The membrane versus bubble oxygenator controversy

Haemocompatibility of membrane and bubble oxygenators

During clinical hypothermic cardiopulmonary bypass (CPB), the haemocompatibility of six groups of membrane oxygenators (Cobe CML2, Shiley M2000, Maxima, Bard HF4000, Bard HF5000, Capiox E has been studied in 60 patients having open-heart surgery. A standardized anaesthetic and perfusion protocol was used, during which the abilityof the perfusionist to achieve target blood gas values (PaO2 20kPa and PaCO2 5.3kPa: alpha-stat) using inline electrodes was assessed. Haemocompatibility was evaluated by measurement of platelet numbers and function, betathromboglobulin (BTG), plasma haemoglobin, complement (C3a des Arg) and white blood cell (WBC) count pre- and post-CPB. Platelet and WBC numbers were also measured every five minutes throughout CPB.

All oxygenators allowed the perfusionist to control blood gases adequately to prescribed levels. There were only minor differences in the degree and pattern of platelet depletion, reduction in platelet aggregation, elevation of BTG and C3a des Arg observed between oxygenator groups, which did not appear to be influenced by membrane type (flat plate versus hollow fibre). The membrane oxygenator haematological data was amalgamated with that obtained in previous clinical studies using membrane and bubble oxygenators (Cobe CML, Polystan Venotherm, Harvey H 1700, Bentley BIO-10, Bentley 1 0B, Bentley 1 OPlus, Gambro 10 and Shiley S100A HED) in which a similar evaluation protocol was employed.

Comparison of the percentage change in platelet count when the pre- and post-CPB values were compared, demonstrated statistically significantly less platelet depletion (p <0.001 ) in the membrane oxygenator groups (-0.2 ± 8.3%) when compared to the bubble oxygenator groups (-21.7 ± 8.7%). A significantly lower percentage rise in BTG was also observed in the membrane oxygenator group when compared to the bubble oxygenator groups (p <0.001 ). All oxygenator groups showed elevation of both WBC count and plasma haemoglobin with a nonspecific fall in platelet aggregation over the period of bypass but no significant differences could be found between the two types of oxygenator.

Membrane oxygenators, when compared to bubble oxygenators, exhibit lower GME production and improved haemocompatibility and allow superior blood gas control. Membrane oxygenators manifestly must be the oxygenator type of choice for clinical CPB.

A clinical evaluation of the performance characteristics of one membrane and five bubble oxygenators: haemocompatibility studies

The haemocompatibility of five different bubble oxygenators (Polystan venotherm, Harvey H-1700, Bentley BIO-10, Gambro 10 and Shiley S-100A HED) and one membrane oxygenator (Cobe CML) have been evaluated during standardized clinical perfusion for open-heart surgery in 48 adult patients. Control of arterial PO2 and PCO2 was an important feature of the evaluation protocol. Over the period of cardiopulmonary bypass (CPB) there was a marked difference in the mean percentage reduction in platelet count in the different oxygenator groups. Only 1% reduction in platelet count occurred with the Cobe CML membrane oxygenator group compared with, in the bubble oxygenator groups, 7% for the Gambro 10, 16% for the Harvey H-1700, 19% for the Shiley S-100A HED, 24% for the Bentley BIO-10 and 31% for the Polystan venotherm. The post bypass platelet count was significantly lower than the prebypass value in all oxygenator groups ( p < 0.05) except the Cobe CML and Gambro 10. The two oxygenator groups with the largest percentage reduction in platelet count (Polystan venotherm and Bentley BIO-1 0) demonstrated a significant reduction ( p < 0.05) in platelet aggregation over the period of bypass. Platelet depletion in the Harvey H-1700. Shiley S-100A HED, Bentley BIO-10 and Polystan venotherm oxygenators was associated with a significant fall ( p < 0.05) in mean platelet volume during the first 35 minutes of CPB due to the removal from the circulation of large, young, functionally more active platelets. Erythrocyte damage was minimal in all oxygenator groups and only a minor degree of leucopenia could be demonstrated during the first five minutes of CPB. Cardiotomy suction was not associated with significant changes in platelet numbers or platelet aggregation. When selecting the oxygenator for use in patients undergoing open-heart surgery, gas transfer characteristics and GME production together with the superior preservation of platelet numbers and function in the membrane oxygenator group and variable degree of platelet depletion and reduction in platelet aggregability demonstrated in the five bubble oxygenator groups, must be taken into account.

A clinical evaluation of the performance characteristics of one membrane and five bubble oxygenators: gas transfer and gaseous microemboli production

The gas transfer characteristics and gaseous microemboli (GME) production of five different bubble oxygenators (Polystan Venotherm, Harvey H-1700, Bentley BIO-10, Gambro 10 and Shiley S-100A HED) and one membrane oxygenator (Cobe CML) have been assessed during standardized clinical perfusion for open-heart surgery in 60 adult patients. The perfusionist attempted to maintain physiological levels of PaCO 2 (5 ± 1 kPa) and PaO2 (12 ± 2 kPa). Only 3% of blood gas values were within the normal range in the Bentley BIO-10 group compared with 17% for the Gambro 10, 20% for the Shiley S-100A HED, 31% for the Polystan Venotherm, 33% for the Cobe CML and 36% for the Harvey H-1700. The number of GME detected in the arterial line was significantly lower in the Cobe CML membrane oxygenator when compared with any of the five different bubble oxygenators (p < 0·001). The Polystan Venotherm released significantly less GME (p < 0·02) than the other oxygenators and the Shiley S-100A HED released significantly more GME (p < 0·02) than the other oxygenators except the Gambro 10. Low gas-blood flow ratios were not necessarily associated with low GME levels and inadequate oxygenation. This study provides meaningful data on which to base the choice of oxygenator, for clinical perfusions.

Hematological advantage of a membrane oxygenator over a bubble oxygenator in long perfusions

To determine whether the large volumes of cardiotomy suction which occur during long perfusions can obscure the hematological advantage of the membrane oxygenator (MO) over the bubble oxygenator (BO), we studied 23 patients undergoing a coronary artery bypass grafting operation with an expected perfusion time of 3 hours (MO group, N = 10, SciMed spiral coil; BO group, N = 13, Shiley 100-A). During MO perfusion we found significantly higher platelet numbers, better platelet function (adenosine diphosphate-induced platelet aggregation), and less hemolysis (plasma hemoglobin), than during the BO perfusion. After the MO perfusion we measured significantly shorter bleeding times (Simplate II) and fewer transfusions of blood products. However, blood loss and whole-blood transfusions 18 hours after perfusion did not differ significantly between both groups. So in coronary artery bypass grafting operations with long perfusion times (mean, 3 hours), the MO still causes significantly less platelet and erythrocyte damage than the BO, despite the large volumes of cardiotomy suction known to occur during these operations.

Initial clinical experience with a low pressure drop membrane oxygenator for cardiopulmonary bypass in adult patients

The new Travenol oxygenator is composed of 80 parallel blood pathways. Microporous membrane separates the blood and gas compartments. The membrane surface area is 3 m2, with a pore size of 0.01 microns. Venous blood drains directly from the patient through the oxygenator, then through an integral heat exchanger and into a reservoir, from which a single arterial pump returns the blood to the patient. The advantage of this configuration of membrane oxygenator is simplicity of setup and operation. A disadvantage that we have observed is an apparent variation in resistance to blood flow through the oxygenator during clinical perfusion. Construction changes in a later version of the oxygenator have reduced the resistance to flow through the blood pathway. This device has been used for 20 perfusions at moderate hypothermia (mean 31.8 degrees C) in patients up to 2.1 m2 body surface area for up to 313 minutes. Blood flow was 2.1 to 5.6 liters/min, partial arterial oxygen pressure 100 to 394 torr, partial arterial carbon dioxide pressure 19 to 57 torr (mean 37 torr) and, arterial pH 7.29 to 7.56 (mean 7.41). Oxygen transfer was as high as 230 ml/min. This integral oxygenator-heat exchanger-reservoir is operated like a bubble oxygenator, with direct venous drainage through the device and a single pump, but it uses a membrane oxygenator for gas exchange to eliminate the detrimental effects of bubbles.

Initial in vitro evaluation of a pediatric vortex-mixing membrane lung

A new design for a pediatric membrane lung is described in this paper. The lung consists of eight blood compartments, each having six U-shaped blood channels, with microporous PTFE membranes supported on rigid plates in such a way that the membranes form furrowed blood channels. Two rolling diaphragm pumps are attached to the open ends of the U-shaped blood channels; these pumps are operated in antiphase. Mean flow is provided by a roller pump placed at the inlet end of the membrane lung. Pulsatile blood flow within the blood channels produces successive vortex formation and ejection, leading to good blood mixing and high efficiency in gas transport. The design of the rolling diaphragm piston pumps ensures that the blood prime volume is low (280 ml), and the grouping of the pumps at one end of the oxygenator allows the driving mechanism to be simple and compact. The relatively wide blood channels (minimum width 0.5 mm) and vortex mixing make priming the membrane lung particularly easy. The membrane area is 0.39 m2. Preliminary performance testing of the pediatric membrane lung was undertaken by pumping blood around a circuit containing a roller pump, the membrane lung, and a bubble oxygenator (to adjust the blood gases at the inlet to the membrane lung). In five such experiments it was shown that the membrane lung transferred 80 ml O2/min and 120 ml CO2/min at a blood flow rate of 1.5 L/min.