Continuous Renal Replacement Therapy
Stuart L. Goldstein
When acute renal failure occurs in critically ill patients, severe cardiovascular, respiratory, and metabolic instability may contraindicate standard intermittent dialysis techniques. In such situations, patients generally are monitored in intensive care units, and continuous renal replacement therapy (CRRT) may be required to minimize cardiovascular instability. Under these conditions, effective renal replacement must (a) provide adequate blood purification from uremic toxins; (b) correct fluid, electrolyte, and acid–base derangements; (c) maintain homeostasis; and (d) protect the kidneys from further injury.
Transition from the use of adaptive continuous renal replacement therapy (CRRT) equipment to hemofiltration machines with volumetric control that allows for accurate ultrafiltration flows has likewise led to a change in pediatric renal replacement therapy modalities. Accurate ultrafiltration (UF) and blood flow rates are crucial for pediatric CRRT, because the extracorporeal circuit volume can comprise more than 15% of a small pediatric patient’s total blood volume, and small UF inaccuracies may represent a large percentage of a small pediatric patient’s total body water. Polls of U.S. pediatric nephrologists demonstrate increased CRRT use over peritoneal dialysis as the preferred modality for treating pediatric acute renal failure (ARF). In 1995, 45% of pediatric centers ranked PD and 18% ranked CRRT as the most common modality used for initial ARF treatment. In 1999, 31% of centers chose PD versus 36% of centers reporting CRRT as their primary initial modality for ARF treatment.
Continuous venovenous hemofiltration (CVVH) is performed using blood pumps and double-lumen venous catheters to avoid the complications found in previous arteriovenous treatments. The use of countercurrent dialysate flow can be used alone (CVVH-D) or in combination with hemofiltration (CVVH-DF) and is subject to local standards of care, because no data to date demonstrate improved patient outcome using one form of therapy versus another. High clearances can be obtained during continuous therapy, and adequate blood purification can be achieved in severely catabolic patients. The difficulties and complications encountered with these techniques are related to anticoagulation practices, the establishment of adequate vascular access, and the selection of an appropriate hemofilter for the system. The ideal machine should have a small volume, an interface that is easy to use, and high flexibility. The machine must be freestanding and easily transportable to the bedside.