A 75 kg, 70-year-old male with a history of hypertension, coronary artery disease, and benign prostatic hypertrophy gets admitted to the ICU after a partial colectomy and liver resection for colon cancer. Placement of indwelling urinary catheter placement was challenging due to his enlarged prostate causing some hematuria. The operative procedure was complicated by bleeding, requiring 4 units of packed red blood cell transfusion. Postoperative hemoglobin was 8.2 g/dL.
Over the next 12 hours, his abdominal drain produced 700 mL of sanguineous output and his urine output decreases from 80 mL/h in immediate postoperative period to 20, 10, and 5 mL, respectively, in the last 3 consecutive hours. His vitals 12 hours after admission are now:
Which of the following would be LEAST likely in this patient?
A. A serum BUN:Cr ratio of 10:1Correct Answer: D
Important causes for low urine output may be classified into prerenal, intrarenal, and postrenal causes. Based on presentation sanguineous output from abdominal drain, hypotension, and tachycardia are consistent with a prerenal etiology. A common cause of prerenal disease is volume depletion, which may occur from hypovolemia caused by hemorrhage, dehydration, diuretics, or gastrointestinal fluid losses (vomiting/diarrhea). Prerenal physiology and renal hypoperfusion may also occur due to heart failure causing poor cardiac output, or cirrhosis causing splanchnic venous pooling and systemic vasodilation. Prerenal physiology is often characterized by azotemia, caused by increased sodium and urea absorption in the proximal tubule in an attempt to increase circulating blood volume. In such patients, the BUN is often increased to a greater proportion than the creatinine such that the serum BUN:Cr ratio is greater than 20:1 (Answer A). In prerenal disease, the FENa is typically less than 1% due to decreased urinary sodium excretion (due to reabsorption of sodium) in an attempt to retain water and circulating blood volume (Answer B). Urine with a high specific gravity (Answer C) indicates very concentrated urine, which occurs when the kidney is trying to retain additional free water. RBC casts in the urine (Answer D) are virtually diagnostic of some form of glomerulonephritis or vasculitis and indicate microscopic bleeding in the kidney. They are not associated with trauma to an enlarge prostate on insertion of a foley catheter and are not associated with prerenal or acute tubular necrosis. Acute tubular necrosis typically results in “muddy brown casts” consisting of renal tubular epithelial cells.
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