טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentKishon Amir
SubjectStudy of the Processes Involved in Cardiac Stimulation by
Ultrasound Pulses
DepartmentDepartment of Biomedical Engineering
Supervisor Professor Emeritus Dan Adam
Full Thesis textFull thesis text - English Version


Abstract

Ultrasound is known to be safe and cause almost no permanent harm. This is the major reason why usage of ultrasound medical imaging is widespread. Studies have shown that a single pulse of high intensity ultrasound can produce effects on the heart. There is evidence of Premature Ventricular Contractions (PVCs) in human caused while being subjected to strong ultrasound fields. PVC may also occur while using ultrasound contrast agent combined with imaging at high Mechanical Index (MI). It has been hypothesized that mechanical waves such as ultrasound waves, can influence cardiac tissue by the Mechano-Electrical Feedback (MEF) phenomenon that characterizes this tissue. In this study a model has been suggested to describe two processes associated with the MEF in a cardiac cell, in order to understand the cause of PVC during ultrasound radiation. The first is the current through stretch-activated channels (SAC) and the second describes changes in affinity between calcium and Troponin-C (TnC), which occur during mechanical changes (stretch-compression). The model is based on the Luo-Rudy model. The relationship between an external force and sarcomere length is modeled, and then influence of the ultrasonic pressure is studied - whether it may be strong enough to create PVC, according to the phenomena mentioned above. The results of the simulation show that during stretch SACs cause the Action Potential Duration (APD) to expand while the changes in the calcium-TnC affinity cause the APD to shorten. During compression, on the other hand, SAC cause the APD to become shorter, while the calcium-TnC affinity cause the APD to expand and even to cause early depolarization. Analysis of the model demonstrates that the major influence is of the calcium-TnC affinity, thus the APD becomes shorter in stretch and expands during compression. The time of producing the stretch affects the result as well; stretch during the last period of the action potential increases the probability for early after-depolarization. The ultrasound radiation force, as modeled here without the amplification produced by contrast agents, did not cause early after-depolarization, but affected the APD in a manner that may cause an early after-depolarization. Thus, this study confirms the interaction between ultrasound insonation and the potential generation of PVC’s.