Plasmapheresis with hemodialysis equipment

INTRODUCTION

Therapeutic plasma exchange (TPE, plasmapheresis) is usually performed with centrifugation devices used in blood blanking procedures. (See "Prescription and technique of therapeutic plasma exchange".) These devices permit selective cell removal (cytapheresis) but are often associated with posttreatment thrombocytopenia [1]. An increasingly popular and often more efficient alternative is to perform plasma exchange by membrane plasma separation (MPS) [2-5].

MPS utilizes a highly permeable membrane and a standard dialysis machine used in its ultrafiltration mode, dialysis-bypass mode (similar to the technique in hemoperfusion). The most commonly used membrane in the United States is the Plasmaflo from Asahi Medical. This filter can be used with most dialysis machines, but the Cobe 3 and Hospal machines are incompatible with the filter's tubing. The published American experience is very limited even though thousands of treatments are performed in the United States each year [6,7].

TECHNICAL CONSIDERATIONS

The Plasmaflo filter should be operated with a very modest transmembrane pressure (TMP <75 mmHg) and blood flow (50 to 150 mL/min) in order to minimize the tendency to hemolysis and filter clotting [6,8]. Limiting TMP can be most easily achieved by limiting blood flow, usually via use of a double roller pump on the filtrate output of the filter. This double pump also regulates the infusion of replacement fluid (such as albumin or fresh frozen plasma), matching its rate of return to the volume of plasma removed, thereby eliminating the risk of hypotension or fluid overload.

It is important to appreciate that rapid removal and infusion of protein-containing solutions produce volume changes that are essentially limited to the intravascular compartment. Thus, even small mismatches in net plasma removal rate will have a far greater effect on systemic blood pressure than would be produced by an equivalent water shift during standard hemodialysis [9].

The protein permeable membranes, such as the Plasmaflo, are very susceptible to concentration polarization, a phenomenon whereby protein layering on the inner surface of the membrane rapidly limits the maximum achievable filtration rate [3]. As a result, inappropriately high transmembrane pressures will not yield higher filtration rates but will increase the tendency for the filter to clog (graph 1). Dialysis nursing issues have been the subject of several reviews [8,10,11].