Mapping and radiofrequency ablation of ventricular tachycardia

Abstract

Radiofrequency catheter ablation has become the nonpharmacologic treatment of choice in patients with a variety of supraventricular arrhythmias. Small discrete lesions are produced by delivering 20-40 W of unmodulated 500 kHz RF energy to the tip of a standard 4 mm electrode catheter. Resistive heating of cardiac tissue occurs at the point of tissue contact. Successful treatment of these arrhythmias may be achieved in greater than 90% of cases. The results of RF catheter ablation for the treatment of ventricular tachycardia (VT) are variable. RF catheter ablation in patients with normal hearts who may have either idiopathic left VT arising from right ventricular outflow tract is highly effective with success rates approaching 100%. These tachycardias usually arise from a small focus and therefore the area required for ablation is small and easy to target. Unfortunately, most patients who have VT have abnormal ventricular function, frequently a previous myocardial infarction. In these patients, the tachycardia circuits may be large and complex. The efficacy rate of RF ablation for VT using current technology is much lower. This presentation will focus on our development of a strategy for successful ablation of VT post myocardial infarction (MI). Accurate analysis of the VT substrate is crucial for successful ablation. A post-MI model of sustained VT was created in swine by injecting agarose gel beads following PTCA balloon occlusion of the LAD coronary artery. Surviving animals returned for programmed electrical stimulation 4-6 weeks later. Stable sustained VT was induced in 35 animals. This VT could be reproducibly initiated and terminated. A multielectrode "basket" catheter was percutaneously inserted prior to VT induction to map endocardial electrical activation. The "basket" catheter (Constellation, EP Technologies, Sunnyvale, CA) consists of eight self-expanding nitinol struts with 64 symmetrically arranged electrodes. The catheter is capable of both recording and pacing. Using this system we prospectively analyzed the induced VTs in these animals. Bipolar endocardial signals were obtained from the catheter during sinus rhythm and VT. Signals were filtered at 30-500 Hz and recorded multichannel recorder (EP LabSystem, Corp.). Endocardial recordings demonstrated fractionated electrical activity in the zone of infarction during sinus rhythm. Early presystolic activity was recorded during VT as well as middiastolic potentials. Reset of VT was seen in 5 animals. Features of classic entrainment as well as concealed entrainment were demonstrated in 12 animals. These features suggest that the mechanism of VT is endocardial reentry, as in humans. RF ablation was performed by guiding a large-tip ablation catheter to the appropriate "basket" electrode by means of a "homing device" Successful RF ablation of VT was demonstrated in this model. Computer algorithms for analysis of the zone of slow conduction are being developed. Clinical post-myocardial-infarction VT is now mapped and treated in patients using this system.