Basic design
of bubble oxygenators
Constructed to include 3 primary sections of operation:
1.
Bubble column
a) Sequential design variant
i) Bubble column is distinctly separate but in series with the debubbler/defoamer and arterial reservoir
b) Concentric design variant
i) Bubble column is surrounded by the debubbling/defoaming area and the arterial reservoir respectively
c) A gas dispersion device separates the blood entry point from the gas inlet port
i) The sparging plate may be a polycarbonate plate perforated with holes of precise dimension, or porous silicate
ii) Purpose of the sparging plate is to break the bulk gas into small bubbles to allow gas exchange
iii) Actual bubble size as determined by the disperser, is a function of:
a) Orifice diameter
b) Gas flow rate
c) Blood viscosity
d) Surface tension
d) May include heat exchanger
2.
Debubbling/defoaming
area
a) Made of polyurethane mesh sponge coated with a silicon compound (antifoam A)
i) Break surface tension of bubbles causing them to collapse
b) Includes polyester fabric surrounding mesh sponges acting as a screen filter - a final mechanism for the disruption of bubbles
c) Bubble elimination within the debubbler/defoamer & the arterial reservoir is a function of:
i) Mechanical disruption
ii) Surface area of defoaming section
iii) Residence time in the defoaming section
iv) Velocity of flow
v) Surface active agents
vi) Filtration
vii) Buoyancy
viii) Absorption
3.
Arterial reservoir
a) Always positioned after the debubbling/defoaming area
b) Arterial outlet must be designed to prevent vortexing of blood when the device is operated at low reservoir volumes
1.
Principles of gas
transfer
a) Gas bubbles are dispersed into the venous blood in the bubble column & act as vehicles for the transfer of both CO2 & O2
b) Oxygen
i) Diffuses from the bubble into the blood film surrounding the bubble
ii) This transfer is limited by the thickness of the blood film around the bubble and the diffusion coefficient of oxygen in the plasma
iii) Adequate quantities of oxygen are transferred by diffusion owing to the high partial pressure gradient that exists between the gas in the bubble (initially 100% O2) and the inlet blood pO2
c) Carbon dioxide
i) Diffuses from the blood film into the bubble which acts a vehicle for CO2 transport until the bubble bursts & the gas is released (defoamer section)
ii) Although this transport is limited by the low partial pressure gradient between the blood film & the bubble, adequate CO2 is removed owing to the high CO2 coefficient of CO2
d) Bubble size
i) Critical to adequate gas transfer
ii) As the bubble column has fixed dimensions, only a certain volume of bubbles can be accommodated
iii) Surface area of bubbles a radius2
iv) Volume of bubbles a radius3
v) Doubling radius of bubble
a) Increase surface area ´ 4
b) Increase volume ´ 8
vi) Many small bubbles in a fixed column dimension
a) Large surface area
b) Efficient O2 transfer
c) Poor CO2 transfer
(1) As the oxygen diffuses from the bubble the bubbles dissolve & are not available for CO2 transfer
(2) Also, if the bubbles did not fully dissolve, the CO2 would rapidly equilibrate between the CO2 of both the blood & the bubble thereby limiting the volume of CO2 transported
d) Small bubbles are more difficult to eliminate in the debubbling section
vii) Small number of large bubbles in a fixed column dimension
a) Relatively small surface area
b) Improved CO2 transfer
c) Reduced O2 transfer
viii) Ideal bubble size
a) Compromise between optimal surface area for O2 transfer and volume for CO2 transfer
b) A gas dispersion plate is unlikely to produce bubbles of a precise size
c) Bubbles are usually made 3 — 7 mm to optimise both CO2 & O2 gas transfer
2.
Sparging Plate
a) A gas dispersion device separates the blood entry point from the gas inlet port
i) The sparging plate may be a polycarbonate plate perforated with holes of precise dimension, or porous silicate
ii) Purpose of the sparging plate is to break the bulk gas into small bubbles to allow gas exchange
iii) Actual bubble size as determined by the disperser, is a function of:
a) Orifice diameter
b) Gas flow rate
c) Blood viscosity
d) Surface tension
3.
Blood gas
management
a) Ventilating gases are a mixture of oxygen or carbon dioxide
b) Nitrogen mixtures are not recommended due its the low solubility posing an emboli risk
i) Therefore some degree of patient denitrogenation will occur
c) Oxygen management
i) Since the FiO2 of the ventilating gas is fixed (100%), direct adjustments to the PaO2 cannot be made in most bubblers
d) Carbon dioxide management
i) Controlled by adjusting the gas-to-blood-flow ratio
a) The higher the gas-to-blood-flow ratio, the more volume is available for CO2 transfer resulting in reduced PaCO2
b) Important to use the lowest gas flow as high gas flows are associated with:
(1) Increased gaseous microemboli
(2) Excessive haemolysis
(3) Protein denaturation
ii) Also controlled by titrating carbon dioxide into the gas source
a) Addition of carbon dioxide will increase the PaCO2
1.
Types of
dispersing(diffuser) units
a) Various type of dispersion plates exist these may consist of;
b) plastic tubing with holes to dispensers
c) Holes in plastic discs some may have large holes and some may have small holes to provide different sized bubbles.
d) some dispersing units comprised of a number of different sized balls which were partially glued this allowed bubbles of differing sizes to be produced ie; either large or small bubbles.(Sparger plate) (nose:pp88)
2. Ratio and effect of bubble sizes for O2 and CO2 transfer
a) Carbon dioxide is approximately 20X more diffusible than oxygen . Oxygen transfer requires large surface area hence smaller bubble sizes per given volume. Whereas on the other hand larger bubbles are required for the removal of the carbon dioxide.
3. Transit time through the oxygenators.
a) The transit time through the bubble oxygenator is critical in determining the efficiency of oxygenation ie; exposure to gas and also a longer transit time is beneficial in that it aids in the defoaming of the oxygenated blood.
b) As more (defoaming substrate)polyurethane foam is added to an oxygenator it takes more time for the blood to transit through the device. More volume is held up in the oxygenator this is known as the dynamic volume hold-up.
4.
Designs of
diffusers and methods of creating or controlling bubble size
a) The tube with holes size of the holes controls the bubble size.
b) The plastic diffuser plate controls the size of the bubbles by various plastic sized holes.
5.
Gas to blood flow
rates
a) The gas to blood flow rates are important because this determines the carbon dioxide tension in the blood.
b) If there is high gas flow excess carbon dioxide removal takes place. This is the reason that carbon dioxide is usually added to the oxygen so that at least 2-5% of carbon dioxide is attained.
c) With low gas flow rates there is ample time for the carbon dioxide to equilibrate(due to its high diffusion rate) and hence no carbon dioxide is required in the ventilating gas.
d) At least equal gas/blood flow rates are necessary.
1. Debubbling/defoaming area
a) Made of polyurethane mesh sponge coated with a silicon compound (antifoam A)
i) Break surface tension of bubbles causing them to collapse
b) Includes polyester fabric surrounding mesh sponges acting as a screen filter [100 - 200 microns] - a final mechanism for the disruption of bubbles
c) Bubble elimination within the debubbler/defoamer & the arterial reservoir is a function of:
i) Mechanical disruption
ii) Surface area of defoaming section
iii) Residence time in the defoaming section
iv) Velocity of flow
v) Surface active agents
vi) Filtration
vii) Buoyancy
viii) Absorption
2. Arterial reservoir
a) Always positioned after the debubbling/defoaming area
b) Arterial outlet must be designed to prevent vortexing of blood when the device is operated at low reservoir volumes
GME transmission
Dependent on rated blood flow rate
1.
Types and surface
areas defoaming materials
a) Silicone defoaming agent is often used . The surface tension of the bubbles is reduced by the silicones, causing the bubbles to break and thus preventing the buildup of foam.
b) Various materials such as beads, sponges(polyurethane),shreds, meshes, fabrics are coated with the defoaming agent and installed in the debubbling chamber.
c) The materials are usually coated with a silicone compound by the process of dipping, the coating should be kept as thin as possible because of the chance of antifoam emboli. Some oxygenation also occurs in the defoamer.
d) Methods of defoaming may involve the flow through open cell sponge with Antifoam agent, Flow through mesh(wire, plastic), Flow through fabrics(mon-multifilament) or flow alongside the antifoam surface.
e) also the flow may occur through.
f) In summary debubbling the blood may includes action of :
g) surface active substances
h) settling
i) trapping
j) filtration/centrifugation
2.
Methods of
defoaming
a) Several methods (designs ) have been advocated for this they are based on bubble traps :
i) Upflow-defoaming
a) The blood carrying gas bubbles enters at the lower part of a glass or plastic cylinder this rises through a relatively narrow path then cascades into another cylinder located within the first one. There is an outlet at the top of the device which permits removal of excess gas.
ii) Downflow defoaming
a) The blood carrying gas bubbles is led against the wall and runs into a pool at the bottom of the chamber. Antifoam coated baffles are often inserted to assist.
iii) Helix blood settling column
a) A helical separating column allows for the separation of gas bubbles from blood.
3.
Filter sizes and
materials
a) Swank filter Dacron/nylon screen.
b) Bentley polyfilter polyurethane foam.
c) Patterson-Pall filter polypropylene and a filter element made of polyester 25-40um material
d) stainless steel mesh
e) polyurethane foam
f) nylon tricot mesh 100um
4.
Volume displacers
a) Volume displacers such as that in the Travenol sheet oxygenator (inflatable cuff which is used primarily to decrease the volume.
5.
Gas micro emboli
emission
a) Gaseous microemboli production detectable in the arterial line was directly related to the pO2(arterial) and inversely related to the arterial reservoir level and the gas to blood flow ratio. As the gas to blood flow ratio goes down the efficiency goes up so to does the emboli count.
b) As some secondary oxygenation does occur in the defoamer this can be a source of gaseous emboli as well.
c) Note that Gaseous emboli are only removed by defoamer and time.
Bubble oxygenators should not be used in CPB without a cardiotomy reservoir.
If an integral reservoir is to be used it should have sufficient defoaming and filtering material to cope with the additional volume and be able to filter the cardiotomy blood. In should be vented in order not to pressurise. Blood dynamic hold-up is usually increased with an integral reservoir. If an integral reservoir is to be used it should be filtered prior to oxygenating. If the reservoir is blocked then it is difficult to change if this is an integral reservoir.
a)Types, soft/hard shell
b) Filter sizes/materials
c) Air venting
d) Gas/blood ports sampling ports
e) Blood dynamic hold up
a) Types , plate,
coil, torpedo etc
Plate heat exchangers involved two sheets of metal usually coated with a non thrombogenic substance. This configuration offers good efficient transfer of heat. The plates may be flat, corrugated (Travenol heat exchanger has an air interface between the water and the blood therefore if there is a leak of the warming fluid it would be to air rather than to the blood.)
Coil configuration this consists of a hollow coil through which water circulates the disadvantage of this is that in a non disposable unit it is very difficult to clean.
Torpedo heat exchangers are less efficient it is basically a torpedo shaped object which is inserted inside of an arterial reservoir. This can be made more efficient by
the introduction of fins
b) Heat transfer
efficiency (coefficiency)
c = tbin -tbout/tbin-twater
c) Priming volume
With an integral heat exchanger this can be incorporated in with the oxygenator so that it reduces the priming volume. The use of fins in the heat exchanger also helps to reduce the priming volume.
The external heat exchanger obviously takes up more priming volume.
d) optimal water
flow/pressure
If water taps are used then sometimes pressure reducing devices need to be inserted. A satisfactory water flow through the heat exchanger is usually 15 to 20 LPM.
Pressure ratings range from 40 - 60 p.s.i (3 atm and up).
e) Theory of
countercurrent heat exchanger flow
Countercurrent heat exchanger flow allows for more efficient transfer of heat . Due to the gradients that are setup along the heat exchanger. If the blood and the water enters the same end of the heat exchanger a 30% reduction in efficiency occurs.
f) The effect of the
position of the heat exchanger in the oxygenator and perfusion circuit.
The position of the heat exchanger is the perfusion circuit can vary either on the venous side prior to oxygenation or in the arterial reservoir itself if the heat exchanger is in the arterial reservoir care must be taken to avoid creating a large gradient when rewarming due to the risk of air emboli from gases coming out of solution.
1.
Independence of CO2/O2
removal
a) Oxygen management
i) Since the FiO2 of the ventilating gas is fixed (100% ; N2 must not added), direct adjustments to the PaO2 cannot be made in most bubblers
2.
Vortex action in
arterial reservoir
a) Arterial outlet must be designed to prevent vortexing of blood when the device is operated at low reservoir volumes
b) Applicable to membrane oxygenator venous reservoir BUT blood still has to go via oxygenator
3.
Plasma haemoglobin
production
a) Important to use the lowest gas flow as high gas flows are associated with:
i) Increased gaseous microemboli
ii) Excessive haemolysis
iii) Protein denaturation
4.
Rated Blood flow
a) Maximum flow at which an oxygenator can receive blood at a standard venous inlet conditions & oxygenate it to a PaO2 of 95%
i) Capacity of the debubbling & defoaming area to handle the volume of blood that must be processed
5.
Advantages of
bubblers
a) Bubbler is efficient (compared to a non disposable system)
b) Inexpensive
c) Easy to prime
d) Easy to use
e) Ideal for short term cases