Assessment of oxygen transfer in membrane oxygenators during clinical cardiopulmonary bypass

A theoretical model for evaluation of the design of a hollow-fiber membrane oxygenator

Geometric data are fundamental to the design of a contactor. The efficiency of a membrane contactor is mainly defined by its mass-transfer coefficient. However, design modifications also have significant effects on the performance of membrane contactors. In a hollow-fiber membrane oxygenator (HFMO), properties such as priming volume and effective membrane surface area (referred to as design specifications) can be determined. In this study, an extensive theoretical model for calculation of geometric data and configuration properties, and, consequently, optimization of the design of an HFMO, is presented. Calculations were performed for Oxyphan(®) hollow-fiber micro-porous membranes, which are frequently used in current HFMOs because of their high gas exchange performance. The results reveal how to regulate both the transverse and longitudinal pitches of fiber bundles to obtain a lower rand width and a greater number of windings. Such modifications assist optimization of module design and, consequently, substantially increase the efficiency of an HFMO. On the basis of these considerations, three values, called efficiency factors, are proposed for evaluation of the design specifications of an HFMO with regard with its performance characteristics (i.e. oxygen-transfer rate and blood pressure drop). Moreover, the performance characteristics of six different commercial HFMOs were measured experimentally, in vitro, under the same standard conditions. Comparison of calculated efficiency factors reveals Quadrox(®) is the oxygenator with the most efficient design with regard with its performance among the oxygenators tested.

Functional and biocompatibility performances of an integrated Maglev pump-oxygenator

To provide respiratory support for patients with lung failure, a novel compact integrated pump-oxygenator is being developed. The functional and biocompatibility performances of this device are presented. The pump-oxygenator is designed by combining a magnetically levitated pump/rotor with a uniquely configured hollow fiber membrane bundle to create an assembly free, ultracompact, all-in-one system. The hemodynamics, gas transfer and biocompatibility performances of this novel device were investigated both in vitro in a circulatory flow loop and in vivo in an ovine animal model. The in vitro results showed that the device was able to pump blood flow from 2 to 8 L/min against a wide range of pressures and to deliver an oxygen transfer rate more than 300 mL/min at a blood flow of 6 L/min. Blood damage tests demonstrated low hemolysis (normalized index of hemolysis [NIH] approximately 0.04) at a flow rate of 5 L/min against a 100-mm Hg afterload. The data from five animal experiments (4 h to 7 days) demonstrated that the device could bring the venous blood to near fully oxygen-saturated condition (98.6% +/- 1.3%). The highest oxygen transfer rate reached 386 mL/min. The gas transfer performance was stable over the study duration for three 7-day animals. There was no indication of blood damage. The plasma free hemoglobin and platelet count were within the normal ranges. No gross thrombus is found on the explanted pump components and fiber surfaces. Both in vitro and in vivo results demonstrated that the newly developed pump-oxygenator can achieve sufficient blood flow and oxygen transfer with excellent biocompatibility.

Comparison of three commercially available hollow fiber oxygenators: gas transfer performance and biocompatibility

The new generation of oxygenators have improved blood flow pathways that enable reduction in priming volume and, thus, hemodilution during cardiopulmonary bypass (CPB). We evaluated three oxygenators and two sizes of venous reservoirs in relation to priming volume, gas transfer, and blood activation. To compare priming volume, gas transfer, and biocompatibility of three hollow fiber oxygenators and two different size venous reservoirs, 60 patients were randomly allocated in groups to undergo cardiopulmonary bypass. In each group, an oxygenator with a different surface area and priming volume was used: 1.8 m2 and 220 ml (group 1, n = 23), 2.2 m2 and 290 ml (group 2, n = 20), and 2.5 m2 and 270 ml (group 3, n = 17). In groups 1 and 3, a large soft shell (1900 ml) venous reservoir was used, whereas in group 2, a smaller soft shell (600 ml) venous reservoir was used. Gas transfer was assessed by calculating the oxygen transfer rate for each group and per square meter for each oxygenator group. Partial arterial oxygen pressure (paO2) and partial arterial carbon dioxide pressure (paCO2) between the groups were assessed with forward stepwise regression analysis. Biocompatibility was evaluated through measurement of platelet numbers, complement activation products (C3b/c), coagulation (thrombin anti-thrombin III complex), and fibrinolysis (plasmin anti-plasmin complex). No differences were found in oxygen transfer rate per group. However, when correcting the oxygen transfer rate for surface area, group 1 demonstrated a higher oxygen transfer rate compared with group 2 (p < 0.05) at an FiO2 of 40 and 60% and compared with group 3 at an FiO2 of 60 and 70%. The regression analysis showed that the average arterial PO2 was the highest in group 3, i.e., 79.2 mm Hg higher than in group 1 (p < 0.001) and 73.5 mm Hg higher than in group 2 (p < 0.001). Group 3 also had the lowest average arterial pCO2, 0.57 mm Hg lower than in group 1 (p = 0.004) and 0.81 mm Hg lower than in group 2 (p < 0.001). During CPB, platelet numbers decreased significantly in all groups (p < 0.001), without differences between the groups. C3b/c levels increased in all groups during CPB. At cessation of CPB the C3b/c level in group 2 (398 nmol/L(-1)) was significantly higher compared to group 1(251 nmol/L(-1); p < 0.05) and group 3 (303 nmol/L(-1); p < 0.05). Thrombin anti-thrombin III complexes and plasmin anti-plasmin complex complexes increased during CPB to significantly high levels at cessation of CPB, but there were no differences between the groups. The oxygenator with the smallest surface area and lowest priming volume (group 1) had the highest oxygen transfer rate per square meter and showed the least blood damage, as depicted by complement activation. The oxygenator with the largest blood contact surface area and improved geometric configuration (group 3) showed the lowest oxygen transfer rate per square meter. However, this oxygenator elevated oxygen partial pressure the most and reduced carbon dioxide partial pressure the most. In group 2, where a smaller venous reservoir was used, the highest blood activation was observed.

Clinical evaluation of a new generation membrane oxygenator: a prospective randomized study

A new generation hollow-fibre membrane oxygenator (Spiral Gold) has been introduced by Baxter Healthcare (Irvine, CA, USA). The purpose of this study was to evaluate the operational performance of this device under clinical conditions and to compare it to the Univox Gold membrane oxygenator. Following institutional review board approval, and the obtainment of informed consent, 26 patients undergoing coronary artery bypass grafting were randomly assigned to either a Spiral Gold (Spiral) (n = 13) or Univox Gold (Univox) (n = 13) group. Study parameters were grouped into the following categories: haematological, haemodynamic, oxygenator performance and perioperative outcomes. All patients received identical surgical, anaesthesia and postoperative care. There were no statistically significant differences in either preoperative or operative parameters between groups. During cardiopulmonary bypass, the Spiral group had a significantly lower pressure drop (26.9 +/- 8.2 vs 46.7 +/- 16.2 mmHg, p < 0.001). The Spiral group had significantly lower plasma free haemoglobin levels during all time periods of CPB compared to the Univox group. Heat exchange coefficients were higher during the rewarming period in the Spiral patients (0.59 +/- 0.28) compared to the Univox group (0.36 +/- 0.19), p = 0.06. There were no differences in oxygen transfer between groups, but ventilation gas sweep rates and FiO2 levels were statistically lower in the Spiral group at two of the three sampling time periods. The ratio of ventilating gas sweep rate to blood flow rate was lower in the Spiral group (0.56 +/- 0.12) compared to the Univox group (0.74 +/- 0.23), p < 0.03. The Spiral Gold oxygenator had superior oxygen transfer efficiency and lower haemolysis rates than the Univox Gold oxygenator.

Comparison of the performance characteristics of three generations of membrane oxygenators: Univoxregistered, Univoxreqistered Goldtrademark and SpiralGoldtrademark

With continuous enhancement in oxygenator design, the question is raised as to how these changes actually impact the performance of the oxygenator. The recent addition of two new oxygenators by the Bentley Division of Baxter Healthcare Corporation provided us with a unique opportunity to compare the performance of each device and isolate the impact of each design change on performance. While the basic design and flow patterns have remained the same, application of the Duraflo II treatment has produced the Univox Gold and a change in the fibre-winding technique has produced the SpiralGold. This study compared the effects of heparin coating (Univox to Univox Gold) and fibre-winding (Univox Gold to SpiralGold) on gas and heat transfer and resistance to blood flow (pressure drop). Six oxygenators of each model were evaluated utilizing an in vitro single pass circuit, which first conditioned bovine blood to the Association for the Advancement of Medical Instrumentation (AAMI) venous standards. Blood flows of 4.0, 5.0, 6.0 and 7.0 l/min, FiO2 values of 1.0, 0.8 and 0.6, and gas-to-blood flow ratios of 0.5, 1.0 and 1.5 were chosen as test variables. Data generated included oxygen transfer, carbon dioxide transfer, arterial pO2, resistance to blood flow, and coefficient of heat exchange. The results indicate that the Duraflo II treatment does not have a significant effect on gas and heat transfer or resistance to blood flow. The fibre-winding technique employed with the new SpiralGold, however, has improved significantly gas exchange and arterial pO2 when compared with the previous Univox models. Resistance to blood flow and coefficient of heat exchange were not affected significantly by the winding technique.