ARF - Acute renal failure. Also known as Acute renal insufficiency

If you were to ask a Nephrologist to give you the definition of ARF - he will most likely say - there is none. But that is why I do not want you to study from a nephrologist. For the boards you must know certain figures that define ARF for the generalist.

It is defined as the sudden rise in BUN, creatinine and potassium associated in most cases with oliguria or anuria. The absolute value of creatinine being >1.3 associated with a rise of more than 0.5 over a period less than 6 weeks. It may also be taken to be an acute reduction in GFR.

Regardless of its definition, it is important to know that in most out-patient cases it is reversible and for that the etiology of the ARF is important. On the other hand, in-patient cases of ARF have a high mortality.

Causes:
Pre-renal: dehydration, hemorrhage, sepsis, cardiac failure and burns, drugs (NSAIDs, Cyclosporin, ACE Inhibitors). All pre-renal causes left untreated - progress to intrarenal mechanisms.

Renal: Glomerulonephritis, ATN (ischemic/toxic), toxins (myoglobin, hemoglobin, uric acid are endogenous whereas  aminoglycosides, Amphoterecin B & Acyclovir, IV contrast are exogenous), athero-embolism, Acute interstitial nephritis (Infections and drugs such as Penicillin, Sulfa, NSAIDs and Cephalosporins), vasculitis (TTP, HUS, PAN)

Post-renal: post-renal obstruction and intra-tubular obstruction (e.g. urate crystals, Acyclovir and calcium oxalate)

Differentiating between pre-renal and renal is frequently tested.

Features Pre-renal Renal
Urinary Na <20 >20
FENa 
(UNa/Ucr x PCr/Pna x 100)		 <1 >1
Serum BUN/Creatinine ratio ³ 20 £ 10

It is extremely important to rule out obstruction before delving into differentiating pre-renal from renal as it is so easily treatable and will show the same indices as pre-renal especially BUN/Cr ratio > 20.

Azotemia:

High levels of Blood Urea Nitrogen is called azotemia. It can also be seen in patients with GI bleed, Steroid therapy, high protein diet (particulary TPN).

Azotemia becomes uremia if it causes any of the following things:

  1. Nausea
  2. Metallic taste
  3. Itching
  4. Tremors
  5. Asterixis
  6. Altered sensorium
  7. Pericarditis
  8. Seizures

Indications for urgent dialysis include: YOU MUST know these

  1. Resistant hyperkalemia
  2. Volume overload
  3. Fulminant acidosis
  4. BUN >100 in association with uremic signs and symptoms
  5. Pericarditis due to uremia - irrespective of the BUN level
  6. Toxins e.g Lithium overdose or salicylate overdose etc.