Membrane types & materials

 

·                     The membrane oxygenator mimics more closely the natural pulmonary capillary by interposing a thin membrane between the blood & the gas

·                     Membrane oxygenators have a large surface area (2 - 4 m2) that is either fanfolded, coiled, or shaped into capillary tubes

·                     In order to produce a thin blood film, the blood may be contained within multiple capillary tubes, between plates, or squeezed between multiple capillary tubes

 

 

1.                   TRUE MEMBRANE OXYGENATOR

 

·                     Silicon rubber is a homogenous, nonporous membrane

 

a)                   COIL [Scrolled] TYPE

i)                    ‘True’ membrane (vs microporous)

ii)                   Consists of silicone rubber sheets coiled in a cylindrical fashion

iii)                 The silicone rubber is nonporous; there is a complete barrier between the blood & gas

iv)                 A gas manifold system is designed to distribute the gas into the coil while blood is allowed to flow on the outside. A screen in the gas path ensures proper gas distribution

v)                  Gas transfer depends totally on diffusion of gas through the membrane material

vi)                 Gas transfer through the membrane is dependent on:

vii)               diffusion distance of the gas in blood

viii)              driving pressure of either gas in the membrane

ix)                  permeability of the membrane

x)                   Eg: Sci-Med oxygenator

xi)                  Advantages

a)                   Able to maintain stable CO2 & O2 for long periods of time (weeks)

b)                  Used primarily in ECMO

xii)              Disadvantages

a)                   Costly to manufacture

b)                  High priming volume

 

2.                   MICROPOROUS MEMBRANE OXYGENATOR

 

a)                   Polypropylene is a heterogenous, microporous, hydrophobic membrane

b)                  The microporous membranes are developed by stretching the membrane material and forming ‘rents’ in the substance of the membrane that act as ‘pores’ for gas transfer

c)                   The pores are 0.03 to 0.07 mm in diameter and cover ³ 50% of the membrane surface

d)                  As the surface is hydrophobic & the pores are so small, the blood water is not ultrafiltered; inhibit both gas & serum leakage across the membrane

e)                   Gas may be forced into the blood path if the pressure in the gas path exceeds that in the blood path

f)                    The microporous membrane provides the necessary gas transfer capability via the micropores [without need for excessive surface area], where there is a direct blood-gas interface with minimal resistance to diffusion

 

g)                  FLAT [Parallel] PLATE TYPE

i)                    One of the two primary designs of microporous membrane structure currently used

ii)                   Folded envelope design; the membrane material is folded, accordion style and encased

iii)                 Gas supply is directed to one side of the Z-fold while blood is delivered to the other side

iv)                 Screens are placed in the gas & blood paths to ensure proper distribution of blood & gas supply

 

h)                  HOLLOW FIBRE TYPE

i)                    Two basic types: blood phase may be on the inside or the outside of the fibers — however decreased oxygenator function from thrombosis within the fibers may occur with the blood travelling within the fibers

ii)                   Hollow fibres of membrane material are used to separate the blood & gas phases

iii)                 The number of fibres utilised is dependent on the total surface available for gas exchange

iv)                 Blood flow may be concurrent, crosscurrent or countercurrent to the flow of gas within the fibres

v)                  Major consideration in design is adequate permeation of blood throughout all fibers ie elimination of blood streamlining (direct flow through the oxygenator without gas exchange)

 

i)                    Comparison of Hollow fibre versus Flat Plate

i)                    Equivalent haemocompatibility

ii)                   Equivalent gas exchange performance

iii)                 Different gross air handling [see Perfusion 90; 117]

 

Method of operation of membrane oxygenators

The gas passageways in a membranous lung are akin to the alveoli

 

1.                   Carbon dioxide

a)                   CO2 transfer depends primarily on the permeability of the membrane

b)                  By increasing O2 flow, CO2 is more rapidly blown through and out of the gas pathway

c)                   Increased gas flow dilutes & decreases PCO2 in the gas passages thus increasing CO2 removal from the blood [= increased ventilation in lung]

d)                  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]

 

2.                   Oxygen

a)                   O2 transfer is controlled by the thickness of the film as well as other factors of blood distribution and secondary flow

b)                  Oxygenation of blood is accomplished simply by altering the FiO2 of gas supplied to the oxygenator

c)                   Because the membrane separates gas and blood phases, nitrogen can be safely added without risk of gas emboli formation (as it does with bubble oxygenators)

d)                  For a particular membrane, as the driving force of O2 is exceedingly high, the oxygenating capacity for this membrane is dependent only on the thickness of the blood film — the thinner the blood film the more efficient the oxygenation & the higher the PaO2

e)                 The faster the blood flow, the less mixing of blood around the membrane surface (secondary blood flow), and the lower the PaO2

 

Gas transfer/ diffusion control

 

1.                   Oxygen

a)                   Oxygen, delivered to the gas path of the membrane oxygenator, diffuses along a concentration gradient through the membrane material to the blood flowing through the device

b)                  Oxygen transfer directly varies with

i)                    Total surface area of membrane

a)                   Not controllable on CPB

ii)                   Oxygen gradient developed across membrane

a)                   Controllable on CPB by varying FiO2

iii)                 Permeability of material to oxygen

a)                   Not controllable on CPB

c)                   Indirectly varies with

i)                    Blood film thickness in blood path of device

a)                   Must be limited to have a minimum diffusion distance for oxygen

b)                  Another technique is to augment the primary flow of blood to increase the exposure of blood to the membrane surface

(1)                 Mechanical turbulence (screens)

(2)                 Altering surface of membrane material

(3)                 Shaking oxygenator (vortex blood film)

c)                   Parallel plate

(1)                 Uses blood path screens

d)                  Cylindrical design & blood outside fibres

(1)                 Tightness of membrane packaging to limit blood film thickness

e)                   Blood inside fibres

(1)                 Internal diameter of fibre to limit blood thickness

 

2.                   Carbon Dioxide

a)                   Carbon dioxide delivered in the venous blood diffuses along a concentration gradient through the membrane material to the gas path of the device

b)                  Carbon dioxide transfer directly varies with

i)                    Total surface area of membrane

a)                   Not controllable on CPB

ii)                   Carbon dioxide gradient developed across membrane

a)                   Controllable on CPB by varying flow of gas

iii)                 Permeability of material to Carbon dioxide

 

 

3.                   Water vapour

a)                   Also subject to transfer in membrane oxygenator

b)                  Unique in that its partial pressure varies only with temperature

i)                    Transferred at higher rates at warm blood temperatures

c)                   Water can condense in the cooler gas path of the oxygenator

i)                    Results in reduced membrane surface area for gas exchange

d)                  Gas path design should allow condensed water to be eliminated from device

 

 

Advantages & disadvantages of membranes including: membrane resistance & compliance; membrane rupture; macro/micro emboli; hollow fibre-blood outside vs inside

 

1.                   Hollow fibre Blood inside versus outside

a)                   Membrane/fibre rupture

i)                    Blood phase within fibre + rupture

a)                   Blood will spill into gas phase

b)                  May decrease total surface area available for gas exchange

ii)                   Blood phase outside fibre + rupture

a)                   Blood will only spill into the affected fibres

b)                  Will not significantly affected total surface area for gas exchange

b)                  Visualisation of air bubbles within oxygenator

i)                    Blood phase outside of fibres allows for visualisation of air bubbles during priming or operation

ii)                   Blood phase within fibres does not allow for visualisation of bubbles

c)                   Fibre plugging

i)                    Blood phase within fibre design may have problems when the tubes become plugged, leading to decreased gas transfer 2° decreased blood transit times through the remaining patent fibres

d)                  Priming volumes

i)                    Blood phase outside fibre has allowed small surface area membranes with associated small priming volumes

 

2.                   Membrane resistance

a)                   Because of the high resistance to blood flow through most membrane oxygenators, blood must be actively pumped through the oxygenator

b)                  Excessive pressure through the membrane may cause rupture, creating either a blood leak or air embolism

 

3.                   Oxygenator failure

a)                   Spiral wound silicon membranes

i)                    Prone to membrane displacement

b)                  Parallel Plate

i)                    Poor shim pressure control

ii)                   Variable exhaust back pressure

 

4.                   Membrane Rupture

a)                   Unlikely for air embolism to occur as always more pressure in blood phase - unless have blocked gas outlet (but modern design oxygenators have more then one gas outlet)