Prerenal Acute Renal Failure
- Prerenal Acute Renal Faliure (ARF) is a subcategorization of pathogenic mechanisms by which acute renal failure may develop. It is defined by the development of Acute Renal Failure due to dysregulation of processes prior to the kidney. Thus, the kidneys themselves are not dysfunctional and may exhibit no pathology in cases of Prerenal ARF.
- Recall that Acute Renal Failure is defined by a sudden (hours to days) reduction in the Glomerular Filtration Rate (GFR). Such reductions can occur when there is insufficient pressure perfusing the kidneys and thus the glomeruli. In such cases, there is simply not enough glomerular capillary hydrostatic pressure to maintain sufficient levels of GFR (See: Glomerular Filtration Rate page)
- Ultimately, prerenal ARF is caused by sustained hypotension that reduces renal perfusion. However, many of the etiologies discussed below reduce renal perfusion over courses much longer than hours to days. Given the relatively slow decline in renal perfusion, how can these etiologies precipitate relatively sudden drops in GFR? The answer is that neuroendocrine and autoregulatory mechanisms can usually maintain GFR in the face of falling renal perfusion over a wide range. However, at some point these mechanisms cannot sufficiently compensate and this precipitates a sudden drop in GFR, manifesting as ARF.
- True Hypovolemia
- Conditions which generate true hypovolemia cause hypotension which then translates into reduced renal perfusion pressure. Common true hypovolemic etiologies causing Prerenal ARF include major hemorrhage, dehydration, or hypovolemic shock.
- Deranged Hemodynamics
- Even in the presence of normal or increased ECF volume, conditions which cause major derangements of hemodynamics can result in significant hypotension and thus reduced renal perfusion pressure. For example, sizable reductions in systemic vascular resistance due to systemic vasodilation observed in septic, anaphylactic, or neurogenic shock can induce prerenal ARF. Alternatively, significant reductions in cardiac output due to cardiogenic shock, myocardial infarction or other causes of heart failure can precipitate prerenal ARF. Finally hepatorenal syndrome can yield prerenal ARF although the pathogenesis can yield prerenal ARF through cryptic mechanisms.
- The symptomology of prerenal ARF is described in Acute Renal Failure. Note that in prerenal ARF there is no actual pathology of dysfunction in the kidney itself. However, with sustained renal hypoperfusion, the renal interstitium can become highly ischemic and prerenal ARF may precipitate Ischemic acute tubular necrosis, and thus evolving into Intrinsic ARF