Renal effects of critical illness

Acute renal failure (ARF) is broadly defined as deterioration of renal function over days to weeks, a common occurrence in intensive care unit (ICU) patients; incidence and mortality estimates vary according to ARF definition and associated morbidities.'-"he mortality of ARF is 50-80% in ICU populations, and has not declined significantly since the initial marked benefit of acute dialysis therapy, despite numerous advances in renal replacement technologies and critical care over several decade^.^ Although multiple system organ failure (MSOF) and other comorbidities contribute to its high mortality rate, ARF independently increases morbidity and m~rtality.~"he mechanisms by which ARF contributes to nonrenal organ dysfunction and death are incompletely understood. Emerging data suggest that isolated renal ischemia-reperfusion injury causes injury and dysfunction of distant organs in experimental ARE9-" Apart from obviously lethal ARF sequelae such as severe hyperkalemia, the identities and precise effects of 'uremic' toxins have not been identified, particularly in ARE Accordingly, it is likely that renal replacement therapy in its current form is only a partial solution to the multisystem problems caused by ARF. Prevention or early diagnosis and amelioration of ARF are clearly the preferred approaches to diminishing the contribution of renal dysfunction to adverse outcomes in critically ill patients. Most ARF in ICU patients is caused by either prerenal azotemia (reversible renal insufficiency due to renal hypoperfusion) or acute tubular necrosis (ATN). ATN results from a variety of ischemic and nephrotoxic insults, often in additive or synergistic combination.'? Multiple causes of renal hypoperfusion and injury are commonplace in the ICU, including systemic hypoperfusion (shock), the effects of mechanical ventilation,'~enal vasoconstriction in patients with cirrhosis and sepsis,14,15 increased intra-abdominal pressure,lh and a variety of nephrotoxins (endogenous and exogenous). Because renal hypoperfusion plays a role in the pathogenesis of prerenal azotemia and ATN, and shock is common in critically ill patients who develop ARF, much attention is paid to optimization of renal perfusion, attempting to prevent or ameliorate ARF in the ICU. This chapter reviews the effects of critical illness on renal perfusion and function, and the impact of renal insufficiency on outcome in the ICU.