• Surface area of gas exchange in oxygenator [<10 m2] are considerably less than that in the lung [70m2]
• To compensate [to enhance gas diffusion], membrane lungs have:
1) Increased blood path length (the distance that the blood
travels past the gas exchange area), thereby increasing the time available for blood exposure to the gas exchange area
2) Decreasing diffusion path by:
a) Minimising blood path thickness as much
as feasible by placing membranes as close together as possible without causing
an excessive pressure drop across the oxygenator
b) Secondary flows (induced eddies)are induced to promote
mixing and the bringing of deoxygenated blood closer to the exchange surface
3) Increasing driving gradient of gas (oxygen) [limited to 760
mmHg]
4) Increasing dwell time of blood in oxygenator [limited by
requirement for increased priming volume]
• Microporous membranes permit the gases to exchange through the
tiny pores in the membrane; this increases the efficiency of the membrane by
enhancing the speed of diffusion
• Blood surface tension prevents gas leakage into the blood via
micropores (providing excessive gas pressures do not occur)
• The CO2 transfer capability is much higher in
teflon or polypropylene microporous membranes than silicone rubber (as
previously used)
• The membranes are non wettable and the pores so small ( eg 0.3mm) that blood cannot be forced through
• The longer a membrane oxygenator is used the lower becomes its
efficiency due to:
i) coating of platelets on membrane surface thereby
increasing
diffusion
distance of gases
ii) water vapour may collect in gas pathway thereby also
increasing diffusion distance of gases
• CO2 transfer depends primarily on the
permeability of the membrane ie the membrane material itself; the thinner the
membrane, the increase in CO2 removal
• Membrane oxygenators have a design flow rate [the flow rate at which
a A—V oxygen saturation difference of 25% is maintained]; this design flow rate
cannot be changed — it is inherent in the specific oxygenator
• The faster the blood flow, the less mixing of blood around the
membrane surface (secondary blood flow), and the lower the PaO2
• Oxygen transfer rate increases with decreasing blood flow
• Increased gas flow dilutes & decreases PCO2 in the gas passages thus increasing CO2 removal from the blood [∫ increased ventilation in lung]
• Decreased gas flow increases PCO2
in the gas passages thus decreasing CO2
diffusion gradient thereby increasing CO2
retention in the blood [∫
reduced ventilation in lung]
• The boundary layer refers to the region of minimal blood flow
immediately around a stationary object (capillary membrane) within the blood
flow path; thus a diffusion boundary is generated which impedes gas exchange
• The major resistance to gas diffusion occurs in the blood phase;
efforts to improve gas exchange have focused on reducing this diffusion barrier
• Diffusion distances in the oxygenator [100-500mm] are much greater than in the lung [10mm]
• Thicker blood films require longer exposure to oxygenating
membrane and greater secondary flow
(mixing)
• Thinner blood films need high flow rates per unit area of
membrane surface
• This results in a high resistance, high pressure lung unless the
blood flow path is made short
• O2 transfer is controlled by the thickness of
the film as well as other factors of blood distribution and secondary flow
• As the driving force for oxygen across the membrane is very
high, the oxygenating capacity of a particular membrane is only dependent on
the thickness of the blood film
• Increasing the total gas flow rate changes ventilation (CO2 elimination) by reducing the gas phase CO2 partial pressures & by decreasing the gas phase
boundary layers for CO2 transfer
• In order to produce a thin blood film, the blood is squeezed
between and around multiple capillary tubes
• To compensate (partially) for the reduced surface area of the
membrane lung versus natural lung, secondary flows are induced to promote
mixing and the bringing of deoxygenated blood closer to the exchange surface
• These induced eddies or secondary flows of blood from the
primary stream into the diffusion boundary layer thereby decrease the thickness
of the diffusion boundary layer and thus increase gas transfer
• Eddies may be created by:
a) Making surface of membrane irregular
b) Positioning the elements within the flow stream to
disrupt the smooth flow
• The creation of these induced eddies is the major advance in
enhancing gas transport in a membrane lung
• Disadvantages of secondary flows are:
a) increased shear stress in boundary layers resulting in
cells & protein destruction
b) increased blood pressure drop across oxygenator
• Oxygenators with blood flow outside the fibers, blood flows
either perpendicular to the fibre bundle ( cross-current) or in the direction
of the fibre bundle (concurrent or counter-current to gas flow); cross-current
flow offers the advantage of producing secondary flows
• Elimination of blood streamlining through the oxygenator (direct
flow through the oxygenator without gas exchange) by adequate manifolding of
both inlet & outlet blood flows is a primary concern in designing the
oxygenator; particularly in hollow fiber designs where blood flows outside the
fibers
|
Comparison
of physical characteristics of membrane lung vs natural lung |
||
|
Characteristic |
Membrane
lung |
Natural
lung |
|
Surface area (m2) |
0.5
- 4 |
70 |
|
Blood path width (m ) |
200 |
8 |
|
Blood path length (m) |
250,000 |
200 |
|
Membrane thickness (m) |
150 |
0.5 |
|
Max O2
transfer (ml/min) STP |
400
- 600 |
2,000 |
KCPotgerÓ