Treating Cardiac Diseases Based On Catheter Based Tissue Heating

Abstract

In microwave ablation, electromagnetic energy would be delivered via a catheter to a precise location in a coronary artery for selective heating of a targeted atherosclerotic lesion. Advantageous temperature profiles would be obtained by controlling the power delivered, pulse duration, and frequency. The major components of an apparatus for microwave ablation apparatus would include a microwave source, a catheter/transmission line, and an antenna at the distal end of the catheter .The antenna would focus the radiated beam so that most of the   microwave energy would be deposited within the targeted atherosclerotic lesion.

 Microwave Cardiac Ablation

Another application of catheter based microwave heating is the treatment of abnormal heart rhythm, or cardiacarrhythmia .this life threatening disease , which affects over 300,000 Americans yearly, is caused by anomalous electrical activity in certain areas of the heart. Although drugs can be used to control the excessively rapid heart beat, mechanically removing or destroying  section of this tissue is more effective in curing arrhythmias. Selective catheter fed ablation, or excessive heating of tissue, destroys the region of the heart responsible for the anomalous  electrical activity.

Effects Of Electric Field

When oriented parallel to artery walls, the electric fields are the same on both sides of each LWC/ HWC boundary. Dissipated power is equal to  |E|2 / 2.Since the conductivity is much greater in HWC than LWC tissue, more power is deposited on the HWC side. Conversely, electric field perpendicular to artery walls are greater on the LWC side by the ratio H/ L so the power is preferentially dissipated on the LWC side by the ratio (ΣL/ ΣH )(H/ L)2 .Figure 2 shows schematically  relative sizes of the electric fields (arrow lengths)and deposited power (box volumes) for the two field orientation. Using the normal electric field polarization ensures that waves with radially polarized electric field deposit more power in the plaque layer than in the healthy artery wall.

Mca Applicators

        Microwave catheter ablation (MCA) antenna applicators have been used experimentally for cardiac ablation. These applicators are grouped in to two categories: the monopolar antennas and helical coil antennas. Both types radiation the normal  mode, with waves propagating perpendicular to the axis of the helix. Further, monopole antennas are usually one-half of the tissue wavelengths in length and generate a well-defined football-shaped heating pattern along its axis.

 Introduction

For decades, scientists have been using electromagnetic and sonic energy to serve medicine. But, aside from electro surgery, their efforts have focused on diagnostic imaging of internal body structures—particularly in the case of x-ray, MRI, and ultrasound systems. Lately, however, researchers have begun to see acoustic and electromagnetic waves in a whole new light, turning their attention to therapeutic—rather than diagnostic—applications. Current research is exploiting the ability of radio-frequency (RF) and microwaves to generate heat, essentially by exciting molecules. This heat is used predominantly to ablate cells. Of the two technologies, RF was the first to be used in a marketable device.

Conclusions 

        Two applications of microwave internal biological heating have been discussed. Both MABA and MCA consist of an antenna applicator fed by means of coaxial cable, which passes through a catheter. The antenna designs take advantage of polarization and phase effects of microwaves to create specific  power deposition patterns. MABA with a helix and mode filter balloon  uses the large differences in the dielectric characteristics of HWC and LWC tissue to preferentially heat and weld plaque while sparing healthy artery walls. The wide aperture MCA uses  an unfurlable spiral antenna within a balloon to generate a deep large ablation volume in diseased cardiac tissue. Theoretical studies have been validated with a variety of in-vitro and in-vivo experiments .There is less of a potential for tissue surface charring with microwaves than with RF ablation. Live animal studies indicate that MCA is well tolerated by animals.