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RENAL


NEPHRO SYSTEM

ACUTE RENAL FAILURE IN ICU

Introduction

The mortality rate of acute renal failure (ARF) in isolation is 8% and rises to 69% - 80% when associated with other organ failures. In ICU, the patients usually have ARF as part of the syndrome of multiple organ failure (MOF).

Diagnosis

ARF can be defined as any acute reduction in renal function sufficient to result in retention of nitrogeneous waste.
The clinical diagnosis rests primarily on the increase of plasma level of two final products of protein and muscle metabolism that are excreted by the kidney – urea and creatinine.
The volume of urine is variable and the following terms are sometimes used to differentiate on the basis of urinary output.

Urine Output (mls/day)
Description
<50
Anuric renal failure
<400
Oliguric renal failure
400 - 2500
Non-oliguric renal failure (NORF)
>2500
Polyuric renal failure

Non-oliguric renal failure causes a significantly lower mortality than oliguric renal failure.

In ICU, the most common precipitating factors are infection, hypotension and hypoxia. Iatrogenic contribution to ARF include inadequate and delayed resuscitation; deterioration during inadequately monitored and supervised transfer; aminoglycoside toxicity due to inadequate monitoring of levels; aggressive diuretic use e.g.frusemide leading to dehydration and NSAID toxicity.

CLASSIFICATION

Pre-Renal Renal Failure

This is caused by decreased renal perfusion due to a fall in cardiac output, extracellular  fluid deficit (e.g. shock or dehydration), or decreased intravascular volume as may occur during rapid “ third space” accumulations (e.g peritonitis, pancreastitis and intestinal obstruction). Renal function is impaired without parenchymal damage and the impairment is rapidly reversible with fluid therapy.

In circumstances of hypoperfusion, the kidney concentrates urine in an attempt to conserve salt and water and the urinary sodium concentration may fall to below 20 mmol/l.
Subsequently, as renal failure becomes established and tubular function is lost, the urinary sodium concentration rises to above 30 mmol/l and may approach serum levels. Often , the urinary sodium values are difficult to interpret. Diuretics will nearly always have been used. The difference between blood and urinary urea concentrations may be more useful (urinary urea > 20X blood urea with normal renal function) and is helpful in assessing decline and recovery in renal function and the timing of renal support.

Intrinsic Renal Failure
Intrinsic renal failure is divided into 2 groups:-

  1. Acute tubular necrosis – includes conditions secondary to persisting renal ischemia (i.e.severe, unrelieved pre-renal failure ) or nephrotoxins.
  2. Other causes – A diverse group caused by glomerular or vascular lesions or by severe infections.

  • bilateral cortical necrosis
  • acute glomerulonephritis
  • acute interstitial nephritis
  • malignant hypertension
  • severe pyelonephritis
  • acute activation of chronic renal disease

Post – Renal Failure

This refers to obstruction to the outflow of urine. This may occur anywhere between the renal pelvis and the urethra, but must be bilateral to cause renal failure in the presence of two previously functioning normal kidneys.
In the ICU, the most common cause of urinary tract obstruction is bladder catheter blockage, and this should always be suspected in the presence of complete anuria.

Another important causes of urinary tract obstruction in the ICU is raised intraabdominal pressure (IAP). This occurs commonly as a result of intra-abdominal bleeding, intestinal distension or ascites. The possible mechanisms by which raised IAP causes a reduction in urine output include:

  • Impaired renal blood flow due to reduced venous return and cardiac output particularly in the presence of hypovolaemia.
  • Renal vein compression occurs, with reflex renal artery vasoconstriction and a reduction in RBF and urine output.
  • In patient with raised IAP the proximal renal tubular pressure approximates the IAP.

The intravesical pressure is a close approximation of  the IAP. 100 ml of sterile saline is instilled through the catheter which is then clamped proximal to the sampling port on the catheter tubing. A needle attached to a pressure transducer leveled with the symphysis pubis is inserted through the sampling port diaphragm and the pressure recorded. Anuria and an IAP of 30 – 40 mmHg associated with intra-abdominal bleeding may be an indication for laparotomy.
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