Blood flow within a membrane oxygenator

a) Venous reservoir

• Receives venous return as well as drainage from the cardiotomy reservoir, and serves as the atrium for the arterial pump

• Hard or soft shell

• Consists of a defoaming layer and a filtering screen (100-200 mm)

• Compensates for flow discrepancies between passively flowing venous return & pump driven arterial output

b) Arterial pump

• Pumps blood through the oxygenator and into the patient

• Membrane oxygenators are positioned after the pump because of the high resistance in most membrane oxygenators requires blood to be pumped through them

c) Heat exchanger

• Located proximal to the gas exchanging component to minimise the risk of releasing microbubbles of gas from the blood, which could occur if the blood is warmed after being saturated with gas

• Composed of tubular stainless-steel

• May be located directly before pump (after venous reservoir) or after pump (immediately before oxygenator)

d) Membrane oxygenator

• eg composed of microporous polypropylene

• Blood flows around semipermeable capillary tubules

• Gas flows within semipermeable capillary tubules

• Blood flow may be perpendicular (to induce eddies) or parallel to direction of gas flow (counter-current or co-current)

• Baffles in blood channels cause blood to mix by forming secondary currents

• Blood may exist at the lowest most aspect of the oxygenator thereby minimising pumping of gas emboli to patient

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