Phrenic nerve visualization: a profitable new application for intracardiac echocardiography?

This editorial refers to ‘Visualization and mapping of the right phrenic nerve by intracardiac echocardiography during atrial fibrillation ablation’ by X. Liu et al., https://doi.org/10.1093/europace/euad012. Catheter ablation for atrial fibrillation (AF) has advanced as a major treatment strategy for AF, and cryoballoon (CB) ablation is next to radiofrequency (RF) ablation the most important energy source for pulmonary vein isolation (PVI).1 Besides complications like pericardial tamponade, stroke, atrioesophageal fistula, or access site complications that occur similarly in both RF- and CB-based AF ablation,2 phrenic nerve palsy (PNP) is much more exclusively associated with balloon procedures, including cryo-, laser-, and RF-balloon ablation. Pulsed field ablation is a more recently introduced ablation technique, increasingly used for PVI,3 for which only transient PNP for a few seconds were reported up to now.4 Electrical isolation of the superior vena cava (SVC) via RF or CB ablation is performed mainly for repeat ablation of AF to eliminate extra-pulmonary vein trigger foci5 and is also associated with an increased incidence of PNP. Phrenic nerve palsy mainly occurs during ablation of the septal pulmonary veins (PVs) in PVI procedures due to the short distance between the targeted PV and the right phrenic nerve (RPN) or during SVC isolation procedures. The risk in CB procedures could be reduced with the release of the second-generation CB.6 The incidence rates of persistent PNP until discharge are reported to be as high as 1.2–2.3% in more recent trials.1,7 In SVC isolation, PNP damage evenly occurs in 2.3–5%.5,8 Reported clinical predictors for PNP are shorter body height, lower body mass index, and female sex.9 Different strategies to prevent PNP have been reported and evaluated, mainly in studies of CB-based PVI.10 These strategies all rely on positioning a catheter in the higher SVC that allows for continuous pacing during balloon ablation. To monitor nerve function, the operator either visualizes the diaphragmatic motion via fluoroscopy or intracardiac echocardiography (ICE) or diaphragm excursion is monitored via palpation. Another option is to record and monitor the compound motor action potentials (cMAPs) of the diaphragm via a modified placing of lead I electrocardiogram electrodes.11 A decrease of the cMAP amplitude of more than 35% can indicate imminent PNP.12 Once RPN damage is indicated as reduced cMAP and/or diaphragmic excursion, ablation must be ceased immediately.13 Phrenic nerve function then recovers in most patients.7,9 However, challenges related to RPN pacing can occur and are associated with instability or dislodgement of currently used pacing catheters due to respiratory movement and/or other anatomical reasons. In this situation, reduction of cMAP amplitude or inaccurate diaphragm excursion indicates imminent PNP. Compound motor action potential monitoring is likewise feasible and efficient in RF-based SVC isolation.8 In this issue of Europace, Liu et al.14 report a new application of ICE to visualize the RPN during AF ablation procedures in which the RPN is at risk. An integration of the ultrasound stream via CARTO® Sound into the 3D map of the right atrium (RA) and the SVC allowed for a delineation of the RPN’s spatial course. A visualization of the RPN was successful in 92% of patients. The main reason for visualization failure in the remaining patients was a narrowed space between the SVC and the mediastinal pleura of 1 mm, which was difficult to distinguish from the RPN. The investigators also assessed the RPN’s pacing threshold in relation to its distance to the SVC, which were strongly negatively correlated. Besides the description of a new method, the presented work—as such—gives valuable insights into the spatial course of the RPN and its anatomical relation to the SVC from an electrophysiologist’s point of view: it was located straight lateral in 71%, anterolateral in 16%, and posterolateral in 8% of patients. A nerve course at the anterior wall of the SVC was observed in 5% of patients. Through an integration of the RPN’s spatial course into the 3D map of the SVC and the RA, the investigators were able to design individualized ablation lines for SVC isolation. Hence, a weakened diaphragm movement or cMAP decrease was not observed in any of the included patients. The strengths of the presented strategy are its accuracy in RPN localization and visualization, which is different from a previous study that used pre-procedural computed tomography scans that are subject to map shifting and inaccurate merge, as well as a lower sensitivity for RPN detection.15 The greatest weakness of the method in this study is that it can only visualize the nerve in real time and integrate it into the 3D map, but it does not allow for monitoring its function. Another important clinical issue of the presented work is that only RF-based point-by-point SVC isolation is investigated. Unfortunately, the new strategy has not yet been evaluated in the more frequently performed balloon-based PVI. Of note, 3D mapping and reconstruction of the right-sided PN can increase procedure time and the dosage of anticoagulants, which may potentially lead to increased procedure-associated complications in large patient populations. Finally, the factor of cost-effectiveness for the additional use of a CARTO® sound catheter should be taken into consideration in daily clinical practice. Insurance compensation for use of ICE during electrophysiological procedures is unavailable, and this is extremely challenging for most ablation centres in the European Union. In summary, PNP is still a concern during balloon-based PVI. Constant monitoring of RPN function as well as RPN visualization during the procedure can reduce the incidence of PNP. We congratulate Liu et al. for an innovative, easy, and reproducible workflow for ICE-guided RPN visualization and mapping. The provided method allows for live anatomical correlation of the RPN’s spatial course in relation to the right PVs and SVC. Hopefully, live RPN visualization can be used in balloon-based PVI and further minimize PNP incidence in the future. The authors would like to thank Mirkka Hiort for drawing the Graphical Abstract.