Author’s reply to “Pulsed field ablation and phrenic nerve function: how safe is it?”

Phrenic nerve palsy (PNP) is a complication of pulsed field ablation (PFA),1Franceschi F. Maille B. Pulsed field ablation for paroxysmal atrial fibrillation.N Engl J Med. 2024; 390: 1054-1055Crossref Scopus (0) Google Scholar but its true incidence remains unknown, as diaphragmatic monitoring has never been studied on a large scale. It can be hypothesized that several mechanisms may be involved—electrical if PNP are transient, or traumatic if they are persistent. Dr Ollitrault considers his recent publication2Ollitrault P. Chaumont C. Font J. et al.Superior vena cava isolation using a pentaspline pulsed-field ablation catheter: feasibility and safety in patients undergoing atrial fibrillation catheter ablation.Europace. 2024; 2024; 26euae160Crossref Scopus (1) Google Scholar reassuring. However, his study has several major limitations concerning phrenic nerve monitoring:(1)Stimulation of the phrenic nerve is, surprisingly, performed with the pentaspline catheter, which is not designed for it. Moreover, the method is not detailed. The risk is to stimulate the nerve unreliably or below the conduction block level and thus underestimate the incidence of PNP.(2)Compound motor action potential (CMAP) values are not reported in the study. The authors defined PN stunning as a drop in CMAP amplitude >30%. This threshold is arbitrary, in the absence of published data on CMAP monitoring in PFA procedures. Incomplete CMAP recovery is probably not trivial, as CMAP amplitude reflects the number of diaphragmatic fibers that depolarize. For example, is the fact that 25% of fibers no longer depolarize after a PFA procedure a fact that should not be taken into account? What are the long-term consequences?(3)Chest radiography was performed in 20 patients, meaning probably only 12 patients with PN stunning. This figure is far too low. Moreover, chest radiographs can be falsely reassuring,3Katsume Y, Miwa Y, Ueda A, Soejima K. Underdiagnosis of phrenic nerve palsy caused by cryoballoon ablation for atrial fibrillation with upright position chest radiography: usefulness of supine position dynamic chest radiography. Europace2021;23:1567.Google Scholar and dynamic examinations (ultrasound or fluoroscopic sniff test) should be preferred.(4)Finally, applications in the superior vena cava are parallel to the phrenic nerve, unlike applications in the right pulmonary veins, which are perpendicular to the nerve. Assuming a traumatic mechanism, perpendicular applications should be more deleterious. Under these conditions, no conclusions should be drawn from this study3Katsume Y, Miwa Y, Ueda A, Soejima K. Underdiagnosis of phrenic nerve palsy caused by cryoballoon ablation for atrial fibrillation with upright position chest radiography: usefulness of supine position dynamic chest radiography. Europace2021;23:1567.Google Scholar regarding the incidence of PNP in pulmonary vein isolation with PFA. It is imperative to remain vigilant, as PNP is undoubtedly a complication of PFA.1Franceschi F. Maille B. Pulsed field ablation for paroxysmal atrial fibrillation.N Engl J Med. 2024; 390: 1054-1055Crossref Scopus (0) Google Scholar Diaphragmatic monitoring should be recommended to operators until further notice. Frédéric Franceschi is consultant for the company Circle-Safe. Linda Koutbi, Baptiste Maille, and Jean-Claude Deharo have no conflict of interest. None.