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Key words: cardiac, ablation, arrhythmias, radiofrequency, RF, tech support
Radiofrequency (RF) catheter ablation is a commonly used procedure for the management of cardiac arrhythmias. The procedure uses RF electrical energy delivered through one or several catheter electrode(s) to generate thermal lesions, which disrupt aberrant electrical pathways that cause potentially lethal cardiac arrhythmias. This nonsurgical technique has found popularity within the last decade because it is curative and lacks the problems that arise from either the implantation of a device or the adherence to a specified drug regimen. From 1989 to 1996, the number of cardiac ablation procedures performed annually in the United States has increased from 450 to approximately 25,000. It is estimated that over a million people in the United States suffer from conditions that are treatable with catheter ablation. There still remain many unanswered questions regarding the procedures safety and efficacy. CDRH is concerned about the different parameters affecting the development of lesion sizes and the extent of damage done in the myocardium.
OST is currently studying the safety issue of healthy cardiac tissue damage during ablation. This is an especially important research area as the tools required to treat the newer clinical indications involve electrodes of higher power and larger size, as well as the simultaneous use of multiple electrodes. Previous clinical research demonstrated that lesions produced using constant temperature ablation tend to produce more consistent lesion dimensions than lesions produced in a constant power mode. However, constant-temperature lesions still vary considerably, and the correlation to steady-state temperature is not strong. A second safety aspect being studied is the nonuniform distribution of energy from ablation electrodes.
OST scientists have constructed specialized in vitro systems to measure near-electrode heat transfer phenomena under near physiologic conditions in order to simulate transient lesion development and uncover possible safety concerns. The ablation evaluation system consists of an RF signal source with a frequency that can be set at any clinically used frequency (usually several hundred KHz), power meters, an ablation catheter, arrays of several thermocouple sensor, and a specially designed ablation test chamber that is hydrodynamically controlled. Specially designed solid (gel) and liquid phantom materials were developed to simulate the electrical properties of blood and tissue. The solid phantom material can be shaped to various geometries to produce stable nonlinear flow fields and simulate vascularization in cardiac tissue.
Figure 1 - Cardiac Ablation Test System
Figure 1 shows a diagram of the test system. The system is hydrodynamically regulated by two pressure head tanks, which control the pressure differential across the test chamber. The centerline velocity in the vicinity of the ablation electrode is approximately proportional to the square root of the pressure differential. This arrangement allows for predictable flow patterns, enabling surface boundary layer analysis and detailed examination of near-electrode heat transfer phenomena. Thermistor probes are inserted into the solid phantom at positions immediately below the surface and are separated by approximately 1.5 mm. The temperature at each of these thermistor locations, as well as the amount of power being delivered to the ablation, are monitored through a custom-designed data acquisition system.
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Figure 2 - Ablation Temperature
Figure 2 shows preliminary data generated with an aggregate flow rate of 8.7 L/min and a power application of 4.6 Watts over a period of 20 seconds. The maximum temperature was attained at the thermistor located at the surface (54.75 degrees). Figure 2 also demonstrates that temperature changes near the surface of tissue occur with a faster time constant than in positions deeper in the tissue phantom.
Further analysis of the data indicated that at areas in the tissue phantom greater than 3.5 mm from the surface, there is a latent heating phenomena. The tissue temperature in these regions continues to rise after the power delivery has been discontinued. This phenomenon may be useful in the understanding of lesion development in certain ablation procedures. The transient temperature data was used to develop a preliminary numerical model.
Figure 3 - Model Results![[GRAPH]](img3.gif)
Figure 3 shows results computed using the computer model. This data, while preliminary, provides valuable insight into the complexity of lesion development. Detailed evaluations of a variety of flow patterns and perfusion models will be explored by OST scientists in FY 98.
OST scientists are also studying the safety issue of healthy cardiac tissue damage during ablation by constructing and using a special model that is composed of materials that are electrically and thermally equivalent to the human heart. This system is composed of simulated tissue and blood materials. Measurements of the heat transfer phenomena near the electrodes have been performed under near physiologic conditions. The relative merits of maintaining constant temperature heating by adjusting the RF power delivered to ablation electrodes versus the delivery of constant power are being studied, as well as surface cooling due to blood flow. The different parameters affecting the development
of lesion sizes and the extent of damage done in the myocardium will be identified via experiments with the simulated heart model and with thermodynamic and electro-magnetic computer analysis. Using the findings of their experiments, OST scientists are able to provide relevant requirements for specific data from device manufacturers. These requirements have been included in the latest revisions of the CDRH cardiac ablation device reviewers guidance document.