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Pathophysiology of Acute Renal Failure
The interaction of tubular and vascular events result in ARF. The primary cause of ATN is ischemia. Ischemia for more than two hours results in severe and irreversible damage to the kidney tubules. Significant reduction in glomular filtration rate (GFR) is a result of (1) ischemia, (2) activation of the renin-angiotensin system , and (3) tubular obstruction by cellular debris. As nephrotoxins damage the tubular cells and these cells are lost through necrosis, the tubules become more permeable. This results in filtrate absorption and a reduction in the nephrons ability to eliminate waste.
The clinical course of ARF is characterized by the following three phases:
Phase 1. Onset
ARF begins with the underlying clinical condition leading to tubular necrosis, for example hemorrhage, which reduces blood volume and renal perfusion. If adequate treatment is provided in this phase then the individual's prognosis is good.
Phase 2. Maintenance
A persistent decrease in GFR and tubular necrosis characterizes this phase. Endothelial cell necrosis and sloughing lead to tubular obstruction and increased tubular permeability. Because of this, oliguria is often present during the beginning of this phase. Efficient elimination of metabolic waste, water, electrolytes, and acids from the body cannot be performed by the kidney during this phase. Therefore, azotemia, fluid retention, electrolyte imbalance and metabolic acidosis occurs. The patient is at risk for heart failure and pulmonary edema during this phase because of the salt and water retention. Immune function is impaired and the patient may be anemic because of the suppressed erythropoietin secretion by the kidney and toxin-related shorter RBC life.
Phase 3. Recovery
Renal function of the kidney improves quickly the first five to twenty-five days of this phase. It begins with the recovery of the GFR and tubular function to such an extent that BUN and serum creatinine stabilize. Improvement in renal function may continue for up to a year as more and more nephrons regain function.