Blender setting for different oxygenators and at hypothermia. 1

Use and principle of air-oxygen blenders. 1

Gas exchange capacity of varying surface area membrane oxygenators. 12

Effect of turbulent flow in relation to gas exchange in membrane oxygenation versus laminar flow.. 18

Blender setting for different oxygenators and at hypothermia

1. Bubble oxygenator

a) oxygen

b) Mixtures oxygen & carbon dioxide

i) Mixtures including nitrogen or room air are not recommended

c) Direct adjustments of PaO₂ cannot be made in most bubble oxygenators

d) Adjustments to PaCO₂ are made by altering

i) Gas flow rates

ii) Titration of CO₂ into gas source

a) Increases PaCO₂

2. Membrane oxygenator

a) The ventilating gas used is a mixture of 100% oxygen and room air, allowing alteration in FiO₂

b) The mechanical blender mixes the two gas sources (oxygen & air) to achieve FiO₂ values between 1.0 (100% oxygen) & 0.21 (100% room air)

c) Decreasing the FiO₂ from 1.0 will decrease the arterial PO₂

d) Arterial PCO₂ is controlled as with the bubble oxygenator; by gas flow rate (sweep rate)

_i) Increased gas flow will eliminate more CO₂ from the membrane surface thereby increasing the gradient from the blood-to-gas phase and decreasing the PaCO₂

3. Hypothermia

a) Reduced VO₂ with reduced temperature

b) Issue of reducing FiO₂:

i) Addition of nitrogen?

a) Source of N₂ emboli

ii) See increasing PaO₂

a) But is a problem?

(1) Probably yes with bubbler

(a) Reduced gradient for resorption of O₂ bubbles

(2) Issue of O₂ free radicals?

Use and principle of air-oxygen blenders

· The ventilating gas used is a mixture of 100% oxygen and room air, allowing alteration in FiO2

· The mechanical blender mixes the two gas sources (oxygen & air) to achieve FiO2 values between 1.0 (100% oxygen) & 0.21 (100% room air)

· Screwing in the knob reduces the diameter of the air entry port but simultaneously increases the diameter of the oxygen entry port thereby increasing the percentage contribution of oxygen to the final gas mixture

· Formula used:

Q_oxy: flow of oxygen

Q_air: flow of air

· Membrane oxygenator

Decreasing the FiO2 from 1.0 will decrease the arterial PO2

Gas exchange capacity of varying surface area membrane oxygenators

Maxima Plus PRF 2.3 m² SA

Effect of turbulent flow in relation to gas exchange in membrane oxygenation versus laminar flow

• 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

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