Radiofrequency catheter ablation (RFA) of cardiac tissue has been well-characterized in animal models. Both unipolar (HUA88) and bipolar (TAN91) radiofrequency ablation cause similar acute and chronic effects, the extent of which is determined by the amount of energy applied to the tissue. It has been shown that the application of microwave (LIE98,WOR02), cryogenic (DUB98,WOR02), laser, (WOR02), and ultrasonic (WOR02) energy result in similar cellular damage as radiofrequency energy.
Radiofrequency energy produces cell damage by heating the tissue and causing coagulative necrosis (cell death). These areas of cell death will eventually be replaced with fibrosis (scar tissue). (WOL94,REM98,LIE00) As one increases the energy applied to tissue, one will see an increase in the coagulation area and an increase in thermal damage. (REM98) Interestingly, this technique has been used to stiffen the palate (roof of the mouth) to prevent snoring. (MAI00) It has been speculated that RFA may cause direct valve tissue injury in patients undergoing ablation of accessory pathways.(MIN92) In fact, Wolfsohn et al (WOL94) found that when they accidentally applied energy to one of the valves of the heart they caused a fibrous scar. Another group (TAN91) found pathologic changes in the tricuspid valve when AV node or His bundle ablation was attempted in dogs. These changes included edema, swelling of the spongiosa, degenerative change of collagen fibers, endocardial thickening, and slight fibrosis and destruction of the valvular tissue. Additionally, collagen is the substance in the body that forms the support structure of tissue. Collagen in heart tissue denatures (breaks down) and contracts when heated to >65° CelciusVIC02. In fact, the chordae tendinae also contract when heated to >65° Celcius (VIC02). This contraction of heart tissue has also been noted by another group.LIE00 Though this technique has not been studied on heart valves one can infer that similar cellular changes may occur when this energy is applied to cardiac valves. RFA has been used to stiffen the palate (roof of the mouth) to prevent snoring. (MAI00) In fact, Wolfsohn et al (WOL94) found that when they accidentally applied energy to one of the valves of the heart they caused a similar fibrous scar.
The acute (1-7 days) effects of RFA (50-300J) include a lesion that is round-oval in shape, centrally-pale with varying amounts of a hemorrhagic rim.(HUA88) Occasionally, there is a mural thrombus over the lesion (~20% occurrence). There is central coagulation necrosis, peripheral contraction band necrosis, and interstitial edema and fibrinous material on the endocardium. (TAN91) During days 3-7, one observes a rim of granulation tissue composed of proliferating capillaries and fibroblasts admixed with acute and mononuclear inflammatory cells. (HUA88) Finally, the chronic effects are usually evident at 4-6 weeks after RFA. Chronic lesions are round-oval to irregular in shape, well-circumscribed, and fibrotic.(HUA88) The acute edema has almost disappeared and there are almost mature collagen fibers and a mild increase in elastic fibers.(TAN91) Much of these pathological observations were made in dog models however, Huang et al (HUA88) found that dog tissue damage was very similar to the tissue damage found in postmortem humans. RFA lesion size is, in part, determined by the amount of energy applied (typically, 50-700Joules). However, energy levels of 100-300J have been shown to cause similar lesion sizes (LxWxDepth, ~4.8×4.6×4.3mm) (HUA91). Earlier work by Huang et al (HUA87), demonstrated the application of unipolar RFA with an energy level of 50J caused an average subacute (7-10d) lesion size of 4x4x2mm.
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