During RF catheter ablation, local temperature elevation can result in coagulum formation on the ablation electrode, resulting in impedance rise. A recent study has also demonstrated the formation of a so-called soft thrombus during experimental ablations. This deposit poorly adhered to the catheter tip and did not cause an impedance rise. The mechanism of soft thrombus formation and the role of the natural coagulation system are unknown. The formation of a soft thrombus was investigated experimentally by temperature-controlled RF delivery in heparinized blood at different heparin concentrations and in serum. After 60 seconds of RF delivery in blood with an electrode target temperature of 80 degrees C, a semisolidified mass had formed around the ablation electrode at all heparin concentrations. A smaller but structurally similar deposit had formed after RF delivery in serum. Scanning electron microscopy analysis revealed that these deposits consist of denaturized and aggregated proteins, and not of a classical thrombus. The formation of the so-called soft thrombus resultsfrom heat induced protein denaturation and aggregation and occurs independent of heparin concentration and also in serum. The formation of such deposits may occur at temperatures well below 100 degrees C, which may have important consequences for further development of ablation technologies.

Medical devices, intended for blood contacting applications, undergo extensive in vitro testing followed by animal and clinical feasibility studies. Besides the use of materials known to be intrinsically blood-compatible, the surface of such devices is often modified with a coating in order to improve the performance characteristics during blood exposure. In vitro evaluation of blood-device interactions accompanies the product development cycle from the early design phase using basic material geometries until final finished-product testing. Specific test strategies can vary significantly depending on the end application, the particular study objectives and variables of interest, and cost. To examine the degree to which findings derived from two different in vitro approaches complement one another, this report contrasts findings from a simple multipass loop model with findings from a simulated cardiopulmonary bypass (CPB) model. The loop model consists of tubular test materials, with and without surface modification, formed into valved Chandler loops. The CPB model has an oxygenator with and without surface modification connected to a reservoir and a blood pump. The surface modifications studied in this report are the Carmeda BioActive Surface and Duraflo II heparin coatings. Common blood parameters in the categories of coagulation, platelets, hematology, and immunology were monitored in each model. Ideal models employ the optimal level of complexity to study the design variables of interest and to meet practical cost considerations. In the case of medical device design studies, such models should also be predictive of performance. In the more complex and realistic simulated CPB model, experimental design and cost factors prevented easy/optimum manipulation of critical variables such as blood donor (use of paired samples) and heparin level. Testing in the simpler loop model, on the other hand, readily offered manipulation of these variables, and produced findings which overlapped with observations from the more complex CPB model. Thus, the models described here complimented one another. Moreover, conclusions from consistent findings, such as favorable responses associated with the heparin coatings, between the two models were considered to be more robust.

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objekt innerhalb kurzer Zeit bestimmen. Bei den Messungen wird sinusformig modulierte thermische Energie, z. B. in Form von sinusformig modulierter W-estrahlung, an der gesamten Oberflache des MeRobjekts zugefiihrt. Von der Oberflache breitet sich nach Abklingen des Einschwingvorgangs eine nahezu ebene thermische Welle in das Material hinein aus. Durch Messung der ortlichen Temperaturschwingungen an jedem Oberflachenpunkt und durch Berechnung der ortlichen Phase und Amplitude wird ein Phasenund ein Amplitudenbild fiir die gesamte Oberflache erzeugt. Im Prinzip 1aRt sich nun aus beiden Bildern die ortliche Verteilung des Warmeubergangskoeffizienten oder der Temperaturleitzahl berechnen. Allerdings hangt die Amplitude der Temperaturschwingungen im Gegensatz zur Phase vom Betrag der zugefiihrten thermischen Energie und zusatzlich vom optischen Absorptionskoeffizienten und vom Emissionskoeffizienten ab, falls die anregende Energie in Form von Warmestrahlung zugefiihrt wird. Daher ist die Amplitudenauswertung mit erheblich groReren Schwierigkeiten verbunden, als die Phasenauswertung.Zur Messung der ortlichen Temperaturschwingungen wird ein schneller Infrarotscanner eingesetzt, dessen Abtastfrequenz auf die Frequenz der thermischen Anregung abgestimmt ist. Mit dieser Einrichtung lassen sich die kleinen Temperaturanderungen an der Oberflache beriihrungslos erfassen. Die an den Oberflachenpunkten ermittelten Temperaturdaten werden an-schlieRend mit einem PC ausgewertet. Da eine periodisch variierende GroRe (ortliche Temperatur) mit bekannter Frequenz gemessen wird, konnen bei der Analyse frequenzselektive Techniken zum Einsatz kommen, durch welche sich auch bei hohem Rauschuntergrund genaue Ergebnisse erzielen lassen. Gleichzeitig kann die Frequenz der thermischen Anregung auf die Eigenschaften des MeRobjekts abgestimmt werden, urn in einen moglichst optimalen MeRbereich zu gelangen.