Principles of Extracorporeal Circulation and Transport Phenomena

Blood Access

Most patients receiving RRT in an acute setting do not have an established vascular access. In these patients, access to the circulation usually is provided by a catheter placed in an appropriate blood vessel. A single catheter with two lumens may be used for withdrawal and return of the blood. Less commonly, two single-lumen catheters may be used. If the patient does have an established blood access, such as a fistula or a synthetic graft, 15- to 16-gauge needles may be used. More details on blood access are given in Chapter 167 .

Blood Tubing

Blood is conveyed to and from the blood purification device by a disposable tubing set. It consists of two segments: an inflow (“arterial”) segment that connects the blood access to the inlet port of the purification device, and an outflow (“venous”) segment that connects the outlet port of the blood purification device to the return blood access. Blood tubing sets usually are manufactured from plasticized polyvinylchloride and may be sterilized with ethylene oxide or gamma irradiation. Some systems specifically designed for continuous RRT applications combine the tubing set and the purification device in a system-specific integrated unit. The blood tubing set typically includes ports for the administration of fluids (replacement, dialysate) and anticoagulants, as well as a chamber in the venous line for capturing any air that may enter the blood circuit inadvertently. Specific segments of the lines are designed to be connected to pressure sensors on the machine.

Blood Pump

The central venous pressure of the patient is usually insufficient to provide the desired flow rate of blood through the extracorporeal circuit. Therefore extracorporeal circuits for RRT use a blood pump to provide a controlled flow to the blood purification device. Peristaltic pumps (also known as roller pumps) usually are employed, because they guarantee no direct contact with blood and accurate control of blood flow. A typical peristaltic pump uses a rotating arm fitted with diametrically opposed spring-loaded rollers (two or three) that occlude the tubing accommodated in the pump housing and force the blood in the section of tubing before the point of occlusion to the outlet of the pump as the arm rotates. The rotational speed of the pump depends on the blood flow prescription and is proportional to the stroke volume, defined by the geometry of the pump and the inner diameter of the blood tubing.

Blood Purification Device

Nearly all contemporary blood purification devices used in critical care nephrology are hollow-fiber membrane devices (filters or dialyzers) that allow exchange of solutes by diffusion and/or convection and removal of water by ultrafiltration (see section below). The mechanisms involved in solute removal depend on the selected therapy. Hemodialysis relies mostly on diffusion, particularly for small waste products, electrolytes, and non–protein-bound drugs, whereas hemofiltration, achieved through convection, allows the removal of larger molecular-weight-solutes. Other therapies, such as hemodiafiltration, rely on a combination of diffusion and convection. More details on these processes are given in Chapter 165 .

Another type of blood purification device achieves depuration specifically by adsorption of solutes and consists of a cartridge containing various types of adsorption resins (see also Chapters 193 and 194 ).

Anticoagulation System

All extracorporeal circuits activate coagulation pathways because of contact of blood components with foreign surfaces and air within the circuit. Furthermore, low wall shear stress in the fibers of the filter enhances the risk of this activation. For these reasons, extracorporeal therapy usually requires infusion of an anticoagulant into the extracorporeal circuit. The anticoagulant may have a systemic effect (involving the patient and the circuit) or local (“regional”) effect (involving only the circuit, specifically the filter). Common anticoagulants used for extracorporeal circulation in critical care RRT are heparin and citrate (see Chapter 168 ). Heparin may be administered as an intermittent bolus or as a continuous infusion through a dedicated syringe pump (in the machine or external) that allows highly accurate control of the volume infused. Citrate anticoagulation is regional, involving infusion of a citrate solution into the inflow line and a calcium solution into the outflow line through dedicated pumps.

Other Pumps

RRT achieves depuration by administration of fluids, the nature of which are dictated by the specific therapy prescribed. Administration and balance of these fluids are controlled through a dialysis machine having incorporated pumps with dedicated lines. The replacement/infusion pump controls the rate of replacement fluid infused into the blood inflow line (predilution) and/or into the blood outflow line (postdilution). The dialysate pump controls the rate of dialysate flow into the dialysate compartment of the filter. The effluent/ultrafiltrate pump controls the removal rate of fluid leaving the filter.