Haematological effects of membrane oxygenators vs bubble oxygenators

No direct gas—blood interface

i) Less trauma to red cells

• Haemolysis still occurs secondary to suctioned blood from pericardium & pleura (any suctioned blood is considerably traumatised)

ii) Less trauma to platelets

• Can expect to see a smaller drop in platelet count, and a more reversible situation with less platelet dysfunction and early rebound in numbers

iii) Less trauma to white blood cells

• May relate to the phenomena of complement activation, white cell activation and release of free radical species

• see iv)

iv) Less protein denaturation

• Therefore less complement activation

• However, contradictory results have been published when comparing bubble vs membrane oxygenators: may relate more to materials used in a particular oxygenator than the mechanism of oxygenation per se

Foreign surface—blood interface

• As plasma proteins come into contact with a foreign surface within the extracorporeal circuit (both bubble & membrane oxygenator), denaturation occurs

• Blood contact with foreign surfaces also activates platelets, which leads to aggregate formation and subsequent thrombocytopenia as platelets are consumed in the process

• Membrane oxygenators produce fewer aggregates than bubble oxygenators

• The oxygenator requires a large nonblood surface to effect gas exchange & accounts for 50 - 85% of the SA of the extracorporeal circuit

• Membrane & bubble oxygenators both reduce platelet numbers & compromise their function, but the time course differs

• In membrane oxygenators, platelets react with the huge synthetic surfaces almost instantly; platelets counts drop to 20% of initial levels within 2 minutes of surface contact; however, the platelet levels rises to about 50% of initial levels in 2 hours of bypass (as platelets detach from the surfaces)

• In bubble oxygenators, the initial decline in platelet count is less severe (partly due to haemodilution) presumably due to the smaller synthetic surface area of bubble oxygenators for platelet-synthetic surface interaction. However, each bubble partly denatures plasma proteins at the bubble surface & each bubble exposes the platelets to a new surface. The initial decline in platelet count is irreversible and continues to progressively & exponentially fall with time; During clinical perfusions lasting > 1 hour, membrane oxygenators better preserve platelets & other blood components than bubble oxygenators