In this mode, renal function is supplied continuously while the patient recovers. There are several different CRRT modalities. Historically, arterial circulation was used to provide the force to move blood across a
dialyzer cartridge. Because of the need for arterial access and inherent complications and
the technical improvement in venovenous therapies, these methods have been largely abandoned.
The venovenous modalities differ primarily in the methods of clearance. Slow continuous
ultrafiltration (SCUF) is a method employed to remove volume by taking off fluid
filtered by a dialysis membrane by convective force. Continuous venovenous hemodialysis
(CVVHD) provides clearance using diffusive clearance by running dialysate across the
membrane. Continuous venovenous hemofiltration (CVVHF) removes fluid by convection
(as in SCUF) and then provides replacement fluid back to the patient. Finally, continuous
venovenous hemodiafiltration combines the clearance properties of CVVHD and CVVHF.
These approaches are provided by bedside machines incorporating the required pumps,
control instrumentation, filters, and fluids.