Chronic Renal Failure

CRF is the end result of various disorders. The most common cause for CRF is glomerulonephritis. With CRF, the patient will have azotemia. Azotemia is defined by increased BUN/creatinine ratio (biochemical term). There are three different kinds (or causes) of azotemia.

1. Pre-renal (literally, before kidney): Results whenever there is not enough blood flow to the kidney. The most common cause is CHF. In case of pre-renal azotemia, BUN/creatinine ratio is always greater than 10-20/1.

2. Renal (as a result of disease of the kidney, either glomerular or tubulo-intersitual). In this case, BUN/creatinine ratio is less than 10-20/1.

3. Post-renal: due to obstruction to urinary outflow—again ratio greater than 20/1

Uremia (clinical term) is basically azotemia with clinical signs and symptoms. Remember that azotemia is a biochemical definition of increased BUN/creatinine ratio. There are quite a few manifestations of uremia

1. Impaired volume regulation. A patient can’t concentrate urine and may be dehydrated (losing a lot of free water). Please note that in CRF, a person will actually have increased urine output (kind of opposite of what you would’ve guessed had you not known better ) (whereas, in ARF a person will initially lose ability to make urine and hence decrease urine output; much more on this later). This is a very important distinction between CRF and ARF. Don’t overlook it!

2. Acid/base imbalance, resulting in metabolic acidosis. One can’t excrete hydrogen ions or reabsorb bicarb, resulting in decrease in plasma pH. Remember the infamous Handerson –Hasselbach equation, which can be simplified in the following way:

pH ~ HCO3/pCO2

So, in CRF, we have decreased HCO3- since we fail to reabsorb it, so we have decreased pH. To compensate, a patient will hyperventilate (to blow off CO2). This type of deep breathing is called Koussmaul respiration (remember, this is the type of respiration we would see in a diabetic with DKA).

3. GI: nausea, vomiting, bleeding

4. Carciovascular: HTN, may have CHF as a result of fluid overload and HTN, and also pericarditis (uremia should also be in your differential for pericarditis, remember the other ones?)

5. Hematopoietic: anemia, due to decreased erythropoietin production

6. Ca/PO4 imbalace: due to decreased hydroxylation of vit D, one can’t absorb Ca from the gut. We don’t have active vit D in CRF, so a patient would have low Ca. Due to low plasma Ca, one will have increased secretion of PTH (secondary hyperparathyroidism), which causes osteoclastic activity in the bone, leading to what is known as, renal osteodystrophy.

7. Neurologic: The biochemical mechanism is unknown.

Category: Nephrology Notes , Pathology Notes