 Acute kidney failure is common in children in the Pediatric Intensive Care Unit PICU. You are being asked to participate in this study because your child is being treated for kidney failure with continuous veno-venous hemofiltration CVVH. CVVH is a continuous, gentle form of removing excess fluids and small wastes from the blood, similar to kidney dialysis artificial kidney. It is an accepted therapy for temporary support of kidney failure. In some patients with acute kidney failure, beginning CVVH is followed by a temporary decrease of urine output. The reason why this happens is currently unknown. The purpose of this study is to determine why this happens. 