Cardiac Activation

Eugene Downar, M.D., FRCP(C)
Professor of Medicine, University of Toronto, Faculty of Medicine
Toronto General Hospital, Division of Cardiology
PMCC 3 – 552, 150 Gerrard Street West
Toronto, Ontario, CANADA, M5G 2C4

Historical Background

Although cardiac mapping invokes high-tech images, the origins go back to more than 100 years ago in German and British literature. In 1915, Sir Thomas Lewis published the ventricular activation sequence during sinus rhythm in the canine heart using a string galvanometer. Clinical application of such studies had to wait for Durrer’s classic studies of  detailed 3-D activation in the isolated perfused human heart. Inspired by those studies an intraoperative mapping program was started at Duke University to locate and ablate accessory pathways resulting in the surgical cure of WPW syndrome. Guiraudon subsequently applied the same technique to initiate the first map-directed surgical ablation of ventricular tachycardia.

The traditional mapping technique entails collection of local electrograms during a tachycardia by a simple roving electrode. All the electrograms are then collated to reconstruct a stereotypical activation sequence to identify a target for ablation. The technique is limited by being able to collect only one data point for each cardiac cycle and is therefore very time-consuming. Tachycardias that are unstable hemodynamically or morphologically or that are non-sustained cannot be mapped adequately by this technique. To overcome these shortcomings, we developed multiple electrode arrays which, in conjunction with a multi-channeled recording system, could be used to record simultaneous endo and epicardial activation. With such a system it was possible to detail online beat-by-beat activation sequences of ventricular tachycardia and gain insights into the underlying tachycardia mechanisms.

Intraoperative Mapping

In ventricular tachycardia, due to coronary artery disease, the activation pattern is most often due to endocardial reentry. One of three patterns may be seen: (1) a single broad wavefront of continuous recirculation reminiscent of a vortex; (2) two opposing wavefronts sharing a common narrow isthmus of slow conduction consistent with figure 8 reentry; (3) micro-entry with radial spread.

Details of the zone of slow conduction forming the return tracts show that these are highly variable but can be complex and extensive suggesting a sheet-like structure of surviving myocardium. There can be multiple paths of entry and exit. Spontaneous and induced block can occur within portions of the complex. Intermittent block in one of two paths of entry can cause intermittent cycle changes of a tachycardia without changing its configuration. Block in one exit pathway can result in a shift to an alternate exit path with dramatic changes in ventricular activation and tachycardia configuration. Termination of a tachycardia could result from block close to the entry or exit portion of a return tract. Also different tachycardias can share common portions of a return path. The optimal targets for ablation are sites of spontaneous block at points of entry and exit in a reentrant tract.

Catheter Mapping

The advent of the AICD era marked the effective demise of surgical treatment of ventricular arrhythmias. The challenge went to percutaneous catheter-based techniques to map and ablate problematic persistent ventricular arrhythmias. Standard catheter mapping is similar to traditional surgical mapping techniques in that an exploring catheter electrode is manipulated to collect local electrograms one at a time and the activation sequence is reconstructed by collating all the data. X-ray fluoroscopy is used to guide the catheter. The technique is limited by being very time-consuming and by the fact that precise positioning of the exploring electrode is not feasible with fluoroscopy alone.

Different technical strategies have been used to address these limitations. One approach has been to allow simultaneous acquisition of multiple electrograms from a multi-electrode catheter. The basket catheter provides 64 contact electrodes, which give useful resolution of activation sequence on a beat-by-beat basis. A much higher resolution has been achieved by computing 3,000 virtual electrograms derived from 64 unipolar electrodes mounted on a non-contact balloon catheter. Both techniques allow mapping of hemodynamically unstable and non-sustained arrhythmias.

3-D electro-anatomic mapping using electromagnetic fields for precise spatial orientation has also revolutionized catheter mapping. Although this technique is restricted by sequential data collection, in stable arrhythmias high resolution of activation sequence can be obtained and low-voltage channels of conduction can be identified through scarred myocardium. Successful strategies have been developed for ablation of unstable ventricular arrhythmias. These include voltage mapping in sinus rhythm to locate scarred myocardium. In an approach reminiscent of surgical encircling endocardiotomy, linear RF ablation lesions are delivered to target the border zone and disrupt potential reentry. Future developments are directed at combining electrophysiological mapping information with 3-D imaging techniques. Activation maps will be displayed nested within detailed anatomic reconstructions provided by CT scans and MR angiograms.