Principles of operation of membrane oxygenators: i) coil type; ii) plate type iii) hollow fibre type

• 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

TRUE MEMBRANE OXYGENATOR

i) COIL TYPE

• ‘True’ membrane (vs microporous)

• Consists of silicone rubber sheets coiled in a cylindrical fashion

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

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

• Gas transfer through the membrane is dependent on:

i) diffusion distance of the gas in blood

ii) driving pressure of either gas in the membrane

iii) permeability of the membrane

• Eg: Sci-Med oxygenator

• Advantages

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

• Used primarily in ECMO

• Disadvantages

• Costly to manufacture

• High priming volume

MICROPOROUS MEMBRANE OXYGENATOR

• Hollow polypropylene fibres containing pores less than 1 micron [pores less than one micron are required to inhibit both gas & serum leakage across the membrane

• 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

ii) FLAT PLATE TYPE

• One of the two primary designs of microporous membrane structure currently used

• Folded envelope design

iii) HOLLOW FIBRE TYPE

• 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

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

• 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)

Principles of Operation

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

Carbon dioxide

• CO₂ transfer depends primarily on the permeability of the membrane

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

• 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]

Oxygen

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

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

• 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)

• 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

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