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 computerized system forthe study of oxygen uptake and haemodynamics during cardiopulmonary bypass

A computerized system which incorporated haemodynamic, temperature and arterial pH (pHa) monitoring was developed to record data and to improve temperature and pHa control during hypothermic cardiopulmonary bypass (CPB). The system was evaluated in the course of a study of haemodynamics and metabolism during CPB. It was planned to have three 10 minute periods of study in 24 patients, during which time arterial and nasopharyngeal temperatures would be maintained between 27° and 29°C and pHa between 7.38 and 7.42. Repeated recordings of haemodynamic variables, temperature and pHa were made during each period. Only 59 of a possible 72 periods were obtained due to shortage of time at stable hypothermia. The mean number of total readings was 73.5 (70.1, 77.0) and the mean number of acceptable readings was 71.2 (67.3, 75.1). Good nasopharyngeal temperature control was achieved, with all the 95% confidence intervals of the means falling within the prescribed limits. Control of arterial temperature was less good as there were five periods in which the 95% confidence intervals of the means fell outwith the prescribed range. Poor pHa control was achieved since there were 16 periods in which the 95% confidence intervals of the mean were outside the preset range. The system functioned well in recording data and resulted in good temperature control, but poor pHa control.

Perfusion safety: past, present, and future

Safe cardiopulmonary bypass has been paramount from its first use in the early 1950s until the present. The original perfusion circuits incorporated complex feedback loops and multiple safety devices. As circuits improved and became simpler to operate, advances in safety did not always keep pace. Surveys have illustrated areas that needed improvement and extra attention has been focused on those problems. As the field of perfusion evolved, so has the perfusionist. Perfusion has progressed from on-the-job training to formalized training, certification, and accreditation, and is now approaching national standardization. As the computer age proceeds, the use of safety devices and feedback mechanisms whose developments have been aided by the newly available technologies increases. As the 21st century approaches, cardiopulmonary bypass will continue to become safer, but the perfusionist must continue to stay up-to-date in education and remain vigilant while in the operating room.

Isoflurane and hypothermic cardiopulmonary bypass: vasodilation without metabolic effects

During cardiopulmonary bypass, isoflurane may have beneficial effects on systemic oxygen uptake and vascular resistance. For this reason, the effects of isoflurane during low-flow (1.6 L/min/m2), hypothermic (27 degrees to 29 degrees C) cardiopulmonary bypass on systemic hemodynamics and oxygen uptake were studied in 20 patients in a cross-over experiment. Mean arterial and central venous pressures were measured during two consecutive periods of 10 minutes' duration. Blood samples were aspirated at the end of each period from the arterial and venous lines and analyzed for oxygen content. The concentration of isoflurane in the arterial samples was also determined. Systemic oxygen uptake and vascular resistance were calculated. Isoflurane had no significant effect on systemic oxygen uptake. Significant inverse relationships between blood isoflurane concentration and both mean arterial pressure and systemic vascular resistance were found. It is concluded that isoflurane is a vasodilator under the abnormal conditions of hypothermic cardiopulmonary bypass, but has no effect on systemic oxygen uptake.

Development of a digital adaptive control system for PO2 regulation in a membrane oxygenator

Regulation of gas exchange in artificial lungs (oxygenators) during cardiopulmonary bypass is normally achieved by manual control of the gas composition and flow in response to intermittent sampling of the arterial partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2). Manual control often results in abnormal blood gases which have been implicated in patient morbidity as well as influencing perfusion safety. Fine control of PaO2 and PaCO2 may be achieved by a combination of an in-line blood gas monitoring system and a membrane type oxygenator which is automatically regulated. The overall dynamics of the oxygenation process and control system components are complex and have nonlinear, multivariable and time-varying characteristics. Consequently, an adaptive control system approach is necessary. The implementation of a digital self-tuning control regime for PaO2 is described here. The controller is based on an explicit Linear Quadratic Gaussian (LQG) self-tuning control design which is presented using a polynomial equation approach. The controller performance was investigated in in vitro experiments. The self-tuner performed satisfactorily with various sensor/oxygenator combinations for blood flow and temperature load disturbances. In contrast, a nonadaptive (proportional-integral, PI) type of control system was found to be unsuitable.

Assessment of oxygen transfer in membrane oxygenators during clinical cardiopulmonary bypass

Although functional replacement of the heart and lungs by a pump and oxygenator is a widespread surgical procedure, no widely accepted technique for describing gas exchange in oxygenators exists. In this study, 8 types of commercially available membrane oxygenator (2 flat sheet membrane, 4 gas in hollow fibre membrane and 2 blood in hollow fibre membrane) have been studied during clinical cardiopulmonary bypass. O2 transfer increased with blood flow rate but the O2 transfer at a given blood flow was lower than that obtained by the manufacturers in laboratory studies. Overall O2 transfer coefficients were calculated from the ratio of O2 transfer rate to an O2 difference expressed either as an O2 partial pressure or an O2 concentration. Specific O2 transfer coefficients (overall coefficient divided by membrane area) were similar for oxygenators with a flat sheet or gas in hollow fibre membrane configuration. The two types of oxygenator with blood in hollow fibre membranes had significantly lower (P less than 0.01) specific O2 transfer coefficients. This study shows that oxygenator gas transfer characteristics can be studied in the clinical environment and that O2 transfer coefficients can be related to oxygenator design features.

Priming solutions for cardiopulmonary bypass: comparison of three colloids

The present study was designed to compare the differences in the clinical effects of three colloidal solutions, albumin, urea-linked gelatin, and succinyl-linked gelatin, when used as priming fluids for cardiopulmonary bypass (CPB) under alpha-stat conditions. A consecutive series of 105 patients scheduled for cardiac surgery were randomized into three identically managed groups, except for the CPB prime. Variables relating to acid-base status, oncotic activity, metabolism, coagulation, and postoperative evaluation were measured. Marked differences in acid-base status, colloid osmotic pressure, additional prime requirements, blood lactate, urine output, and the need for buffer solutions occurred among groups, with the succinyl-linked gelatin group having better results than the other groups. Changes in hemodynamics, oxygen consumption, and blood-glucose levels during CPB did not vary among groups. There were also no important intergroup differences in hematologic and clotting variables or postoperative parameters such as blood loss or use of blood products. Electrolyte changes were similar except for a significant increase in ionized calcium that occurred in the urea-linked gelatin group after bypass. The results indicate that succinyl-linked gelatin is an adequate and safe alternative to human albumin for use as a colloid during CPB under alpha-stat conditions.

The effect of high pressure on microporous membrane oxygenator failure

A retrospective study to determine the relationship between early microporous membrane oxygenator (MMO) failure and blood pressure at the MMO outlet (Pmo) was conducted using data collected with 19 dogs (22 +/- 1 kg, mean +/- SEM) undergoing routine normothermic cardiopulmonary bypass. Because gas flow was maintained at a high level, it could not be used to control CO2 exchange. Instead, blood PCO2 was controlled by adding CO2 to the sweep gas. Blood PO2 was controlled as suggested by the manufacturer, by adjusting the %O2 in the gas phase (g). Blood flow was 2575 +/- 54 ml/min; Pmo ranged from 173 to 790 mm Hg; and hematocrit was 33 +/- 1%. O2 exchange was calculated from blood gas parameters. Changes in the diffusion potential of O2 (delta PO2) and CO2 (delta PCO2) and MMO performance (P, taken as oxygen exchange normalized to a diffusion potential of 100 mm Hg) indicated MMO failure. Initial values, taken within 60 min of bypass initiation, were compared to final values taken at 226 +/- 9 min of bypass. Despite higher final delta PO2 (411 +/- 9 vs. 538 +/- 19 mm Hg, p less than 0.0001 paired t-test) and delta PCO2 (18.6 +/- 2.4 vs. 30.5 +/- 4.7 mm Hg, p less than 0.0017), arterial blood PO2 decreased (159 +/- 15 to 89 +/- 6 mm Hg, p less than 0.0005) and PCO2 increased (36.4 +/- 1.5 to 46.1 +/- 3.0 mm Hg, p less than 0.0039), and the performance decreased [24.5 +/- 1.1 to 20.1 +/- 0.7 (ml/min)/(100 mm Hg), p less than 0.0001].(ABSTRACT TRUNCATED AT 250 WORDS)