[Acute kidney failure. Physiopathology--clinical diagnosis--therapy]

Acute renal failure (ARF) is characterized by an acute decrease in glomerular filtration rate (GFR). ARF complicates 4% to 23% of intensive care unit admissions, and is associated with a mortality of approximately 50% among critically ill patients. In the intensive care setting the term ARF is usually applied to acute tubular necrosis (ATN), a form of intrinsic ARF caused by ischemia or nephrotoxins. Pathophysiological mechanisms involved in the decline in GFR include tubular obstruction caused by detachment of tubular epithelial cells from the basement membrane and back-leak of glomerular filtrate as a consequence of disruption of the epithelial cell layer. Vascular mechanisms involved in the pathophysiology of ATN are vasoconstriction due to an imbalance between vasoconstrictive and vasodilatory mediators and vascular obstruction caused by cell aggregation. Currently, there is no real time method to monitor renal function comparable to the real time monitoring of blood pressure or arterial oxygen saturation. Urinary output does not reflect glomerular filtration which may be critically reduced despite normal urine volumes and creatinine clearance still provides the clinically most applicable estimate of GFR. Tubular function can be assessed using the fractional excretion of sodium or the ratio of urinary and serum osmolality; both parameters can be obtained from spot samples of urine and serum and no urinary sampling period is necessary. However, both parameters are strongly affected by the administration of loop diuretics and high fluid and sodium inputs which are common in the intensive care unit. We determined the day to day variability of creatinine clearance, fractional excretion of sodium and the urinary to serum osmolality ratio in critically ill patients without renal dysfunction (i.e. creatinine clearance in the normal range) and found differences of 16% for creatinine clearance, 79% for fractional excretion of sodium and 22% for urinary to serum osmolality ratio. Treatment of ARF is mainly supportive and there is no clinically accepted therapy that attenuates the course of ATN. Treatment of the underlying disease and renal replacement therapy are the main options for the treatment of patients with ARF. In critically ill patients continuous venovenous hemo(dia)filtration is the first choice because it provides more hemodynamic and metabolic stability than intermittent therapy. Acute life-threatening hyperkalemia is an indication for intermittent hemodialysis because of the higher efficacy of dialysis in the clearance of low molecular weight substances.