Major Concepts Defined
Definitions
Azotemia is an increase in nitrogen containing waste
products such as urea and creatinine in the blood.
Uremia is a syndrome characterized by a constellation of
clinical signs caused by the retention of wastes normally excreted by the kidneys.
An azotemic patient is not always uremic but a uremic
patient is always azotemic.
Renal disease implies a pathologic or functional lesion of
any size, distribution, cause or degree of functional impairment in one or both kidneys.
Renal disease may be subclinical due to the large renal reserve.
Renal failure is present when kidney function deteriorates
to a point at which the integrity of the internal environment of the animal can no longer
be maintained. So like the association between azotemia and uremia, animals with renal
failure have concomitant renal disease but the opposite is not true. The abnormalities
associated with renal failure include fluid, electrolyte and acid base imbalances,
excretory deficits and decreased renal hormone production. Because the kidney has so many
functions the clinical signs of failure are diverse and
polysystemic.
Azotemia can be caused by prerenal, postrenal and intrinsic
renal causes. Therefore the presence of azotemia does not always indicate the presence of
kidney disease. Pre renal and post renal disorders can both lead to intrinsic renal
disease if the situations are not corrected.
In order for the kidneys to
eliminate wastes such as creatinine from circulation the following conditions must be met:
(failure to meet any of this conditions can result in waste retention)
- the heart must generate sufficient cardiac output to provide
the kidneys with adequate renal blood flow
- there must be
sufficient blood volume and blood pressure to
generate at least a mean blood pressure of 70 mmHg in order to maintain adequate renal
blood flow
- there must be sufficient numbers of functional nephrons to
filter wastes (normal GFR)
- urine must be able to be eliminated from the bladder (no
obstruction to urine outflow)
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Prerenal azotemia is
due to a decrease in renal blood flow which leads to a reduction in glomerular filtration
rate (GFR). As GFR decreases, there is a smaller amount of waste products presented to the
to the kidneys for elimination, resulting in elevation of waste products in the blood. Pre
renal factors which can lead to decreased renal blood flow and GFR include:
Laboratory abnormalities in patients with pre renal
azotemia usually include:
- increased BUN and creatinine
- increased PCV and albumin reflecting dehydration
- blood sodium may be increased or normal depending upon the
type of dehydration which exists (e.g. isotonic, hypotonic, hypertonic)
- urine specific gravity is high (usually well in excess of
1.035) consistent with the elimination of a small volume of urine which is the appropriate
kidney response to dehydration (to conserve water)
- urine creatinine concentration is high, consistent with the
kidneys excreting as much creatinine as possible in a smaller volume of water
- urine sodium concentration is low (<10mEq/l). In order
for the kidneys to conserve water they must also conserve sodium.
Treatment of pre renal azotemia includes fluid therapy and
correction of the underlying cause. Prognosis is good if the underlying cause is treatable
If the prerenal insult persists the patient may develop intrinsic renal disease.
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Postrenal azotemia
results from an obstruction of the urinary outflow tract or discontinuity of the outflow
tract with extravasation of urine into body compartments. Obstruction can be by calculi,
neoplasms, blood clots or foreign bodies, strictures, herniation of bladder into a
perineal hernia, neurologic abnormalities, or feline urologic syndrome
(FUS) in cats. Prostatic hypertrophy
usually does not cause obstruction in the dog. Prostatic abscess or tumor can cause obstruction.
Signs may include anuria (no urine) if the obstruction is total or
oliguria (small volume) if partial. If urine can be obtained, the laboratory findings are
the same as those of prerenal azotemia.
Diagnostic tests should include a physical exam,
radiographs (survey and contrast), and urethral catheterization. Potassium is commonly
elevated as potassium is a small molecule and is reabsorbed from the urine back into
blood.
Treatment is to treat the underlying cause. The prognosis
is good if the underlying cause can be treated
Postrenal azotemic states can progress to intrinsic renal
disease due to increased pressure in the renal tubules or due to hypotension and prerenal
influences caused by dehydration.
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Intrinsic renal disease or failure can
be divided into Acute renal failure (ARF) and
Chronic
renal failure (CRF).It is important to differentiate acute from chronic renal failure to determine
prognosis. ARF has a poor short term prognosis but good long prognosis. CRF has a good
short term prognosis but poor long term prognosis. Some differentiating features include:
|
Acute renal failure |
Chronic renal failure |
| History |
- recent drug administration, toxin exposure,
- surgery/hypovolemia
|
polyuria, polydipsia |
| Urine output |
oliguria |
polyuria |
| Kidney size |
normal to large |
small |
| Anemia |
absent |
present |
| Metabolic bone disease |
absent |
present |
| Urine specific gravity |
1.007-1.017 (isosthenuric) |
1.007-1.017 (isosthenuric) |
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Summary- Classification of
azotemia
|
Prerenal |
Postrenal |
Renal |
|
Urine specific gravity |
>1.025 dog
>1.030 cat |
- >1.025 dog
- >1.030 cat
|
1.007-1.017 |
|
Urine sodium |
<20 mEq/L |
<20 mEq/L |
>40 mEq/L (variable) |
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College of Veterinary Medicine,
Washington State University, Pullman, WA,
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Copyright Washington State University
Revised July 26, 2007
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