Mechanisms of  Renal Failure

     
       

 

         
       

What are the mechanisms responsible for acute renal injuries?

Renal injuries are caused by damage and destruction of renal tubules. This may occur due to a variety of different mechanisms, ultimately all lead to hypoxia and ischemia:

  1. Medullary ischemia due to hypoxic injury to the thick limb of the loop of Henle. This leads to reduction in tubular filtration, via feedback mechanisms.
  2. Tubular obstruction due to casts, which are damaged tubular cells. The tubular pressure builds up and glomerular filtration is inhibited.
  3. Interstitial edema: tubular fluid leaks in between damaged cells.
  4. Vasoconstriction: due to release of vasoactive agents in critical illness.

 

 Tubular obstruction, with backflow of tubular fluid causing oliguria

It is thought that the mechanism behind oliguria is ischemic injury to tubular cells which subsequently slough off as casts and block the passage of fluid through the nephron. Pressure builds up in the pre-obstruction area, and is reflected backwards towards the Bowmans Capsule. As the pressure increases, less and less is filtered, and oliguria becomes pathological. When the pressure in the Capsule equals the net filtration pressure, filtration stops and the patient becomes anuric. One of the reasons why some experts recommend the use of diuretics is to flush debris out of the nephrons, and allow filtration and passage of urine. The efficacy of this technique has not been established.

When renal physicians come to consult on our patients, they retrieve a sample of urine and look at it under a microscope, why do they do this?

Urinary microscopy is a useful diagnostic technique for acute renal failure in the medical setting, particularly in the early stages. The presence of different cells or casts indicate the etiology of the disease process.  We dont get very excited about this in ICU as the majority of our patients develop ATN due to hypovolemia, hypotension and sepsis. However the test is inexpensive and worth thinking about, especially if there is a diagnostic dilemma. Table 2  summarizes the various things that can be seem under the microscope.

Table 2 Urinalysis Findings In Acute Renal Failure

  Injury

Urinalysis

  Prerenal

Benign or hyaline casts

  Acute Tubular Necrosis

Hemegranular or epithelial cell casts

  Acute Interstitial Nephropathy

WBCs, WBC casts, eosinophils, proteinuria

  Acute glomerulonephritis

RBCs, dysmorphic RBCs, RBC casts, proteinuria

  Postrenal

Benign hematuria

WBC = white blood cell; RBC = red blood cell.

 

         
                   
       

         
     

       
       

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