Methods of optimal routine occlusion of a twin roller pump with consideration of: i) variation by roller; ii) raceway tolerance variation

• The occlusiveness must be set before each perfusion in order to avoid excessive haemolysis and to ensure a reasonable accuracy in calibrations [flow rates]. Excessive compression aggravates haemolysis and tubing wear. Too little occlusion also aggravates haemolysis and compromises forward flow.

• It appears that haemolysis is increased with a pump that is non occlusive. This is due to: high backflow velocities, necessary increased RPM, & high kinetic energies transferred to the blood.

A tighter nonocclusive setting may not be associated with more haemolysis

Main Pump

• 1) Using a clear priming solution, pump the arterial line full to a convenient level. Hold the arterial line vertical to the floor and in such a way that the top of the priming solution is about 30 inches above the pump. Vary the occlusion on the roller until the fluid level falls at about 1cm/min. This is the proper occlusion for blood

2) Pump occlusivity is calibrated to allow a 1 cm/min drop along a 1 meter high saline column created with 3/8 inch tubing ( 2 cm/min with 1/4 inch tubing)

Suction Lines

• Clamp the inlet line and, with the pump running at about 40% capacity, increase the occlusiveness until the inlet tubing just begins to collapse

• The occlusion should be checked with each roller at three different positions on the backing plate. It is not unusual to find that the occlusion is not consistent with each roller or at each position on the backing plate. The backing plate will get “out of round” with age. It is usually possible to average out the occlusiveness. However, if a large discrepancy is found the pump should be repaired.

• Roller pumps create an area of vacuum & cavitation in the blood in the tubing behind the moving roller. This is a minimal amount of damage and can be further minimised by optimally setting the occlusion of the rollers. To reduce the vacuum that trails the moving roller, it is advantages to have the level of the pump head below that of the venous reservoir. The higher level of blood will exert a slight positive pressure in the inflow side of the pump.

1) Ideal Occlusion

· A properly set pump, one that is just occlusive, produces acceptable hemolysis rates [note that most hemolysis is produced by the suctioning]

· If the arterial pump head is properly occlusive, the blood flow rate is not dependent upon peripheral resistance either from the ptient’s vascular system or the circuit elements peripheral to the arterial pump head

2) Under Occluded

· Haemolysis is increased when the pump is non occlusive: this is due in part to high backflow velocitie gradients and excesssive shear stresses and necessary increased RPM causing hemolysis

3) Overoccluded

· Total occlusion is not used because of increased hemolysis and excessive tubing wear will result

4) Technique

· Occlusiveness must be set before each perfusion to minimise hemolysis & ensure accuracy in flow rates