Radiofrequency Energy Ablation

Radiofrequency catheter ablation is performed by delivery of a continuous, unmodulated, sinusoidal, high frequency (500,000 cycles per s) alternating electrical current

From: Contemporary Cardiology: Management of Cardiac Arrhythmias Edited by: L. I. Ganz © Humana Press Inc., Totowa, NJ

Fig. 1. (A) Endocardial radiofrequency ablation lesion demonstrating a round shape with sharp borders. (B) Histologic section of lesion in A, demonstrating hemispheric shape and extensive fibrosis. (From Jumrussirikul P, et al. Prospective comparison of temperature guided microwave and radiofrequency catheter ablation in the swine heart. PACE 1998;21[7]:1364-1374. Reproduced with permission.) (See color plate appearing in the insert following p. 208.)

Fig. 1. (A) Endocardial radiofrequency ablation lesion demonstrating a round shape with sharp borders. (B) Histologic section of lesion in A, demonstrating hemispheric shape and extensive fibrosis. (From Jumrussirikul P, et al. Prospective comparison of temperature guided microwave and radiofrequency catheter ablation in the swine heart. PACE 1998;21[7]:1364-1374. Reproduced with permission.) (See color plate appearing in the insert following p. 208.)

between the tip of an electrode catheter and a ground plate positioned on the back or chest. Because the ground plate has a much larger surface area than the tip, current density is focused at the smaller electrode. Current flows from the active electrode into the underlying tissue in alternating directions at high frequency. As a result of ionic agitation in the tissue, resistive heating ensues. Thus, the tissue underlying the ablation electrode, rather than the electrode itself, is the source of heat generation. This contrasts with a thermal probe or soldering iron, in which a resistive element positioned within the probe is the source of heat generation.

The tissue undergoing resistive heating transfers heat to surrounding tissues by conductive heat transfer. Since direct resistive heating falls precipitously with increasing distance from the ablation electrode, it is responsible for heating only a very narrow rim of tissue extending approx 1 mm beyond the ablation electrode (3). The majority of lesion volume is determined by the relative contributions of conductive heat exchange into surrounding tissue and convective heat loss toward the relatively cooler moving blood.

The principal mechanism of tissue destruction during radiofrequency catheter ablation procedures is thermal injury. Elevation of tissue temperature leads to denaturation of proteins and evaporation of fluids, resulting in subsequent tissue destruction and coagulation of tissue and blood (4,5). Temperature-dependent depolarization of myocardial tissue and loss of excitability occurs at temperatures greater than 43°C. Between tissue temperatures of 43°C and 50°C, there is reversible loss of excitability. Once the tissue reaches a temperature greater than 50°C, irreversible tissue injury occurs (6). Electrode-tissue interface temperatures in excess of 100°C cause tissue desiccation and plasma protein denaturation, which result in the formation of coagulum. The development of a coagulum results in a rapid increase in impedance, which leads to a dramatic decrease in current density, thereby limiting further lesion growth. As a result, the lesions created during radiofrequency catheter ablation procedures have well-demarcated borders and are small: 3-6 mm in width, depth, and length (see Fig. 1).

It is well known that deeper ablation lesions can be obtained by increasing the radiofrequency power, since this increases both the volume of resistive heating and the depth of passive conductive heating. However, because coagulation necrosis at the tissue-electrode interface results in a rise in impedance once temperatures reach 100°, the degree to which radiofrequency power can be increased is limited (7). Recently, methods for improved cooling of the electrode have been developed to allow delivery of higher radiofrequency power. These include the use of larger (8-mm) electrodes (7,8), which receive greater convective cooling by the blood, and saline-irrigated electrode tips, in which the electrode is actively cooled (9,10). The use of these catheters has resulted in higher success rates for selected arrhythmias, including ventricular tachycardia (VT) and isthmus-dependent atrial flutter (11-15).

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