrogen-rich compounds in the blood
. It is largely related to insufficient filtering of blood
by the kidneys.
(BUN) and serum
creatinine
concentrations. The BUN-to-creatinine ratio
(BUN:Cr) is a useful measure in determining the type of azotemia. A normal BUN:Cr is less than 15.
, and narrowing of the renal artery among other things.
The BUN:Cr in prerenal azotemia is greater than 20. The reason for this lies in the mechanism of filtration of BUN and creatinine. GFR levels are decreased due to hypoperfusion, leading to a general greater increase in BUN than creatinine. Since the kidney is functioning correctly, the response to decreased GFR is to increase reabsorption rates. The increased reabsorption of Na leads to increased water and urea reabsorption from the proximal tubule
s of the kidney back into the blood. In contrast, creatinine is actually secreted in the proximal tubule. This generally leads to a BUN:Cr ratio > 20 and a fractional excretion of Na of < 1% and an elevated urine osmolarity.
. It is an intrinsic disease of the kidney, generally the result of renal parenchyma
l damage. Causes include renal failure
, glomerulonephritis
, acute tubular necrosis
, or any other kind of renal disease.
The BUN:Cr in renal azotemia is less than 15. In cases of renal disease, glomerular filtration rate decreases. So nothing gets filtered as well as it normally would. However, in addition to not being normally filtered, what urea does get filtered is not reabsorbed by the proximal tubule as it normally would be. This results in lower levels of urea in the blood and higher levels of urea in the urine. Creatinine filtration decreases, leading to higher amount of creatinine in the blood. Third spacing of fluids such as peritonitis, osmotic diuresis, or low aldosterone states such as Addisons Disease.
in an area below the kidneys results in postrenal azotemia. It can be caused by congenital abnormalities such as vesicoureteral reflux
, blockage of the ureters by kidney stone
s, pregnancy
, compression of the ureters by cancer
, prostatic hyperplasia, or blockage of the urethra
by kidney or bladder stones. Like in prerenal azotemia, there is no inherent renal disease. The increased resistance to urine flow can cause back up into the kidneys, leading to hydronephrosis
.
The BUN:Cr in postrenal azotemia is >15. The increased nephron
tubular pressure causes increased reabsorption of urea, elevating it abnormally relative to creatinine.
A urinalysis
will typically show a decreased urine sodium level, a high urine creatinine
-to-serum creatinine ratio, a high urine urea
-to-serum urea ratio, and concentrated urine (determined by osmolality and specific gravity). None of these is particularly useful in diagnosis.
In pre-renal and post-renal azotemias, elevation of the BUN
exceeds that of the creatinine (i.e., BUN>12*creatinine). This is because BUN
is readily absorbed while creatinine is not. In congestive heart failure
(a cause of pre-renal azotemia) or any other condition that causes poor perfusion of kidneys, the sluggish flow of glomerular filtrate results in excessive absorption of BUN and elevation of its value in blood. Creatinine
, however, is not absorbable and therefore does not rise significantly. Stasis of urine in post-renal azotemia has the same effect.
Prompt treatment of some causes of azotemia can result in restoration of kidney function; delayed treatment may result in permanent loss of renal function. Treatment may include hemodialysis
or peritoneal dialysis
, medications to increase cardiac output and increase blood pressure, and the treatment of the condition that caused the azotemia.
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
-containing compounds, such as urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
, creatinine
Creatinine
Creatinine is a break-down product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body...
, various body waste compounds, and other nitrogen-rich compounds in the blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
. It is largely related to insufficient filtering of blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
by the kidneys.
Types
Azotemia has three classifications, depending on its causative origin, but all three types share a few common features. All forms of azotemia are characterized by a decrease in the glomerular filtration rate (GFR) of the kidneys and increases in blood urea nitrogenBlood urea nitrogen
The blood urea nitrogen test is a measure of the amount of nitrogen in the blood in the form of urea, and a measurement of renal function. Urea is a by- product from metabolism of proteins by the liver and is removed from the blood by the kidneys.-Physiology:The liver produces urea in the urea...
(BUN) and serum
Blood serum
In blood, the serum is the component that is neither a blood cell nor a clotting factor; it is the blood plasma with the fibrinogens removed...
creatinine
Creatinine
Creatinine is a break-down product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body...
concentrations. The BUN-to-creatinine ratio
BUN-to-creatinine ratio
In medicine, the BUN-to-creatinine ratio is the ratio of two serum laboratory values, the blood urea nitrogen and serum creatinine . Outside the United States, particularly in Canada and Europe, the truncated term urea is used and the units are different...
(BUN:Cr) is a useful measure in determining the type of azotemia. A normal BUN:Cr is less than 15.
Prerenal azotemia
Prerenal azotemia is caused by a decrease in blood flow (hypoperfusion) to the kidneys. However, there is no inherent kidney disease. It can occur following hemorrhage, shock, volume depletion, congestive heart failureCongestive heart failure
Heart failure often called congestive heart failure is generally defined as the inability of the heart to supply sufficient blood flow to meet the needs of the body. Heart failure can cause a number of symptoms including shortness of breath, leg swelling, and exercise intolerance. The condition...
, and narrowing of the renal artery among other things.
The BUN:Cr in prerenal azotemia is greater than 20. The reason for this lies in the mechanism of filtration of BUN and creatinine. GFR levels are decreased due to hypoperfusion, leading to a general greater increase in BUN than creatinine. Since the kidney is functioning correctly, the response to decreased GFR is to increase reabsorption rates. The increased reabsorption of Na leads to increased water and urea reabsorption from the proximal tubule
Proximal tubule
The proximal tubule is the portion of the duct system of the nephron of the kidney which leads from Bowman's capsule to the loop of Henle.-Structure and appearance:...
s of the kidney back into the blood. In contrast, creatinine is actually secreted in the proximal tubule. This generally leads to a BUN:Cr ratio > 20 and a fractional excretion of Na of < 1% and an elevated urine osmolarity.
Primary Renal azotemia
Renal azotemia (acute renal failure) typically leads to uremiaUremia
Uremia or uraemia is a term used to loosely describe the illness accompanying kidney failure , in particular the nitrogenous waste products associated with the failure of this organ....
. It is an intrinsic disease of the kidney, generally the result of renal parenchyma
Parenchyma
Parenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...
l damage. Causes include renal failure
Renal failure
Renal failure or kidney failure describes a medical condition in which the kidneys fail to adequately filter toxins and waste products from the blood...
, glomerulonephritis
Glomerulonephritis
Glomerulonephritis, also known as glomerular nephritis, abbreviated GN, is a renal disease characterized by inflammation of the glomeruli, or small blood vessels in the kidneys...
, acute tubular necrosis
Acute tubular necrosis
Acute tubular necrosis or is a medical condition involving the death of tubular cells that form the tubule that transports urine to the ureters while reabsorbing 99% of the water . Tubular cells continually replace themselves and if the cause of ATN is removed then recovery is likely...
, or any other kind of renal disease.
The BUN:Cr in renal azotemia is less than 15. In cases of renal disease, glomerular filtration rate decreases. So nothing gets filtered as well as it normally would. However, in addition to not being normally filtered, what urea does get filtered is not reabsorbed by the proximal tubule as it normally would be. This results in lower levels of urea in the blood and higher levels of urea in the urine. Creatinine filtration decreases, leading to higher amount of creatinine in the blood. Third spacing of fluids such as peritonitis, osmotic diuresis, or low aldosterone states such as Addisons Disease.
Postrenal azotemia
Blockage of urine flowHydronephrosis
Hydronephrosis is distension and dilation of the renal pelvis calyces, usually caused by obstruction of the free flow of urine from the kidney, leading to progressive atrophy of the kidney...
in an area below the kidneys results in postrenal azotemia. It can be caused by congenital abnormalities such as vesicoureteral reflux
Vesicoureteral reflux
Vesicoureteral reflux is an abnormal movement of urine from the bladder into ureters or kidneys. Urine normally travels from the kidneys via the ureters to the bladder...
, blockage of the ureters by kidney stone
Kidney stone
A kidney stone, also known as a renal calculus is a solid concretion or crystal aggregation formed in the kidneys from dietary minerals in the urine...
s, pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
, compression of the ureters by cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
, prostatic hyperplasia, or blockage of the urethra
Urethra
In anatomy, the urethra is a tube that connects the urinary bladder to the genitals for the removal of fluids out of the body. In males, the urethra travels through the penis, and carries semen as well as urine...
by kidney or bladder stones. Like in prerenal azotemia, there is no inherent renal disease. The increased resistance to urine flow can cause back up into the kidneys, leading to hydronephrosis
Hydronephrosis
Hydronephrosis is distension and dilation of the renal pelvis calyces, usually caused by obstruction of the free flow of urine from the kidney, leading to progressive atrophy of the kidney...
.
The BUN:Cr in postrenal azotemia is >15. The increased nephron
Nephron
The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....
tubular pressure causes increased reabsorption of urea, elevating it abnormally relative to creatinine.
Signs and symptoms (prerenal azotemia)
- Decreased or absent urine output (oliguria or anuria).
- Fatigue
- AsterixisAsterixisAsterixis is a tremor of the wrist when the wrist is extended , sometimes said to resemble a bird flapping its wings. This motor disorder is characterized by jerking movements and is associated with various encephalopathies due especially to faulty metabolism...
- Decreased alertness
- Confusion
- Pale skin color
- TachycardiaTachycardiaTachycardia comes from the Greek words tachys and kardia . Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heart rate...
(rapid pulse) - Dry mouth (xerostomiaXerostomiaXerostomia is the medical term for the subjective complaint of dry mouth due to a lack of saliva. Xerostomia is sometimes colloquially called pasties, cottonmouth, drooth, or doughmouth. Several diseases, treatments, and medications can cause xerostomia. It can also be exacerbated by smoking or...
) - Thirst, swelling (edemaEdemaEdema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...
, anasarcaAnasarcaAnasarca, also known as "extreme generalized edema" is a medical condition characterised by widespread swelling of the skin due to effusion of fluid into the extracellular space....
) - Orthostatic blood pressure (rises or falls, significantly depending on position)
- Uremic frost, a condition when ureaUreaUrea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
and urea derivatives are secreted through the skin in sweat, which evaporates away to leave solid uric compounds, resembling a frostFrostFrost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air as well as below the freezing point of water. Frost crystals' size differ depending on time and water vapour available. Frost is also usually...
.
A urinalysis
Urinalysis
A urinalysis , also known as Routine and Microscopy , is an array of tests performed on urine, and one of the most common methods of medical diagnosis...
will typically show a decreased urine sodium level, a high urine creatinine
Creatinine
Creatinine is a break-down product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body...
-to-serum creatinine ratio, a high urine urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
-to-serum urea ratio, and concentrated urine (determined by osmolality and specific gravity). None of these is particularly useful in diagnosis.
In pre-renal and post-renal azotemias, elevation of the BUN
Bun
A bun is a small, usually sweet, bread. Commonly they are hand-sized or smaller, domed in shape, with a flat bottom. A bun can also be a savory bread roll similar to a bap or barmcake....
exceeds that of the creatinine (i.e., BUN>12*creatinine). This is because BUN
Bun
A bun is a small, usually sweet, bread. Commonly they are hand-sized or smaller, domed in shape, with a flat bottom. A bun can also be a savory bread roll similar to a bap or barmcake....
is readily absorbed while creatinine is not. In congestive heart failure
Congestive heart failure
Heart failure often called congestive heart failure is generally defined as the inability of the heart to supply sufficient blood flow to meet the needs of the body. Heart failure can cause a number of symptoms including shortness of breath, leg swelling, and exercise intolerance. The condition...
(a cause of pre-renal azotemia) or any other condition that causes poor perfusion of kidneys, the sluggish flow of glomerular filtrate results in excessive absorption of BUN and elevation of its value in blood. Creatinine
Creatinine
Creatinine is a break-down product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body...
, however, is not absorbable and therefore does not rise significantly. Stasis of urine in post-renal azotemia has the same effect.
Prompt treatment of some causes of azotemia can result in restoration of kidney function; delayed treatment may result in permanent loss of renal function. Treatment may include hemodialysis
Hemodialysis
In medicine, hemodialysis is a method for removing waste products such as creatinine and urea, as well as free water from the blood when the kidneys are in renal failure. Hemodialysis is one of three renal replacement therapies .Hemodialysis can be an outpatient or inpatient therapy...
or peritoneal dialysis
Dialysis
In medicine, dialysis is a process for removing waste and excess water from the blood, and is primarily used to provide an artificial replacement for lost kidney function in people with renal failure...
, medications to increase cardiac output and increase blood pressure, and the treatment of the condition that caused the azotemia.