4.2 | Efficacy of VOM-EI
In our study, we achieved mitral isthmus block in all patients undergoing VOM-EI. Other groups described comparable results of mitral isthmus block in 98.7-100% of cases.10,13,16
Importantly, ancillary radiofrequency ablation was necessary in 63% of our patients, mainly on the annular side of the mitral isthmus both endocardially and epicardial from within the CS. As previously described and expected for anatomical reasons, the main impact of VOM-EI is on the pulmonary venous side of the mitral isthmus, sparing the annular aspect.9 The pulmonary venous side of the mitral isthmus is generally thicker and protected from endocardial radiofrequency ablation by adipose tissue.17,18Epicardial ablation via the CS is often required for mitral isthmus ablation. Because of the course of the CS, epicardial ablation usually targets the annular side of the mitral isthmus. VOM-EI can elegantly overcome the limitations of radiofrequency ablation by targeting primarily the pulmonary venous side. Correspondingly, several reports from the Bordeaux group demonstrated that endocardial radiofrequency ablation at the annular aspect of the mitral isthmus was mainly required to achieve mitral isthmus block after VOM-EI.13,19 The ablation time to achieve mitral isthmus block was significantly shorter with supplement VOM-EI compared to radiofrequency ablation alone. Termination of ongoing perimitral flutter during VOM-EI has been reported in 26-56% of cases.9,10,15 Consistent with these results, perimitral flutter terminated or slowed during VOM-EI in 58% of our cases.
Radiofrequency ablation of atrial fibrillation increases cardiac biomarkers like hs-TnT. Haegeli et al. reported hs-TnT levels of 850 ng/L six hours after atrial fibrillation ablation whereas Reichlin et al. observed hs-TnT levels of 1996 ng/L 24 hours after the procedure and found a significant correlation of hs-TnT levels with total radiofrequency time and energy delivery.20,21 In our study, hs-TnT increased significantly from 330 to 598 ng/L within 24 hours after the procedure. Tissue necrosis induced by VOM-EI is therefore not excessive, as compared to a regular atrial fibrillation ablation procedure.
Low-voltage areas induced by VOM-EI can have various sizes, and probably depend on the vascular tributaries of the vein of Marshall as well as volume and infusion rate of ethanol injection.9,22Some cases of chemical ablation have been reported that resulted in large low-voltage areas, including the left atrial appendage or the posterior wall.23,24 In previous studies, mean low-voltage areas induced by VOM-EI have been described in the range of 7.7±3.2 cm2 to 12.7±8.3 cm2.8,9,10,25 Generally, 2 to 4 ml of ethanol were injected in these studies, but some groups reported higher volumes of ethanol injection up to 12 ml.10After injection of a median of 4 ml of ethanol, we observed a low-voltage area of 13.2 cm2. Low-voltage area in our study correlated significantly with the volume of ethanol injected (P = 0.03).