Unstable monomorphic VT

Two approaches are being evaluated for ablation of scar related VT that is difficult to map with a roving catheter because of haemo-dynamic instability or instability of the re-entry circuit. One approach involves defining the area of scar from its low amplitude sinus rhythm electrograms (fig 25.2, top panels); then selecting portions of the scar likely to contain a part of the re-entry circuit based on the VT QRS morphology or pace mapping; and then placing a series of anatomically guided ablation lesions through the abnormal region.15 16 Ellison and colleagues targeted the likely re-entry exit region in five patients with frequent unmappable VT. All three patients with prior myocardial infarction were free of recurrent VT during follow ups of 14-22 months. The procedure was not successful in the two with non-ischaemic cardiomyopathy.15 Marchlinski and colleagues applied a more extensive series of RF ablation lines through regions of scar in 16 patients with recurrent unmappable VT (prior myocardial infarction in nine patients).16 During a median follow up of eight months 75% remained free of VT recurrences. One patient suffered a stroke, emphasising the potential risk of placing extensive lesions in the left ventricle.

Table 25.1 Ventricular tachycardia mechanisms and ablation considerations

Mechanism

Ablation efficacy

Complication risk

Idiopathic VT RV outflow tract LV verapamil sensitive

Automaticity Re-entry

Low, but rare fatalities Low

Post-MI "mappable" VT Reduction in VT episodes Prevention of all VT

Re-entry

Post-MI "unmappable"

Other scar related VTs

RV dysplasia + RV dilation Non-ischaemic cardiomyopathy

Bundle branch re-entry VT

Re-entry

Palliative

Re-entry through bundle 100% branches

AV block

AV, atrioventricular; LV, left ventricular; RV, right ventricular; MI, myocardial infarction; VT, ventricular tachycardia.

VT that is unmappable with a single roving catheter may be mapped with a system that simultaneously records electrograms throughout the ventricle during one or a few beats of the unstable VT, following which the VT can be terminated to allow ablation during stable sinus rhythm. Multielectrode basket catheters have been successfully deployed through a long sheath into the ventricle, but have somewhat limited sampling.17 An alternative system (Endocardial Solutions, St Paul, Minnesota, USA) records electrical potentials from an electrode grid array within the cavity of the ventricle. Electrical potentials at the endocar-dial surface some distance away are calculated. Sites of early endocardial activity, which are likely adjacent to re-entry circuit exits, are usually identifiable; in some cases, isthmuses have been identified.1819 Schilling and colleagues used this system to guide ablation in 24 patients (20 with prior infarction) and recurrent VT. During a mean follow up of 18 months, 64% were free of recurrent VT. In 15 patients Strickberger and associates achieved ablation of 15 of 19 (78%) VTs that were selected for ablation in 15 patients with prior infarction; 10 were free of recurrent VT during a short one month follow up. Major complications of stroke, perforation, and death from pump failure occurred in three patients. Further evaluation with regards to safety and efficacy are warranted.

Bundle branch re-entry causes only 5% of all sustained monomorphic VTs in patients referred for electrophysiologic study, but is important to recognise because it is easily cur-able.20 In its usual form the excitation wave-front circulates up the left bundle branch, down the right bundle branch, and then through the interventricular septum to re-enter the left bundle (fig 25.3), causing VT with a left bundle branch block configuration. Less commonly, the circuit revolves in the opposite direction. This VT occurs in patients who slowed conduction through the His Purkinje system and is usually associated with severe left ventricular dysfunction. The sinus rhythm ECG usually displays incomplete left bundle branch block. The VT is often rapid, commonly causing syncope or cardiac arrest. Ablation of the right bundle branch is relatively easy and effective. AV conduction is further impaired by ablation, necessitating implantation of a pacemaker or defibrillator with bradycardia pacing in 15-30% of patients. Bundle branch re-entry VT coexists with scar related VTs in some patients; implantation of a defibrillator is usually considered.

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