Current brands of Pre-bypass filters- advantages & disadvantages

1. Pall Pre-Bypass Plus Filter

a) 0.2 microns

b) Nylon

c) Remove particulate & bacterial contamination down to 0.2 microns

d) Claims to have a pressure drop only slightly higher than current 5 micron filters

2. Gelman

a) 0.2 microns

b) High pressure drop

c) Cheaper than Pall

3. Bentley

a) 5 microns

· Must be removed prior to blood in circuit: must then drain whole circuit & reprime

Rationale for using pre-bypass filters

1. Types of particles identified

a) Glass; rubber; cellulose; talc; cotton fibres; metallic particles; plasticisers

b) Bacteria & endotoxins

c) Gaseous microemboli

i) Will not be able to pass via a wetted 0.2 micron filter

2. Harmful effects

a) Potential systemic effects of particulate matter in crystalline intravenous solutions

i) Occlusion of finer blood vessels

b) Bacterial contamination

3. Need for filtration at time of administration

a) Many particles appear to arise during storage

b) A particle free solution is impossible for the manufactures to produce

c) Many particles are a consequence of spallation 2° action of pump head on tubing

4. Effective removal with pre-CPB filter

a) The major proportion of particles arising from the circuit & spallation can be effectively removed by recirculation of the pump prime for approx 15 minutes via a pre-CPB filter

b) The prebypass filter is removed prior to going on CPB

Various screen sizes used in pre-bypass filters, and their potential limitations to flow

1. Membrane filtration

a) Clear fluids (crystalloids & colloids) do not contain cellular elements, therefore filtration can be taken to a much lower level

b) Includes removal of submicron particles & microbes

c) Formed rigid structures, appear like paper

d) Have a relative depth for microbes

i) Eg: 0.2 micron filter thickness is approximately 250 x diameter of bacteria

e) Mechanism of direct interception & adsorption

f) Increased resistance to flow

i) Resistance to flow is improved by:

a) Increasing the density of pores

b) Increasing the surface area (pleating)

g) Used in prebypass & cardioplegia filtration

2. Screen sizes

a) Most of the particles found in clear fluids used prebypass & cardioplegia are < 5 microns

b) 0.5 microns

i) Allows adequate flow rates

ii) Removes significant number of particles & microbes

c) 0.2 microns

i) Provides full bacterial retention

ii) Removes endotoxins

Correct method of using pre-CPB filters

1. The major proportion of particles arising from the circuit & spallation can be effectively removed by recirculation of the pump prime for approx 15 minutes via a pre-CPB filter

2. The prebypass filter is removed prior to going on CPB

a) Importance of removing filter by holding it with the inlet side uppermost prior to resecting it from A-V CPB loop to prevent filtered debris re-entering prime

b) Do not use with blood as sub-micronic pores will not permit the passage of blood cells resulting in massive haemolysis and excessive inlet pressures

c) Ca be used with colloid (eg albumin) prime