<qa>  What is the underlying pathophysiology of the decrease urine output seen with CVVHAcute renal failure is common in children in the pediatric intensive care unit. Renal replacement therapies such as peritoneal dialysis PD, intermittent hemodialysis IHD, and continuous venovenous hemofiltration CVVH have been used in the management of acute renal failure. CVVH is becoming increasingly utilized in pediatric acute renal failure. However, in patients who have acute renal failure, the institution of CVVH is often followed by a progression to oliguria or anuria. The underlying pathophysiology of this change is unknown. We believe that this progression is influenced by changes in the renin-angiotensin axis, cytokine response, and other modifiers of renal hemodynamics. By serially measuring components of those systems, this study will attempt to elucidate the pathophysiology of the decreased urine output seen with institution of CVVH. Once this process is understood, future studies should focus on prevention and treatment of this complication. 