pt. has had a urine output of only 60 cc of urine in 24hrs

[kailash]

You are called to the cardiac intensive care unit to evaluate a 73-year-old patient who underwent an emergent cardiac catheterization for an acute myocardial infarction yesterday. You are told that the patient has had a urine output of only 60 cc of urine over the last 12 hours. Prior to this, his urine output had been within normal limits. His temperature is 37 C (98.6 F), blood pressure is 130/70 mm Hg, pulse is 60/min, and respirations are 14/min. He has no complaints at this time and his physical examination is unremarkable. Reviewing the medical chart, you learn that his medical history is significant for benign prostatic hypertrophy and hypertension. You catheterize his bladder and get 20 cc of dark urine, which you send for urine analysis and culture. You deliver a 500 cc normal saline fluid bolus and start intravenous fluid at 150 cc/hour. After 4 hours his urine output does not improve. Laboratory studies show:
Sodium 144 mEq/dL
Potassium 5.3 mEq/dL
Chloride 98 mEq/dL
Bicarbonate 21 mEq/dL
BUN 28 mg/dL
Creatinine 2.2 mg/dL (*Admission Creatinine- 1.3 mg/dL)
Urinalysis
Color Muddy brown
Specific gravity 1.020
Osmolality 55 mOsmol/kg
Leukocyte esterase Negative
Nitrite Negative
Protein Trace
Blood Negative
Microscopic Many muddy brown granular casts
Urine eosinophils None
The most likely cause of this patient's new condition is
A. acute interstitial nephritis
B. acute tubular necrosis
C. postrenal azotemia
D. prerenal azotemia
E. rapidly progressive glomerulonephritis

[kailash]

The answer is B.

This patient has acute tubular necrosis (ATN) and acute renal failure. Remember that when confronted with acute renal failure, the cause is divided into prerenal or "dry", postrenal or "obstructed", and intrarenal. The history and the urinalysis provide us with the correct diagnosis. Patients with ATN can have muddy, granular casts in their urine. They will also have an elevated BUN and creatinine, but in a ratio of less than 20:1 as is seen in patients with prerenal azotemia. The history of recent cardiac catheterization should also provide a clue since the patient received a large ionic dye load during the procedure to image his coronary vessels. Dye is a common precipitant of ATN and needs to be considered in anyone who has an elevated creatinine after a procedure that involves contrast dyes.
Acute interstitial nephritis (AIN) (choice A) is another cause of acute renal failure. It is associated with a transient maculopapular rash, fevers, eosinophiluria, hematuria, and white cell casts. Drugs account for most cases of AIN.
Postrenal azotemia (choice C) would also be reasonable in our patient since he has a history for BPH (which is the most common cause of obstruction). If a Foley catheter is placed and a large amount of urine is found in the bladder, obstruction is a likely cause of poor urine output. Since this patient had a virtually empty bladder, it would be unlikely that he has a postrenal cause of his renal failure.
Prerenal azotemia (choice D) should be your first thought when evaluating a patient with dressed urine output because it is the most common cause of acute renal failure. This patient was given a fluid challenge and still did not increase his urine output. Therefore, prerenal azotemia should be lower on the differential. Other clues that this patient is not prerenal is that his BUN to creatine ratio is less than 20:1 and his urinalysis revealed granular casts.
Rapidly progressive glomerulonephritis (choice E) is a rare cause of acute renal failure. It is associated with hypertension and edema. Urinalysis reveals dysmorphic red blood cells, red cell casts, and mild proteinuria.