Gas exchange efficiency in bubble oxygenators

Bubble Size

• The area of oxygenating surface is the combined surface area of the bubbles

• Each bubble is a capsule of oxygenating gas enclosed in a blood film

• Large bubbles oxygenate less efficiently & remove too much CO₂

• Small bubbles oxygenate well but remove too little CO₂

• Doubling a bubbles surface area will increase its volume by almost 4 times; a slight increase in oxygenating surface vastly increases the carbon dioxide gradient between the blood film and the enclosed gas [the mass transfer of CO₂ would be increased because of the greater volume of CO₂-free gas that must be brought up to diffusion equilibrium]

• As the bubble decreases, more bubbles may be contained in a given bubble column. Since it is the bubble volume that is most profoundly affected, the combined surface area of all the bubbles increases the area exposed to oxygen

• At a constant flow: oxygen exchange varies with the surface area of the bubbles; carbon dioxide exchange is roughly proportional to the gas-volume flow

• Pore diameter of the bubble diffuser principally determines the bubble size [other factors are a consideration]

Viscosity

• The thickness of the blood film in a bubble varies according to viscosity

• Less viscous blood forms a thinner blood film

• The thinner the blood film, the smaller the amount of blood to be oxygenated by the volume of gas within the bubble

Gas/blood Flows

• High gas flows with relatively low blood flow rates can result in large bubbles

• Excessive gas flows can cause excessive turbulence & associated blood trauma

• An excessively high oxygen flow rate blows off too much CO₂ & may unnecessarily raise the PO₂ [arterial pO₂ > 150 - 300 mmHg may facilitate gaseous microemboli formation especially on rewarming]; a low flow oxygenates insufficiently while CO₂ increases [hypercapnia dilates cerebral arteries resulting in cerebral oedema]

• Generally, a gas flow rate that equals the blood flow rate is considered ideal

Temperature

• Dilemma in that during hypothermia, as the patient’s oxygen demands are reduced, the perfusionist may reduce the flow of oxygen to maintain a desired pO₂, however may now not have enough removal of CO₂ [as fewer bubbles are produced per unit time]

• For this reason, when changes in gas flows are used to vary oxygen exchange, carbon dioxide may be added to the ventilating gases to minimise wide swings in CO₂ & pH.

Factors governing gas exchange
	Bubble size
	Gas volume
	Blood film thickness
	Blood film surface area
	Mean transit time
	Bubble column volume
	Gas flow rate
	Blood flow rate

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