6. Transport gases in blood

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1 6. Transport gases in blood

2 Artificial Lung G a s E x c h a n g e 2

3 Gas Exhange The absolute transfer rate of oxygen and carbon dioxide through the artificial lung is dependent on a number of factors:- The degree of convective mixing achieved in the blood phase, which is dependent on the particular configuration and orientation of the fiber bundle gas exchange surface area of the fiber bundle blood flow rate through the device characteristics of the blood itself, such as hemoglobin concentration, viscosity, oxyhemoglobin saturation and carbon dioxide content 3

4 Gas Exhange The absolute transfer rate of oxygen and carbon dioxide through the artificial lung is dependent on a number of factors:- fiber length or gas flow rate through the fiber bundle composition of the oxygenating and ventilating gas 4

5 Gas Exhange In order to maximize gas exchange, it is critical to achieve excellent convective mixing of the blood as it passes through the fiber bundle of the artificial lung The designer of artificial lungs has several parameters that may be manipulated to achieve excellent gas transfer and maintain low blood side pressure losses 5

6 Gas Exchange Consider blood flowing perpendicular to a bank of uniform fibers of outer diameter d The frontal area, A f, is the product of the overall height, H and the width, W The void fraction, or porosity of the fiber bundle, is defined as:- V f = void volume/total volume 6

7 Gas Exchange The tighter packed the fibers, the lower the porosity 7

8 Gas Exchange Smaller diameter fibers allow the designer to increase the effective gas exchange surface area for a given artificial lung configuration In general, to achieve low pressure losses and high gas transfer, the porosity of the fiber bundles in artificial lungs should be in the range from about 0.5 to

9 Gas Exchange A cross-flow configuration, in which the blood path is perpendicular to the fiber bundle, is important to achieve good convective mixing The diameter of the fibers may also vary, but in the case of microporous polypropylene hollow fibers, there are a limited number of sizes typically available The outer diameter usually falls within the range of 0.02 to 0.04 cm 9

10 Gas Exchange This can be seen by the relation:- in which As, the fiber surface area in this example, also increases with frontal area, blood path length or lower void fraction 10

11 Gas Exchange Using microporous polypropylene hollow fibers, the amount of surface area required to achieve clinically significant amounts of gas transfer is between 1.5 to 2.0 m 2, but may be less if small diameter fibers are used 11

12 Gas Exchange By specifying the void fraction and fiber diameter, The design engineer may vary the frontal area and path length to meet certain size constraints and surface area requirements However, the blood path length must not be inordinately long, otherwise the blood-side pressure losses will be excessive 12

13 Gas Exchange It is critical to keep the gas side pressures well below the blood side pressures in order to avoid the consequences of gas embolism 13

14 Gas Exchange According to Poiseuille s Law:- 14

15 Gas Exchange The gas side pressure drop (DP g ) will be dependent on the following:- the gas flow rate (Q g ) viscosity of the flowing gas (μ) the length of the individual fibers (l) radius of each fiber (r) total number of fibers (n) 15

16 Artificial Lung H e m o d y n a m i c C o m p a t i b i l i t y 16

17 Hemodynamic Compatibility In addition to adequate gas exchange capability, the artificial lung must be hemodynamically compatible with the cardiovascular system 17

18 Hemodynamic Compatibility Two approaches may be followed:- The artificial lung derives its blood flow directly from the heart, thereby avoiding the need for an external, mechanical blood pump A composite artificial heart-lung system may be devised, incorporating a mechanical pump to provide blood flow through the lung 18

19 Hemodynamic Compatibility The latter approach has been taken by Japanese investigators, who have used a pneumatically driven blood pump to generate the pressure to force blood through the hollow fiber artificial lung 19

20 Hemodynamic Compatibility The advantages of having an incorporated blood pump are that the blood-side pressure losses across the lung become less of a design issue and adequate blood flows are easily achieved 20

21 Hemodynamic Compatibility On the other hand, the addition of a blood pump to the lung introduces another level of complexity to the device Makes the device less compact Increases the potential for mechanical device failure Associated with higher rates of thromboembolism Direct trauma to the blood elements 21

22 Hemodynamic Compatibility Compliance of the artificial lung is another important design feature The pulmonary circulation is characterized by low impedance and high compliance The compliance of the natural pulmonary circulation acts as a windkessel to dampen the pulmonary arterial pulse and to allow blood to flow through diastole Lack of compliance would lead to greatly diminished flow during diastole and the development of RV strain 22

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