|M.Sc Student||Roytberg Anat|
|Subject||Study of Efficient Pacing of the Myocardium Using External|
High Intensity Focused Ultrasound
|Department||Department of Biomedical Engineering||Supervisor||Professor Emeritus Dan Adam|
|Full Thesis text|
External pacing of the myocardium may be advantageous in two clinical aspects: cardiac arrest leading to sudden death and application of Cardiac Resynchronization Therapy (CRT) in congestive heart failure patients. In cardiac arrest, there is no electrical activity and no contractions of the myocardium, and it does not respond to defibrillation shocks. Fast non-invasive pacing is crucial for saving the patient under such emergency conditions. The CRT technique is used to increase the cardiac output. Selection of the proper pacing site is critical to CRT's success and could be made non-invasively prior to the pacemaker implantation. The myocardium is susceptible to mechanical stimulation, as explained by the mechano-electrical feedback.
In this study mechanical stimulation was applied by using high intensity focused ultrasound (HIFU), which enables localization of the transmitted energy without significant energy absorption in the surrounding tissues. In order to minimize energy losses in the non-target area, a differentiation of the target area properties from its surroundings was achieved by creation of free micro-bubbles. The study hypothesis is that in order to generate efficient myocardial activation, HIFU can be used for localized production of micro-bubbles in-vivo, immediately followed by generation of a mechanical impulse. This sequence may achieve high probability of producing a Premature Ventricular Contraction (PVC's).
In-vitro studies were performed to optimize the conditions of generating focused pulses at a combined sequence (a mostly-negative pulse, followed by a mostly-positive pulse). In order to optimize the parameters of the mostly-negative pulse, an ability to distinguish between the stages of cavitation was developed. While using a mostly-negative pulse, creation of stable cavitation requires a peak negative pressure between 2.04-4.08 MPa.
The in-vivo study was performed on animals (rats) with healthy, beating heart. ECG and aortic blood pressure were measured and R-wave detection was made in order to synchronize the ultrasonic pulse with the ECG signal. PVC's were documented as an immediate response to the ultrasonic pulse. The results imply that the best timing for applying the ultrasound pulse is during the end of T-wave. It also appears that increasing the duration of the mechanical thump makes HIFU more effective in creation of the relevant bio-effect.
Most of the PVC’s were detected while applying the combined sequence. This implies that the main mechanism for triggering PVC’s is radiation force, but it doesn’t disqualify the role of cavitation effects.