Flow-induced platelet activation in mechanical heart valves

Background and aim of the study: A study was conducted to measure in vitro the procoagulant properties of platelets induced by flow through mechanical heart valves. Methods: The procoagulant activity of platelets was measured using a real-time assay of platelet activation state (PAS), which was based on a modification of the prothrombinase method. Acetylated prothrombin was used instead of normal prothrombin in this assay in order to eliminate the positive feedback effect of thrombin. This enabled a direct comparison between thrombin generation rates in the assay and the flow stresses that induce platelet activation. Gel-filtered platelets (10(5) per mul) were circulated through a left ventricular assist device with two Bjork-Shiley monoleaflet mechanical heart valves mounted in opposition, and platelet activation state was measured over 30-min time courses. The results were compared with two configurations in which the leaflet motion of one of the valves was restricted (severely restricted and mildly restricted), mimicking defective function of a compromised valve in vivo, and with a control lacking valves. Results. The severely restricted valve activated the platelets at a rate eight-fold higher than with unrestricted valves, and three-fold higher than with mildly restricted valves. Both restricted valves activated platelets at rates significantly higher than either the control (no valves) or the unrestricted valve. Conclusion: Flow through compromised mechanical heart valves causes platelet activation, which can be measured with a modified prothrombinase assay system. The ability to perform sensitive quantitative measurements in cardiovascular devices in vitro may have a significant impact on the design and development of these devices.