Despite recent advances in the understanding and therapy of atrial fibrillation (AF), it remains a significant challenge. Antiarrhythmic drug therapy is ineffective for many patients and, despite early enthusiasm, radiofrequency ablation is characterized by recurrence of the arrhythmia and the need for repeat procedures. It is known, however, that some patients with AF do better than others after ablation. Clinical predictors of benefit include age, persistence of AF, and markers of structural change such as left atrial enlargement. Many practising electrophysiologists know of cases where patients with an apparently good predicted outcome respond poorly to AF ablation and others vice versa. In a paper presented at the recent ESC meeting n Amsterdam, a group from Salt Lake City announced that they have recently identified that the response to AF ablation may have more to do with the underlying atrial substrate — as judged by the magnetic resonance imaging (MRI)-derived fibrosis score — than clinical factors. 
The study shows that in patients with atrial fibrillation, delayed enhancement magnetic resonance imaging (DE-MRI) performed before ablative treatment can stage the degree of atrial fibrosis and help predict whether treatment will be successful or not, according to results of the “Delayed Enhancement – MRI determinant of successful Catheter Ablation of Atrial Fibrillation (DECAAF)” trial. The lead investigator was Dr N Marrouche, MD, from the CARMA Center at the University of Utah in Salt Lake City, USA.
“The results show that the stage of atrial fibrosis prior to ablation is a new, powerful, independent predictor of outcome,” said Marrouche
“By performing this imaging before ablative treatment we can triage patients according to the likelihood of treatment success, and avoid ablative procedures in those patients for whom it is unlikely to work. If a patient has late stage 3 or stage 4 fibrosis their chance of being cured is only 30-35% which is really low, but if they’re in an early stage their chance of cure is 60-80%,” he said.
High resolution Delayed Enhancement MRI (DE‐MRI) was performed up to 30 days before ablation in 260 atrial fibrillation (AF) patients with atrial fibrosis who were undergoing ablation. The MRI exam was carried out to determine the presence and extent of atrial fibrosis, while post-ablation DE-MRI was performed at 90-days follow-up in 177 of the subjects, to determine the extent of residual ablation. At the 90-day follow-up, recurrence of arrhythmia was noted in 88 of the 260 patients. Specifically, patients whose pre-ablation fibrosis was stage 1 (defined as less than 10% damaged atrial tissue) had an 85.8% success rate, those with stage 2 (10% – 20% damage) had a 63.3% success rate, those with stage 3 (20% – 30% damage) had a 55% success rate and those with stage 4 (more than 30% damage) had a 31% success rate.
For every increased percentage of fibrosis before ablation, there was 6.3% increased risk of recurrent symptoms after ablation said Dr. Marrouche. Similarly, for every percentage of residual fibrosis there was an 8.2% increased risk of recurrent symptoms.