Emboli from an extraluminal blood flow hollow fiber oxygenator with and without an arterial filter during cardiopulmonary bypass in a pig model

Extracellular vesicles: biomarkers and regulators of vascular function during extracorporeal circulation

Extracellular vesicles (EVs) are generated at increased rates from parenchymal and circulating blood cells during exposure of the circulation to abnormal flow conditions and foreign materials associated with extracorporeal circuits (ExCors). This review describes types of EVs produced in different ExCors and extracorporeal life support (ECLS) systems including cardiopulmonary bypass circuits, extracorporeal membrane oxygenation (ECMO), extracorporeal carbon dioxide removal (ECCO₂R), apheresis, dialysis and ventricular assist devices. Roles of EVs not only as biomarkers of adverse events during ExCor/ECLS use, but also as mediators of vascular dysfunction are explored. Manipulation of the number or subtypes of circulating EVs may prove a means of improving vascular function for individuals requiring ExCor/ECLS support. Strategies for therapeutic manipulation of EVs during ExCor/ECLS use are discussed such as accelerating their clearance, preventing their genesis or pharmacologic options to reduce or select which and how many EVs circulate. Strategies to reduce or select for specific types of EVs may prove beneficial in preventing or treating other EV-related diseases such as cancer.

Current Understanding of How Extracorporeal Membrane Oxygenators Activate Haemostasis and Other Blood Components

Extracorporeal membrane oxygenators are used in critical care for the management of severe respiratory and cardiac failure. Activation of the coagulation system is initiated by the exposure of blood to synthetic surfaces and the shear stresses of the circuit, especially from device pumps. Initial fibrinogen deposition and subsequent activation of coagulation factors and complement allow platelets and leucocytes to adhere to oxygenator surfaces and enhance thrombin generation. These changes and others contribute to higher rates of thrombosis seen in these patients. In addition, bleeding rates are also high. Primary haemostasis is impaired by platelet dysfunction and loss of their key adhesive molecules and shear stress causes an acquired von Willebrand defect. In addition, there is also altered fibrinolysis and lastly, administration of systemic anticoagulation is required to maintain circuit patency. Further research is required to fulyl establish the complexities of the haemostatic changes with these devices, and to elucidate the mechanistic changes that are mainly responsible so that plans can be made to reduce their complications and improve management.

Bubble tracking through computational fluid dynamics in arterial line filters for cardiopulmonary bypass

Gaseous embolism is still a concern in cardiopulmonary bypass, and the use of arterial line filters (ALFs) is widespread because of their recognized role in increasing safety. Currently, the methods used for the optimization/evaluation of ALF designs are based on a trial-and-error approach. In this work, we propose a method to objectively assess the air-handling capabilities of ALFs, using computational fluid dynamics (CFD) simulations to track the trajectory of large numbers of bubbles traversing the device under examination. We applied the CFD method to ALF prototypes, whose design featured the classical purge/screen configuration, to establish the relative roles of the bubble-trap and bubble-barrier deairing effects. Simulations were run at the maximum rated blood flows. Clusters of hundred bubbles in the micro- to macroembolic size range (10-1,000 microm diameter) were tracked. The results quantified the relative amount of bubbles whose fate is either to reach the purge line or to be intercepted by the filter screen. For microbubbles, the analysis detailed how the screen barrier is exploited, mapping the percent distribution of the intercepted bubbles along the screen surface. We conclude that the method proposed increases knowledge and awareness in designing an optimal exploitation of the filtration mechanisms involved in ALFs.

Dynamic bubble trap can replace an arterial filter during cardiopulmonary bypass surgery

Objective: The arterial filter (AF) and the dynamic bubble trap (DBT) reduce the number of air microbubbles passing through these devices. The aim of the study was to confirm that the DBT diminishes microbubbles in the arterial line similar to, or better than, the AF, and can replace it.

Methods: In a clinical study, we evaluated 60 patients undergoing cardiopulmonary bypass surgery, divided into two groups (30 patients each). In the first group, we used an open extracorporeal system, and in the second group, a closed system. For 15 patients in each group, the AF was incorporated, the other 15 patients received the DBT. The microbubbles were counted before and after the AF or DBT, using two-channel-ultrasonic Doppler devices.

Results: The exposure of patients to small bubbles (<45μm) is significantly higher in the AF than in the DBT group. The DBT reduces large bubbles (<45μm) better than the AF, with a rate exceeding 16%.

Conclusion: The use of the DBT instead of the AF yields higher air micro-bubble removal efficacy, allowing replacement of the AF, assuming the AF is used for air removal purpose only.

Prebypass filtration of cardiopulmonary bypass circuits: an outdated technique?

Filtration of cardiopulmonary bypass (CPB) priming fluid before connection of the circuit to the patient was first accomplished by arterial line filtration. When dedicated prebypass filters (PBFs) with smaller pore sizes became available, a large number of particles could be found on the filter surface. In recent years, modern manufacturing methods for CPB circuit components were believed to be associated with a reduced number of particles found in components of extracorporeal circuits, making separate filtration of CPB priming solution unnecessary. Microemboli generated during the preparation and priming procedure of the CPB circuit may consist of either solid particles or gaseous emboli and may contribute to patient morbidity. Endotoxins found in infusion solutions and CPB priming solutions may trigger inflammatory responses when administered into the circulatory system. Filtration of crystalloid CPB priming solutions with a PBF consisting of a filter membrane with a pore size of 0.2 microm was found to effectively reduce the number of microemboli. Infusion filters with a filter pore size of 0.2 microm were found to reduce the endotoxin contamination in infusion solutions. Prebypass filtration with filters containing pores of 0.2 pm should be a necessity for contemporary perfusion practice.

In vitro comparison of blood pump induced platelet microaggregates between a centrifugal and roller pump during cardiopulmonary bypass

Platelets are consumed during cardiopulmonary bypass (CPB) and mechanical ventricular assistance, at least partly as a result of the formation of platelet microaggregates in the blood pump. There is no commonly accepted method currently available to detect platelet microaggregates during the use of CPB or left ventricular assist devices (LVAD). The purpose of this study was to develop a flow cytometric method for the quantification of platelet microaggregates generated in blood pumps, and to evaluate the effect of cellular fragments from hemolyzed erythrocytes on the perioperative assessment of platelet counts during CPB.

METHOD

Fresh human anticoagulated blood (1IU heparin/mL, activated clotting time 250+/-24 sec.) was circulated for 120 minutes in an artificial circulatory system, containing either a centrifugal pump (CP) or roller pump (RP). Whole blood was used to quantify platelet consumption and to detect circulating platelet microaggregates in a flow cytometer. Platelet consumption was additionally analyzed using an automated "Coulter" blood cell counter. Hemolysis was analyzed by measurement of plasma free hemoglobin (fHb), as well a by flow cytometric detection of red blood cell (RBC) fragments.

RESULTS

Flow cytometric analysis demonstrated significantly more circulating platelet aggregates and platelet consumption in the RP than in the CP (p<0.01). Quantification of RBC fragments and plasma free hemoglobin (fHb) levels also indicated significantly increased hemolysis in the RP than in the CP (p<0.01). In contrast, the Coulter count data indicated less platelet consumption in the PP compared to the CP.

CONCLUSION

Fragments from hemolyzed erythrocytes have the same size distribution as intact platelets and the number of RBC fragments correlates with the extent of pump-induced hemolysis during CPB. Our data suggest that assessment of platelets by "Coulter counting" cannot distinguish platelets from RBC fragments and may underestimate platelet consumption in the presence of hemolysis during CPB. We conclude that flow cytometry is more accurate in the perioperative assessment of platelet count and platelet aggregation during CPB and LVAD support.

Can an oxygenator design potentially contribute to air embolism in cardiopulmonary bypass? A novel method for the determination of the air removal capabilities of neonatal membrane oxygenators

At present, air handling of a membrane oxygenator is generally studied by using an ultrasonic sound bubble counter. However, this is not a quantitative method and it does not give any information on where air was entrapped in the oxygenator and if it eventually was removed through the membrane for gas exchange. The study presented here gives a novel technique for the determination of the air-handling characteristics of a membrane oxygenator. The study aimed at defining not only the amount of air released by the oxygenator, but also the amount of air trapped within the oxygenator and/or removed through the gas exchange membrane. Two neonatal membrane oxygenators without the use of an arterial filter were investigated: the Polystan Microsafe and the Dideco Lilliput. Although the air trap function of both oxygenators when challenged with a bolus of air was similar, the Microsafe obtained this effect mainly by capturing the air in the heat exchanger compartment while the Lilliput did remove a large amount of air through the membrane. In conclusion, the difference in trap function was most striking during continuous infusion of air. Immediate contact with a microporous membrane, avoidance of high velocities within the oxygenator, pressure drop, transit time and construction of the fibre mat all contribute to the air-handling characteristics of a membrane oxygenator.