Urinary Disorders Q 120

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Which statement correctly distinguishes renal failure from prerenal failure?

  

    A. With prerenal failure, vasoactive substances such as dopamine (Intropin) increase blood pressure.

    B. With prerenal failure, there is less response to such diuretics as furosemide (Lasix).

    C. With prerenal failure, an IV isotonic saline infusion increases urine output.

    D. With prerenal failure, hemodialysis reduces the BUN level.

    

    

Correct Answer: C. With prerenal failure, an IV isotonic saline infusion increases urine output.

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Prerenal failure is caused by such conditions as hypovolemia that impairs kidney perfusion; giving isotonic fluids improves urine output. Vasoactive substances can increase blood pressure in both conditions. The cells in the macula densa are sensitive to the increased delivery of NaCl and activate Type 2 adenosine receptors resulting in vasoconstriction of the glomerular arterioles and retraction of glomerular tufts. As a consequence urine output is decreased and urinary excretion of sodium is reduced providing a diagnostic flag of the tubular ischemic process.

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Option A: In other clinical scenarios, renal hypoperfusion can be present even in the presence of normal blood pressure. Normotensive patients are predisposed to renal hypoperfusion when intrinsic renal structural changes from premorbid conditions interfere with the reserve mechanisms, or extrinsic factors impair the compensatory mechanisms.

Option B: Under normal circumstances, almost 80% of the NaCl is reabsorbed by the end of the thick ascending limb of Henle’s loop. If there is a failure in the tubular reabsorption mechanism, more NaCl will reach this point. Loop diuretics reduce effective intravascular volume and impair the autoregulatory mechanism by interfering with the reabsorption of NaCl by the macula densa cells.

Option D: The most important parameter to distinguish prerenal failure secondary to volume depletion or hypotension from ATN is the response to the fluid expansion. The return of the renal function to the previous baseline within 24 to 72 hours is considered to represent prerenal disease, whereas persistent renal failure is called ATN.

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