Current brands of arterial filters: advantages & disadvantages

 

1.                   Pall ‘stat prime’ blood filter

a)                   Uses process to enhance wettability of filter & housing without use of bonded heparin-surfactant

b)                  micron absolute polyester screen using patented lock woven medium

c)                   Low shear stresses

d)                  Low pressure drop

e)                   Results in negligible platelet removal and no increase in haemolysis

 

 

2.                   Pall ‘autovent’ blood filter

a)                   microns

b)                  Easy prime polyester screen filter with self venting membrane

c)                   As blood enters the filter housing it is directed in a circular flow

d)                  Centrifugal forces separate the lighter gas emboli which move up towards the hydrophobic, self venting membrane

e)                   As the pressure is greater than outside the gas escapes through the membrane into the atmosphere

f)                    Blood will not pass the hydrophobic membrane

g)                  Automatic vent overcomes limitations of vent lines

i)                    Superior air venting & handling of blood foam compared with conventional vent line filters

ii)                   No loss of blood through a vent line

iii)                 No compromise to venting by monitoring from a vent line

h)                  Always require positive blood pressure or will drain

 

 

 

Materials in used in construction

 

 

Filter type

Filter material

Bonded

Outer housing material

Filter [mm]

Bentley AF-10

Screen

Polyester

Heparin coated easy prime

clear polycarbonate

25

Pall ultipor

Screen

Polyester

Surface modified easy prime

opaque polypropylene

40

Delta

Screen

Nylon

 

clear polycarbonate

37

Swank

Depth

Dacron wool

 

 

13

 

1.                   Dacron wool (depth) filters may cause significantly more haemolysis and thrombocytopaenia; and develop channelling & saturation breakthrough

a)                   Often lose bubble retaining capacity due to channelling

 

2.                   Screen filters are made of pleated polyester strands which are lock woven with a defined pore size

a)                   Polyester has a low reactivity with blood

b)                  Pore size remains a balance between microemboli & gaseous emboli removal and damaged to formed blood elements

c)                   Most arterial filter has intermediate pore size (Approx 30 mm)

d)                  The surface area must be large enough to assure blood flows with negligible trauma and a low pressure gradient at both high & low flow rates

 

3.                   Filter housing should be clear to facilitate visualisation of air

 

4.                   Pall hydrophobic arterial filter

a)                   Disperses air to atmosphere automatically

b)                  Disadvantage: must always have a positive blood pressure or will drain

 

5.                   25 vs 40 m: 25 m has better air handling capability [BPP etc]; no difference in cost

 

 

Various screen size specifications - advantages & disadvantages

 

1.                   Particulate matter removal

a)                   Smaller the pores, the more foreign particulate matter removed

b)                  Limited by size or blood cellular components

i)                    rbc: 8 microns

ii)                   wbc: < 18 microns

 

2.                   Air removal

a)                   As pore diameter decreases, pressure required to force a bubble via pore increases

 

3.                   Excessive back pressures

a)                   Pore size must be > 20 microns for adequate blood flows without excessive back pressures

b)                  Increased back pressures - increased transfilter pressure - exceed BPP - filtration of microemboli

 

 

Pressure point theory of screen filters

 

Screen filters block micro­scopic air emboli because the pores are filled with liquid that is maintained by surface active forces; ex­cessive pressure could overcome this barrier by exceeding the bubble point pressure
 

 

1.                   Bubble removal in liquid filtration

a)                   Liquid filters are also effective at removing bubbles once they are wetted (primed) as long as a critical pressure (the bubble point pressure BPP) is not exceeded

2.                   Bubble point pressure

a)                   Pressure required to push a bubble through a filter

b)                  Inversely related to pore size

i)                    The smaller the pore, the more pressure is required to force the bubble through the filter

c)                   Mechanism

i)                    Differential pressure across the membrane acts on the circular area on the bubble, where it deforms at the pore

ii)                   In opposition, surface tension acts to try to keep the bubble spherical and the adsorptive force across of the liquid across the pore resists dewetting

iii)                 When the pressure pushing the bubble across the membrane overwhelms the forces keeping the bubble spherical will the bubble be forced via the pore

d)                  BPP for 40 micron filter

i)                    ? mmHg

ii)                   At a flow of 5 l/min, the pressure difference between the inlet & the outlet of the 40 micron filter is 30 mmHg, however the pressure difference across the screen filter is only a few mmHg - it is the differential pressure across the filter that is relevant to BPP

 

Methods & advantages of filter purging

 

1.                   Air venting

a)                   Enhances bubble removal capacity of filters

b)                  Use of vent ports

c)                   Allows air to escape more rapidly

d)                  If air was allowed to enter a filter and build up

i)                    the area for liquid filtration would be reduced (air would be blocking a proportion of the filter)

ii)                   a substantial rise in pressure could then occur across the filter if the flow were maintained

iii)                 potential to exceed BPP - resulting in air traversing filter

iv)                 Buildup of pressure can be minimised by venting out any entrapped air thereby maintaining the area available for liquid filtration

 

2.                   Vent lines

a)                   Ensure that a vent port is present on the upstream side of the filter housing

i)                    Bubbles are naturally buoyant & so will rise & coalesce in the upper portion of the housing where they can exit from the vent port

b)                  The port is attached to a vent line that is attached to a low pressure component of the circuit eg cardiotomy reservoir

i)                    Use of a one-way valve further reduces the risk of retrograde flow into the filter housing

ii)                   Ideal vent has largest possible diameter and shortest length to minimise resistance to flow but would shunt too much blood - require a compromise

a)                   Compromise between reasonable venting capacity & tolerable recirculation losses (4-6%)

 

3.                   Housing design

a)                   Improved housing designs to encourage passage of bubbles to the vent port

 

4.                   Automatic venting mechanisms

a)                   Avoids dependency on a patent vent line

b)                  Relies on use of a hydrophobic  membrane at the apex of the filter

i)                    Permanent open vent

c)                   Automatic vent overcomes limitations of vent lines

i)                    Superior air venting & handling of blood foam compared with conventional vent line filters

ii)                   No loss of blood through a vent line

iii)                 No compromise to venting by monitoring from a vent line

 

Methods to test efficiency of arterial filters

 

1.                   Filter Examination

a)                   Gross examination

i)                    Little value as filter removes microemboli

b)                  Microscopic examination

i)                    Identify nature & estimate size of filtered particles

ii)                   What about particles not  filtered by filter?

 

2.                   Weight & volume of filtered particles

a)                   Requires data about the composition of the pre & post filtered blood to be meaningful

 

3.                   Assessing pulmonary function

a)                   Aggregates may be filtered out in lungs

i)                    Resulting in pulmonary insufficiency

b)                  Determine pulmonary function and pulmonary ultrastructural changes in filter versus control group

 

4.                   Screen filtration pressure

a)                   The filtered blood is subjected to a screen filter of known surface area & pore size

b)                  The increase in pressure gradient across the screen is proportional to the increase in resistance caused by the occlusion of the pores

c)                   Therefore the more effective the trialed filter is in removing particulate matter, the reduction in the rise in pressure gradient across the screen

 

5.                   Counting aggregates contained in blood pre & post filter

a)                   Use of Coulter Counter

i)                    Measures numbers & sizes of aggregates down to 10 microns

b)                  In vivo assessment potential

 

6.                   Assessment of filters inability to remove desirable cellular components

 

7.                   Filter efficiency

a)                   Measure of the number of particulates retained by the filter as a function of the total number & size of the challenging particles and differential pressure

b)                  Generally, the lower the challenge level & pressure, the more efficiently the filter performs

c)                   Expressed as a percentage of retention of predetermined particle size

 

8.                   Filter life

a)                   Measure of duration of a filter’s useful service based on the amount of standard contaminant required to cause differential pressure to increase to an unacceptable level - typically 2-4 times the initial differential pressure, or a 50-80% drop in initial flow, or a down stream measure of unacceptable particulate

 

9.                   Inlet-outlet ratio

a)                   Eg 4:1 inlet to outlet ratio places 80% of the total priming volume on the inlet side of the filter

b)                  Increases margin of safety

c)                   The resistance of the filter to the passage of air provides a secondary reservoir for temporarily holding a bolus of air

d)                  Allows perfusionist to clamp arterial line

e)                   The larger the inlet volume, the longer the reaction time for clamping the arterial line

 

 

“heparin/wetting agent” coatings used in arterial filters and their advantages & disadvantages

 

1.                   Housing treatments & coatings to facilitate filter priming

a)                   If the wettability of the filter is increased the priming liquid will spread on the surface more easily and air will be expelled more rapidly

b)                  Heparin - coated filters

i)                    Attach heparin to filter via a surfactant (benzalalkonium chloride)

ii)                   Advantages

a)                   One technique to increase wettability of filter

b)                  No improvement in biocompatibility

c)                   But improvement in quickness of priming & debubbling

d)                  Reduce platelet aggregation & loss

iii)                 Disadvantages

a)                   Heparin-benzalkonium may leak during priming

(1)                 Rendering it ineffective

(2)                 Release of surfactant may reduce the surface tension of the blood passing through the filter thereby decreasing the BPP

c)                   Alternative methods to increase wettability

i)                    Physical process that can modify surfaces which do not entail use of heparin-surfactants

ii)                   Increase in wettability associated with an increase in BPP

iii)                 Eg Pall ‘stat prime’ blood filter

 

 

Essential requirements of arterial filters

 

1.                   Requirements for arterial line filter include:

a)                   Ability to function in a high flow (>6 L/min) without undue haemolysis

b)                  Ability to be primed and evacuated of air with ease (high wettability)

c)                   Provision for continuous monitoring of the pressure gradient across the filter to determine filter occlusion

d)                  A bypass line to be employed in case of filter occlusion & failure

e)                   Clear housing to aid priming & debubbling

 

2.                   Desirable characteristics of arterial filters

a)                   Very low resistance

b)                  Easy to debubble

c)                   Not a source of emboli

d)                  Removes both particulate & gaseous emboli

e)                   Produces minimal blood trauma

f)                    Easily purged of trapped air

g)                  Wettable table

h)                  Clear housing

i)                    Minimal hold up volume

 

Position of the arterial filter in the CPB circuit and the safety precautions to be taken

 

1.                   Filter location

a)                   Most filters in the arterial line are screen filters

b)                  A bypass line around the arterial line filter is recommended

i)                    Should the filter become obstructed

ii)                   Rapid repriming of drained circuit

c)                   Pressure drop across filter should be monitored to detect filter obstruction

d)                  Bypass line around filter is usually occluded by a clamp

e)                   Always after oxygenator: distal in circuit