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. Continuous techniques were originally described in the 1970s as experimental treatment for diuretic-resistant, hypervolemic patients whose clinical picture made them inappropriate candidates for either peritoneal dialysis (PD) or intermittent hemodialysis (IHD). Since then, the presence of CRRT in the intensive care unit (ICU) has evolved both in scope and complexity, and as a result, CRRT techniques are quickly becoming the standard of care for critically ill patients with acute renal failure (ARF). This chapter will review 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.
Distinct CRRT techniques are generally defined by the vascular access used and the mechanism of solute/water removal relied upon to maintain the desired clinical parameters (Figure 52–1).
The mechanisms of hemofiltration and hemodialysis with filtration. No attempt has been made to represent the automatic control of the rates of filtration and infusion of replacement fluid. (Reproduced with permission from Forni LG, Hilton PJ: Continuous hemofiltration in the treatment of acute renal failure. N Engl J Med 1997;336:1303.)
In arteriovenous (AV) techniques the pressure gradient for solute and water removal is supplied by the difference in pressures between the patient's arterial and venous vasculature. These techniques necessitate arterial cannulation with its attendant risks of arterial thrombosis, limb ischemia, hemorrhage, and atheroembolism (among others). Furthermore, the amount of solute and water removal is restricted by each individual patient's hemodynamic status, making both delivery of adequate therapy and standardization of delivered therapy difficult, especially in hypotensive critically ill patients. The major advantage of AV access is the ability to deliver therapy without complicated external machinery and support. However, because of the associated high risk-to-benefit ratio and lack of control over blood flow rates, AV techniques have fallen out of favor in most tertiary care centers.
In venovenous (VV) techniques the pressure gradient for solute and water removal is supplied by an occlusive peristaltic pump. VV access allows avoidance of arterial cannulation and greater control over and greater reliability of the rate of blood flow into the extracorporeal circuit. Disadvantages of VV access include incorporation of more external devices (ie, pumps/air traps) and longer circuits that are more prone to clotting. Nevertheless, because of the superior safety and control associated with VV access as compared to AV access, VV techniques are used in the majority of clinical settings ...