Technical considerations

Circulatory access

This is predicated on the separation technique employed. A blood flow of 40 to 50 ml/min is required for a centrifugal device and can often be achieved via a large peripheral vein. Conversely, when a hollow-fiber technique is used, a blood flow of 100 ml/min is necessary; this can only be sustained by a suitable large-caliber double-lumen central venous catheter. The femoral position may reduce patient mobility, but has the theoretical advantage of allowing relatively hypocalcemic blood to mix with normocalcemic blood before reaching the arteriovenous node, thus reducing the risk of arrhythmias.

Separation technology

Centrifugal devices separate blood into its components according to size, density, and function either continuously or discontinuously.

The intermittent-flow instrument intermittently removes small volumes of blood (200-300 ml) which are then anticoagulated and centrifuged, separating the cellular elements from plasma. The plasma fraction is eliminated, and the cellular elements are returned to the patient together with replacement fluid. This is repeated until the desired volume has been exchanged. The continuous-flow apparatus performs the same task while simultaneously withdrawing blood from the patient, centrifuging it, and returning the cellular fraction to the blood stream. This is accomplished using a loop-shaped ring with sampling ports for plasma and individual cellular elements. Hollow-fiber devices similar to hemodialyzers but with larger pore sizes cannot perform white cell, red cell, or platelet pheresis, and thus are limited to plasmapheresis. However, they offer several advantages. The plasma fraction can be passed through successive devices with decreasing molecular weight cut-offs, allowing different plasma fractions to be separated out (cascade plasmapheresis). The albumin fraction can be returned to the patient, and other fractions (immunoglobulins) selected out. A relatively high blood flow is required with hollow-fiber devices.

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