Continuous renal replacement therapies (CRRTs) involve either dialysis (diffusion-based solute removal) or filtration (convection-based solute and water removal) treatments that operate in a continuous mode [1-4]. Variations of CRRT might run 12 to 14 hours, especially during daytime periods of full staffing. This regimen has become more prevalent in Europe and has been called "go slow dialysis." Other variations of this technique are discussed below and probably should be called hybrid therapies because they are a merging of intermittent and continuous duration. The longer duration of CRRT makes it quite different from conventional intermittent hemodialysis in which each treatment lasts four to six hours or less.
The major advantage of continuous therapy is the slower rate of solute or fluid removal per unit of time. Thus, CRRT is generally better tolerated than conventional therapy, since many of the complications of intermittent hemodialysis are related to the rapid rate of solute and fluid loss. (See "Renal replacement therapy (dialysis) in acute kidney injury (acute renal failure): Metabolic and hemodynamic considerations".)
There are many variations of CRRT and the remainder of this topic will provide a general overview of the nomenclature that has been developed. The different modalities are categorized according to the access characteristics — blood or peritoneal, venovenous or arteriovenous (table 1).
Arteriovenous or venovenous — Arteriovenous (AV) refers to the use of an arterial catheter that allows blood to flow into the extracorporeal circuit by virtue of the systemic blood pressure. A venous catheter is placed for return. Venovenous (VV) is an alternative modality in which both catheters or one dual lumen catheter are placed in veins. An extracorporeal blood pump is required to circulate blood through the extracorporeal circuit.
The advantage of arteriovenous access is that it is simple to set up and does not require an extracorporeal blood pump. It does, however, require arterial puncture with an attendant risk of arterial embolization. Blood flow may also be unreliable in patients who are hypotensive or have severe peripheral vascular disease.