Contemporary Oxygenator Design: Shear Stress-Related Oxygen and Carbon Dioxide Transfer

Design of contemporary oxygenators requires better understanding of the influence of hydrodynamic patterns on gas exchange. A decrease in blood path width or an increase in intraoxygenator turbulence for instance, might increase gas transfer efficiency but it will increase shear stress as well. The aim of this clinical study was to examine the association between shear stress and oxygen and carbon dioxide transfer in different contemporary oxygenators during cardiopulmonary bypass (CPB). The effect of additional parameters related to gas transfer efficiency, that is, blood flow, gas flow, sweep gas oxygen fraction (FiO(2)), hemoglobin concentration, the amount of hemoglobin pumped through the oxygenator per minuteQhb, and shunt fraction were contemplated as well. Data from 50 adult patients who underwent elective CPB for coronary artery bypass grafting or aortic valve replacement were retrospectively analyzed. Data included five different oxygenator types with an integrated arterial filter. Relationships were determined using Pearson bivariate correlation analysis and scatterplots with LOESS curves. In the Capiox FX25, Fusion, Inspire 8F, Paragon, and Quadrox-i groups, mean blood flows were 4.8 +/- 0.9, 5.3 +/- 0.7, 4.9 +/- 0.7, 5.0 +/- 0.6, and 5.7 +/- 0.6 L/min, respectively. The mean O-2 transfer/m(2) membrane surface area was 44 +/- 14, 51 +/- 9, 60 +/- 10, 63 +/- 14, and 77 +/- 18, respectively, whereas the mean CO2 transfer/m(2) was 26 +/- 14, 60 +/- 22, 73 +/- 29, 74 +/- 19, and 96 +/- 20, respectively. Associations between oxygen transfer/m(2) and shear stress differed per oxygenator, depending on oxygenator design and the level of shear stress (r=0.249, r=0.562, r=0.402, r=0.465, and r=0.275 for Capiox FX25, Fusion, Inspire 8F, Paragon, and Quadrox-i, respectively, P<0.001 for all). Similar associations were noted between CO2 transfer/m(2) and shear stress (r=0.303, r=0.439, r=0.540, r=0.392, and r=0.538 for Capiox FX25, Fusion, Inspire 8F, Paragon, and Quadrox-i, respectively, P<0.001 for all). In addition, O-2 transfer/m(2) was strongly correlated with FiO(2) (r=0.633, P<0.001), blood flow (r=0.529, P<0.001), and Qhb (r=0.589, P<0.001). CO2 transfer/m(2) in contrast was predominately correlated to sweep gas flow (r=0.567, P<0.001). The design-dependent relationship between shear stress and gas transfer revealed that every oxygenator has an optimal range of blood flow and thus shear stress at which gas transfer is most efficient. Gas transfer is further affected by factors influencing the O-2 or CO2 concentration gradient between the blood and the gas compartment.