Acute renal failure (acute kidney injury)

Question 1

In normal nephron

Answer 1

BUN reabsorbed (for countercurrent multiplication) but creatinine is NOT.

Question 2

Acute renal failure definition

Answer 2

Abrupt decline in renal function with increase in creatinine and BUN over a period of several days

Question 3

Prerenal azotemia: pathophys

Answer 3

As a result of low RBF(hypotension), low GFR

Na+/H2O and urea retained by kidney in attempt to conserve volume
so BUN/creatinine ratio increases

Question 4

Prerenal azotemia: values

Answer 4

Urine osmol: >500
Urine Na: 20
FeNa: 20

Question 5

Intrinsic renal failure: pathophys

Answer 5

Due to acute tubular necrosis or ischemia/toxins;
less commonly due to acute glomerulonephritis

Patchy necrosis leads to debris obstructing tubule and fluid backflow across nerotic tubule=> low GFR

Urine has epithelial/granular casts
BUN reabsorption impaired so low BUN/creatinine ratio

Question 6

Intrinsic renal failure: values

Answer 6

Urine osmol: 40
FeNa: >2%
Serum BUN/cr: <15

Question 7

Postrenal azotemia: pathophys

Answer 7

Due to outflow obstruction (stone, BPH, neoplasia, congenital anomalies)

Develop only with bilateral obstruction

Question 8

Postrenal azotemia: values

Answer 8

Urine osmol: 40
FeNa: >2%
Serum BUN/cr: >15 (vs. intrinsic)