Intramural Needle Ablation for the Treatment of Refractory Ventricular Arrhythmias

Radiofrequency (RF) ablation is the most commonly employed method for the catheter treatment of cardiac arrhythmias. Myocardial scar serves as the most frequent substrate for the genesis of both atrial and ventricular arrhythmias. Such scar frequently contains surviving myocyte bundles interspersed with fibrotic tissue, which leads to slow conduction. Areas of denser fibrosis cause conduction block. When appropriately arranged, conduction through or around these scars leads to the creation of a "reentry" circuit through which an arrhythmia is generated and maintained. Each reentry circuit contains within it an area called the isthmus, a portion of the circuit located in a position intimately related to the scar border zone. Electrical activation travels slowly through the isthmus before breaking out into normal myocardium. Ablation at the site of an isthmus will terminate a reentrant tachycardia.

A variety of techniques, including electroanatomic mapping and activation, entrainment, and substrate mapping, are employed during electrophysiologic (EP) study to identify areas of myocardial scar and potential isthmus sites. Points or lines of ablation using RF energy are then created in an attempt to interrupt the reentry circuit. Typically, RF energy is applied via a catheter tip electrode to the endocardial or epicardial surface of the heart and grounded via an electrode pad placed on the patient's skin. RF energy in this setting is dispersed through the entirety of the tissue between catheter tip and grounding pad. The standard 7-French, 4-mm tip catheters are highly successful at ablating circuits located within a few millimeters of the catheter tip. A focal, 1mm area of resistive heating occurs within the myocardium immediately in contact with the catheter tip; myocardial cell death occurs several millimeters more deeply through passive, conductive heating, which spreads outward from the contact point.

While the standard catheter is effective at the ablation of superficial arrhythmias, it has proven more problematic when used for deep myocardial sites or for creating transmural lesions. A number of alternatives have been developed in an attempt to access these sites. 8-mm or 10-mm catheter tips are able to create larger zones of resistive heating, delivering direct RF energy to a larger area of myocardium. A larger interface between catheter tip and blood improves cooling and allows for the delivery of more power without a rise in impedance.The clinical use of these larger catheters can, however, be limited by rapid temperature rises at the catheter-tissue interface, resulting in thrombus formation, char, and "steam pop" rupture of the endocardial surface. The use of irrigated ablation catheters have improved upon the ability to deliver RF energy without a sustained rise in impedance. Both open irrigated- and closed-loop irrigated catheters circulate saline along the catheter tip-myocardial interface, allowing for continued delivery of RF current without thrombus formation at the endocardial surface. Intramyocardial temperature rises accordingly without a concomitant endocardial temperature surge, creating larger and deeper myocardial ablation zones. Transcoronary ethanol ablation has also been employed with moderate success in patients with arrhythmias resistant to endocardial catheter ablation.This technology, however, grants only limited control over the size of the resulting infarct and is restricted by the need for perfusion of the scar zone by an accessible coronary artery.

Nevertheless, there remain occasions in which an arrhythmia cannot be eliminated by standard ablation techniques. This is seen most frequently due to deep intramural ventricular tachycardia, sometimes encountered following myocardial infarction. Both standard and alternative ablation strategies are frequently either unavailable or inadequate for termination of these arrhythmias.

Initial experience with an electrically active needle electrode have demonstrated that radiofrequency ablative energy can effectively create lesions of homogeneous necrosis. Needle electrodes have been used experimentally from the epicardial surface, from the endocardium ex vivo and in vivo in an internally irrigated form. It has been shown that the use of a narrow-gauge non-irrigated endocardial needle ablation catheter creates very narrow but deep lesions due to the small electrode size. Catheters featuring a retractable needle tipped electrode with intramyocardial saline infusion have also shown promise as a means of accessing deep myocardial circuits in ventricular tachycardia ablation.18

The proposed study will further examine the role of INA in patients with ventricular arrhythmias resistant to standard ablation techniques.