Renal replacement therapy (RRT) can be provided as hemodialysis, hemofiltration, or a combination thereof and as intermittent, or as continuous therapy. Continuous renal replacement therapy (CRRT) represents a number of technically distinct modalities characterized by slow per-minute solute clearance and ultrafiltration rates that are spread over most or all of the day to minimize wide metabolic or volume shifts. The longer duration of therapy has been associated with greater hemodynamic stability and a higher likelihood of kidney recovery compared to standard intermittent hemodialysis (IHD). CRRT encompass several methods of hemofiltration and hemodialysis and is being increasingly utilized for managing patients with AKI in the intensive care unit (ICU) setting. This chapter reviews the major terms used in the delivery of CRRT, the most prominent clinical and technical issues encountered by the healthcare teams, and promising technologies that may improve the efficacy and future applicability of the relevant modalities.
CRRT TERMINOLOGIES AND MODALITIES
Continuous renal replacement therapy is actually an umbrella term for the four different continuous modalities: slow continuous ultrafiltration (SCUF), continuous venovenous hemofiltration (CVVH), continuous venovenous hemodialysis (CVVHD), and continuous venovenous hemodiafiltration (CVVHDF). Solute removal in these techniques is achieved by either convection, diffusion, or a combination of both these methods.
Convective techniques, including ultrafiltration (UF) and hemofiltration (H), depend on solute removal by solvent drag.
Diffusion-based techniques, similar to intermittent hemodialysis (IHD), are based on the principle of a solute gradient between the blood and the dialysate.
Hemodiafiltration (HDF) processes use both diffusion and convection in the same technique. In this instance, both dialysate and a replacement solution are used, and both small and middle molecules can be removed easily.
The predominant mechanism of achieving clearance defines the modality of CRRT.
Simple diffusion: continuous venovenous hemodialysis (CVVHD)
Convection: continuous venovenous hemofiltration (CVVH)
Combination of both diffusion and convection (continuous venovenous hemodiafiltration [CVVHDF]) Table 51–1 and Figure 51–1.
Table 51–1.Modalities of CRRT. |Favorite Table|Download (.pdf) Table 51–1. Modalities of CRRT.
| ||SCUF ||CVVH ||CVVHD ||CVVHDF |
|Vascular access ||VV ||VV ||VV ||VV |
Av. blood flow
|100 ||100–200 ||100–200 ||100–200 |
|0 ||0 ||10–30 ||10–30 |
|Replacement fluid (L/day) ||0 ||21.6 ||0 ||16.8 |
|2–8 ||10–30 ||2–4 ||10–30 |
|Anticoagulation ||Yes/no ||Yes/no ||Yes/no ||Yes/no |
|Clearance mechanism ||C ||C ||D ||C + D |
|Urea clearance (L/day) ||1.7 ||16.7 ||21.7 ||30 |
Modalities of continuous renal replacement therapies (CRRT). Commonly available modalities in ICU. CVVH, continuous veno venous hemofiltration; CVVHD, continuous veno venous hemodialysis; CVVHDF, continuous veno venous hemodiafiltration; HVHF, high-volume hemofiltration; K, clearance; Qb, blood flow; Qd, dialysate flow; Qf, ultrafiltration rate; R, replacement; SCUF, slow continuous ultrafiltration.