By Matthew Wright
Over the past 10 years substantial advances have been made in catheter ablation of both paroxysmal and persistent atrial fibrillation (AF). Since the initial studies in the treatment of paroxysmal AF by pulmonary vein isolation, it quickly became apparent that strategies targeting the pulmonary vein – left atrial junction were insufficient to treat patients with longer duration AF. This led to a number of different approaches being tried, incorporating the addition of left atrial linear lesions and the ablation of complex fractionated atrial electrograms. When taken alone each ablation approach resulted in only modest results, with reported success rates of between 20% and 60% off anti-arrhythmic medication.
The combination of these individual approaches has resulted in a high success rate, albeit with the vast majority of patients requiring more than one procedure. The stepwise approach incorporates pulmonary vein isolation, electrogram based ablation and left atrial linear lesions. The AF cycle length, which can be measured from both the surface ECG and from intracardiac catheters, has been shown to predict success of antiarrhythmic medication, DC cardioversion, and recently catheter ablation of AF and is the monitoring tool to assess the impact of ablation of individual targets. The endpoint of the stepwise approach is not only procedural termination of AF and any resulting atrial tachycardias but also confirmation of pulmonary vein isolation and block of all linear lesions with additional lesions being performed if necessary. This ablation strategy is very demanding with procedures lasting four hours or more, however, using this approach 80% of all patients are in sinus rhythm off all anti-arrhythmic medication at three years. In patients in whom AF is terminated during the index procedure almost 90% of patients are in sinus rhythm and off anti-arrhythmic medication, compared to only 25% of patients in whom AF could not be terminated at the index procedure.
In patients with persistent AF in whom procedural termination is achieved, it is unlikely that they will return with AF, however, the vast majority will return with atrial tachycardia. These atrial tachycardias can be mapped conventionally, using activation and entrainment mapping. Macro re-entrant atrial tachycardias and “focal” atrial tachycardias account for approximately half of all atrial tachycardias each. However, of the focal tachycardias, that is centrifugal activity emanating from a discrete region, only 25% are truly focal, with the majority being due to localised re-entry as defined by being able to record electrograms covering more than 75% of the cycle length in an area <2cm. These localised re-entry tachycardias are sensitive to ablation, typically terminating within seconds of applying radiofrequency energy to the appropriate site. This is in comparison to the macro re-entrant tachycardias, which although being quicker to map, are more difficult to ablate, especially when having to perform a mitral isthmus line.
Despite the progress in being able to treat patients with AF with a realistic goal of rendering the majority of patients AF free, this has been achieved using catheter technology that was developed for ablation of discrete targets. Recent advances in cardiac MRI that allow for detection of left atrial fibrosis may aid our understanding of the anatomical substrate underlying AF, but these results require further investigation. How changes in the structure of the left atrium relate to electrophysiological observations, such as fractionated electrograms needs to be examined. The role of dominant frequency mapping, the use of algorithms for detecting complex fractionated atrial electrograms and ablation of ganglionic plexi remains unclear. There are a number of technological advances that may make life easier for the electrophysiologist. These range from advanced imaging software to novel catheters using different energy sources, however, these will all need to be tested in prospective randomised trials, demonstrating clinical benefit, before they can be widely accepted.
Hopefully, future developments in catheter technology and ablation energies along with a greater understanding of the mechanisms underlying persistent AF in individual patients will result in more targeted ablation, shorter procedures, and fewer recurrences.