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Evaluation of Diaphragmatic Electromyograms in Radiofrequency Ablation of Atrial Fibrillation: Prospective Study Comparing Different Monitoring Techniques SHINSUKE MIYAZAKI, M.D.,∗ NOBORU ICHIHARA, M.D.,∗ HIROSHI TANIGUCHI, M.D.,∗ HITOSHI HACHIYA, M.D.,∗ HIROAKI NAKAMURA, M.D.,∗ EISUKE USUI, M.D.,∗ YOSHIHISA KANAJI, M.D.,∗ TAKAMITSU TAKAGI, M.D.,∗ JIN IWASAWA, M.D.,∗ AKIO KUROI, M.D.,∗ KENZO HIRAO, M.D.,† and YOSHITO IESAKA, M.D.∗ From the ∗ Cardiovascular Center, Tsuchiura Kyodo Hospital, Tsuchiura, Ibaraki, Japan; and †Heart Rhythm Center, Tokyo Medical and Dental University, Tokyo, Japan
Diaphragmatic Electromyograms During AF Ablation. Background: The utility of compound motor action potential (CMAP) monitoring for anticipating phrenic nerve injury (PNI) during cryoballoon ablation has been reported. We sought to compare two different CMAP recording techniques and evaluated the feasibility during pulmonary vein antrum isolation (PVAI) and superior vena cava isolation (SVCI) using radiofrequency energy. Methods and Results: Forty-two patients undergoing paroxysmal atrial fibrillation ablation were prospectively enrolled. SVCI was performed following PVAI if SVC potentials were observed. CMAPs were recorded 3 times (before and after PVAI, and after SVCI) simultaneously from surface electrodes (CMAPsuf) and a decapolar catheter in the subdiaphragmatic hepatic vein (CMAPabd). The baseline CMAPsuf and CMAPabd were 0.92 ± 0.36 and 0.65 ± 0.43 mV except in one case with catheter inaccessibility. The CMAPsuf did not correlate with the body mass index, or CMAPabd. In 2 and 9 patients, the CMAPsuf and CMAPabd amplitudes were < 0.5 and < 0.3 mV, respectively. The diaphragm to catheter distance was significantly longer in cases with a CMAPabd < 0.3 mV than one > 0.3 mV (39.2 ± 10.8 vs. 21.5 ± 6.6 mm, P < 0.0001). Two cases with a CMAPsuf < 0.5 mV had larger amplitudes on the CMAPabd. In 1 patient, apparent PNI occurred during the SVCI, and the CMAP disappeared after the SVCI in both techniques. The CMAPs did not significantly decrease after the PVAI and SVCI; however, a >30% decrease was observed in 2 patients in both techniques. In both, no PNI was apparent on fluoroscopy or chest X-ray. Conclusions: Stable evaluable CMAPs were obtained with the CMAPsuf in most patients. Monitoring with the CMAPabd could be an alternative and complementary method. (J Cardiovasc Electrophysiol, Vol. 26, pp. 260-265, March 2015) phrenic nerve injury, diaphragmatic electromyogram, superior vena cava, catheter ablation, atrial fibrillation, pulmonary vein isolation Introduction Electrical isolation of thoracic veins has become a widely accepted strategy in atrial fibrillation (AF) ablation.1-5 However, phrenic nerve injury (PNI)6 is one of the important complications during right pulmonary vein (PV) isolation, and a specific concern during superior vena cava (SVC) isolation because of the anatomical proximity to the course of the right phrenic nerve. Recently, some groups reported the utility of phrenic compound motor action potential (CMAP) monitoring for anticipating PNI during right PV isolation using a cryoballoon.7-10 Two different monitoring techniques have
been introduced to monitor the CMAP. One is recording from the abdominal surface electrodes, which is technically easy but potentially seems to be difficult in obese patients.7,9,10 The other is recording from a multipolar catheter positioned in the subdiaphragmatic hepatic vein.8 However, these monitoring techniques have not been applied to either radiofrequency (RF) ablation or clinical SVC isolation. The objective of this prospective study was to compare the two different monitoring techniques for CMAP recording and to evaluate the feasibility of CMAP recording during PV antrum isolation (PVAI) and SVC isolation using RF energy. Methods
No disclosures.
Study Population Address for correspondence: Shinsuke Miyazaki, M.D., Cardiology Division, Cardiovascular Center, Tsuchiura Kyodo Hospital, 11–7 Manabeshinmachi, Tsuchiura, Ibaraki 300-0053, Japan. Fax: +81-29-826-2411; E-mail: [email protected] Manuscript received 25 August 2014; Revised manuscript received 6 October 2014; Accepted for publication 15 October 2014. doi: 10.1111/jce.12571
This prospective study was comprised of 42 patients who underwent either a first or second catheter ablation procedure for paroxysmal AF using RF energy. In 34 (80.9%) patients who underwent a first procedure, an SVC isolation was performed following the PVAI if SVC potentials were recorded.11 The CMAP was recorded 3 times: before the PVAI, after the PVAI (before the SVC isolation), and after
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the SVC isolation. In the remaining 8 (19.1%) patients who underwent a second procedure, an SVC isolation was performed following the consolidation of the previous PVAI. CMAPs were recorded twice: before and after the SVC isolation. AF was classified according to the HRS/EHRA/ECAS 2012 Consensus Statement on Catheter and Surgical Ablation of AF.5 All patients gave their written informed consent. Electrophysiological Study and Catheter Ablation All antiarrhythmic medications were discontinued for at least 5 half-lives prior to the ablation if they were prescribed before the procedure. Transesophageal echocardiography was performed within 24 hours preprocedurally to exclude any left atrial (LA) thrombi. Enhanced cardiac computer tomography was performed for the evaluation of any relevant cardiac anatomy before the procedure. The surface electrocardiogram (ECG) and bipolar intracardiac electrograms were continuously monitored and stored on a computer-based digital recording system (LabSystem PRO, Bard Electrophysiology, Lowell, MA, USA). Bipolar electrograms were filtered from 30 to 500 Hz. A 7Fr 20-pole three site mapping catheter (BeeAT, Japan Life Line, Tokyo, Japan) was inserted through the right jugular vein for pacing, recording, and internal cardioversion. The 4 proximal electrodes, middle 8 electrodes, and distal 8 electrodes were positioned in the SVC, right atrium, and coronary sinus, respectively. The electrophysiological study was performed under minimal sedation with pentazocine and hydroxyzine pamoate. The details of the ablation protocol have been described previously.12 After one transseptal puncture, two long sheaths (SL0, AF Division, St. Jude Medical, Minneapolis, MN, USA) were introduced into both superior PVs. Pulmonary venography during ventricular pacing and contrast esophagography were performed to obtain the relative locations of the PV ostia vis-a-vis esophagus. A 100 IU/kg body weight of heparin was administered following the transseptal puncture, and heparinized saline was additionally infused to maintain the activated clotting time at 300–350 seconds. Two decapolar circular mapping catheters (Lasso, Biosense Webster, Diamond Bar, CA, USA) were placed in the superior and inferior PVs, and the left- and right-sided ipsilateral PVs were circumferentially ablated under the guidance of a 3-D mapping system (CARTO3, Biosense Webster). Posteriorly, ablation was performed anatomically in the LA, 1–3 cm from the PV ostia. Anteriorly, ablation was performed on the edge of the left PVs guided by the earliest PV potentials. The electrophysiological endpoint of the PVAI was the achievement of bidirectional conduction block between the LA and PVs and the anatomic endpoint was the creation of a complete continuous circumferential lesion around the ipsilateral veins.13 RF current was delivered point-by-point for 30 seconds with a 3.5 mm externally irrigated-tip ablation catheter (Thermocool, Biosense-Webster) with a power of up to 35 W, target temperature 38◦ C and irrigation rate of 30 mL/min. The power was limited to 20 W on the posterior wall close to the esophagus. After completing the PVAI, a 30 mg bolus of adenosine triphosphate was injected to unmask any dormant PV conduction, and any gap responsible for dormant conduction was eliminated by additional RF applications.14 Electrical SVC isolation was added during pacing from the high right atrium. The circular mapping catheter was placed at the level of the lower border of the pulmonary artery above
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the SVC-right atrial junction guided by SVC angiography.15 Before the RF delivery, high output pacing (10 V, 2 ms) was performed at every site, and if diaphragmatic stimulation was observed, ablation was performed with a maximal power of 10 W. The endpoint of the ablation was the elimination of all SVC potentials on the mapping catheter. Recording of the Phrenic CMAP with Different Techniques CMAPs were obtained simultaneously with the different recording techniques described below. First, two additional standard surface ECG electrodes were positioned 5 cm above the xiphoid process and at 16 cm along the right costal margin.7,9,10,16 The CMAP signals were amplified using a bandpass filter between 1 and 2 kHz and recorded on a Bard recording system (Fig. 1). Second, a deflectable decapolar catheter (1.2 mm electrodes spaced 5 mm apart, Snake, Japan Life Line) was placed in a right-sided subdiaphragmatic hepatic vein8 and connected to the Bard recording system. Bipolar CMAP signals were recorded between the distal electrode and one of the proximal electrodes on the decapolar catheter so as to record the highest CMAP amplitude. The signals were amplified and band-pass filtered between 1 and 250 Hz. The distance between the right diaphragm and mapping catheter on the right mammillary line was measured on the fluoroscopic image after the procedure (Fig. 1). The phrenic nerve was paced from the ablation catheter at 60 bpm, using 10-V output for a duration of 2.0 ms. The appropriate pacing site was carefully identified inside the SVC to obtain the highest stable CMAP amplitude, and recording was obtained during shallow respirations according to the operator’s command. It was possible because all procedures were undertaken under minimal sedation and any paralytic agents that inhibit phrenic nerve capture were not administered. Special attention was paid to avoid atrial capture. The phrenic capture was confirmed by abdominal palpation and dynamic diaphragmatic movement on fluoroscopy. The maximum CMAP amplitude was measured from peak to peak. Each measurement involved averaging 4 consecutive phrenic CMAP amplitude values. All measurements and analyses were performed offline after the procedure. Evaluation of PNI The diaphragmatic movement was evaluated throughout the procedure in the supine position, especially at the time of obtaining the CMAP recording. In addition, a chest X-ray (standard PA view) was undertaken in a standing position 1 day before and the next day after the ablation procedure in all patients. If the level of the right diaphragm was significantly elevated on the next day of the procedure relative to that before the procedure, we diagnosed it as PNI. In cases with PNI, the PNI was evaluated by chest X-ray with provocative maneuvers (inspiration and exhalation) during the follow-up. Statistical Analysis Continuous data are expressed as the mean ± standard deviation for normally distributed variables or as median (25th, 75th percentiles) for nonnormally distributed variables, and were compared using a Student’s t-test or Mann-Whitney U-test, respectively. Categorical variables were compared using the chi-square test. A probability value of P < 0.05 indicated statistical significance.
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Figure 1. (A) Representative diaphragmatic compound motor action potential (CMAP) electrograms recorded from surface electrodes (red arrow) and those recorded from a mapping catheter placed in the hepatic vein (green arrow) during phrenic nerve pacing. (B) Decapolar catheter positioned in the subsiaphragmatic vein. The distance between the diaphragm and mapping catheter was 26 mm. CMAPsuf : CMAP recorded from the surface electrodes, CMAPabd : CMAP recorded from a mapping catheter placed in the hepatic vein. For a high quality, full color version of this figure, please see Journal of Cardiovascular Electrophysiology’s website: www.wileyonlinelibrary.com/journal/jce
Results Clinical Characteristics The patient characteristics are summarized in Table 1. The CMAP was recorded during the initial and second ablation procedures in 34 and 8 patients, respectively. Among the 34 patients who underwent an initial procedure, an SVC isolation was not performed in 2 patients because no SVC potentials were recorded in 1 patient and cardiac tamponade occurred during the LA ablation unrelated to the CMAP recording in the other patient. Therefore, a successful PVAI and SVC isolation was achieved in 33 and 32 patients, respectively. As a result, the CMAP was recorded at baseline, after the PVAI (before SVC isolation), and after the SVC isolation in 34, 33, and 32 patients, respectively. In one patient, right PNI occurred during the SVC isolation, which was recognized on fluoroscopy during the procedure. The PNI completely recovered 8 months after the procedure. Among 8 patients who underwent a second procedure, successful SVC isolation was achieved following consolidation of the PVAI in all. Therefore, the CMAP was recorded before and after the SVC isolation in all 8 patients. No other complications were observed. CMAP Recorded from the Surface Electrodes In all 42 patients, a stable diaphragmatic CMAP could be obtained from abdominal surface electrodes with a mean amplitude of 0.92 ± 0.36 mV. There was no significant difference in the CMAP amplitude between the patients with a normal weight and those that were overweight/obese (0.92 ± 0.35 vs. 0.91 ± 0.39 mV, P = 0.89). No correlation was observed between the body mass index (BMI) and baseline CMAP amplitude (P = 0.96). In 2 (4.8%) cases, the baseline CMAP amplitude was < 0.5 mV, whereas it was > 0.5
mV in the remaining 40 (95.2%) patients. The mean CMAP amplitude after achievement of the PVAI was 0.89 ± 0.39 mV, which was similar to the baseline value (P = 0.79). No patients had a >30% reduction in the CMAP amplitude from baseline after the PVAI. The mean CMAP amplitude after the SVC isolation was 0.84 ± 0.30 mV except for in 1 patient with apparent PNI. In that patient, the CMAP could not be recorded after the SVC isolation, and diaphragmatic paralysis was observed on chest X-ray on the next day (Fig. 2). A >30% reduction in the CMAP amplitude after, relative to before, the SVC isolation was observed in 2 patients, whereas no PNI could be recognized either on fluoroscopy during the procedure, nor on chest X-ray on the next day. CMAP Recorded from a Catheter Placed in the Subdiaphragmatic Hepatic Vein In 1 (2.4%) patient, a deflectable mapping catheter could not be inserted into the subdiaphragmatic hepatic vein. Among the remaining 41 patients, the catheter position was stable without the need for repositioning, and the mean baseline CMAP amplitude was 0.65 ± 0.43 mV, including 9 patients with an amplitude < 0.3 mV. The distance between the diaphragm and mapping catheter on fluoroscopy was significantly longer in the cases with an amplitude of < 0.3 mV than in those with an amplitude > 0.3 mV (39.2 ± 10.8 vs. 21.5 ± 6.6 mm, P < 0.0001). The CMAP amplitude before and after the SVC isolation was 0.60 ± 0.42 and 0.53 ± 0.36 mV (P = 0.14), respectively. Among 32 patients with a baseline CMAP amplitude of > 0.3 mV, a >30% reduction in the amplitude was observed after the SVC isolation in 2 patients. In both patients, a >30% reduction was also observed in the surface electrode recordings. In a case with apparent PNI, the CMAP could not be obtained after the SVC isolation (Fig. 2).
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Figure 2. A case with clinically apparent PNI. (A) CMAP recorded before the SVC isolation. The CMAP recorded from the surface electrodes (red arrow) was smaller than that recorded from a mapping catheter positioned in the hepatic vein (green arrow) at baseline. (B) CMAP recorded after the SVC isolation. The CMAP could not be recorded in either technique. (C) The distance between the diaphragm and mapping catheter placed in the hepatic vein was 19 mm. (D) A chest X-ray 1 day before the procedure. (E) A chest X-ray 1 day after the procedure. Elevation of the right diaphragm was observed. For a high quality, full color version of this figure, please see Journal of Cardiovascular Electrophysiology’s website: www.wileyonlinelibrary.com/journal/jce
Comparison of the CMAP Amplitude Recorded by Different Techniques There was no correlation of the CMAP amplitude obtained by the 2 different techniques (P = 0.67). Among a total of 42 patients, the baseline CMAP amplitude was considerably higher in the recordings from the surface electrodes and those from the hepatic vein in 27 (64.3%) and 8 (19.0%) patients, and was almost the same in the recordings obtained by the 2 different techniques in the remaining 7 (16.7%) patients. In 2 patients in whom the CMAP amplitude recorded from the surface electrodes was < 0.5 mV (0.26 and 0.36 mV), a much higher CMAP amplitude was obtained from the hepatic vein in both (0.74 and 0.74 mV). One of those 2 patients was the patient with apparent PNI during the SVC isolation (Fig. 2). Discussion This study compared 2 different CMAP recording techniques during AF ablation. To the best of our knowledge, the present study is one of the first to evaluate the utility of the CMAP in RF ablation, and during clinical SVC isolation. We found that (1) a stable evaluable CMAP could be easily obtained from the surface electrodes in the vast majority of patients undergoing AF ablation, (2) the amplitude of the CMAP recorded from the surface electrodes did not correlate with the BMI, (3) the amplitude of the CMAP amplitude measured from the subdiaphragmatic hepatic vein highly depended on the anatomical location of the vein, (4) there was no correlation between the CMAP amplitude recorded from
the 2 different techniques, and (5) the CMAP amplitude decreased after the SVC isolation without any clinically apparent PNI in a part of patients undergoing an SVC isolation. Recording the Phrenic CMAP During Catheter Ablation In patients with neuromuscular disorders affecting phrenic nerve conduction, CMAP recordings have provided useful functional information.16 Franceschi et al. initially applied this technique to monitoring the phrenic nerve function during ablation procedures, and demonstrated that the CMAP amplitude could be reliably recorded on modified surface leads during ablation.7 Subsequently, Lakhani et al.9 and Mondesert et al.10 demonstrated that monitoring the CMAP could be an early and reliable method for predicting PNI using cut offs of a 35% and 30% decrease in the CMAP amplitude from the baseline in patients undergoing cryoballoon ablation for AF. This technique is simple and easily applicable; however, it seemed to be limited in obese patients. More recently, Franceschi et al. reported the clinical use of CMAP recordings using a multipolar catheter in the subdiaphragmatic hepatic vein in an effort to overcome the drawbacks of the measurement using the surface electrodes.8 They showed that a stable and reliable CMAP amplitude could be recorded safely in the majority of patients, and no PNI was observed using a cut off value of a >30% drop in the CMAP amplitude. This technique has theoretically some advantages such as (1) a electrode location close to the diaphragm allows recording of high CMAP amplitude values, and (2) the distance between the muscle and electrode does not vary during respirations because the liver moves with the diaphragm.
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TABLE 1 Clinical Characteristics N Age (y) Paroxysmal AF, n (%) Initial procedure, n (%) Female, n (%) Structural heart disease, n (%) Hypertension, n (%) Diabetes mellitus, n (%) Body mass index (kg/m2 ) Normal weight (BMI < 25) Overweight (30 > BMI 25) Obesity (BMI 30) LA diameter (mm) LV ejection fraction (%)
42 60.3 ± 12.0 42 (100%) 34 (80.9%) 10 (23.8%) 0 (0%) 13 (31.0%) 5 (11.9%) 24.5 ± 3.4 24 (57.1 %) 15 (35.7%) 3 (7.1%) 36.8 ± 4.7 66.3 ± 3.7
in whom the amplitude recorded from the surface electrodes was less than 0.5 mV. While recording from the hepatic vein provides reliable signals in the majority of cases, it necessitates an additional venous puncture and electrophysiology catheter, thus incurring additional cost and the risk of complications. In contrast, the use of the surface electrodes is relatively simple, easy, and noninvasive. It is likely that the reproducibility of CMAPs would be higher for surface CMAPs because the best position of the surface electrodes is established,16 but abdominal CMAPs are influenced by the catheter position. Based on our study results, we believe that monitoring the CMAP recorded from the hepatic vein is recommended in patients in whom evaluable large baseline CMAPs could not be obtained from the surface electrodes. Change in the CMAP Amplitude During the PVAI and SVC Isolation
Baseline Amplitude of the CMAP On the surface electrodes, an initial average CMAP amplitude of 0.6 mV in healthy volunteers was previously reported in the neurologic literature,16 and 0.34 and 0.87 mV in Lakhani’s and Mondesert’s studies.9,10 The difference might be explained by the different medications used during the procedure. Patients received paralytics during intubation in the former study, and the procedure was performed under conscious sedation in the latter study. CMAP recordings were unable to be acquired in 7% because there was an initial amplitude of < 0.15 mV in the former study.9 In the present study, the mean amplitude was 0.92 mV under minimal sedation, which was similar to the result of the latter study. Clear CMAP recordings could be obtained in the vast majority of the cases; however, in 2 patients the amplitude was < 0.5 mV. Because the CMAP usually fluctuates with respirations, it is important to obtain a high CMAP amplitude at baseline to evaluate it during the procedure. Although it is likely that effective placement of the surface electrodes can be a challenge in obese patients, no correlation between the CMAP amplitude and BMI was observed. Our study has shown that a stable CMAP was easily recorded in the vast majority of the patients regardless of the patient’s BMI. In Franceschi’s study, a stable baseline phrenic CMAP amplitude could not be obtained in 12% of the patients, because the pacing catheter could not be positioned properly, and the mean CMAP amplitude was 0.64 mV except in those patients.8 In the present study, the mean CMAP amplitude was 0.65 mV except for in one case in whom the catheter could not be placed in the vein. In addition, the amplitude was less than 0.3 mV in 21.4% of the patients, and it seemed to be difficult to use the CMAP to monitor the phrenic nerve function. In such cases, the distance between the catheter and diaphragm was significantly longer than that in the others, which suggested that the CMAP amplitude was highly influenced by the anatomical location of the vein. The main limitation of this technique is the difficulty in predicting the venous anatomy before the procedure. On the other hand, a > 0.3 mV amplitude could be obtained in 76.2% of the patients without any correlation to the amplitude obtained from the surface electrodes. In fact, a significantly higher amplitude could be obtained from the hepatic vein than from the surface electrodes in 19% of the patients including 2 patients
Lakhani’s study presented that, in 38% of cryoballoon applications, the amplitude of the diaphragmatic CMAP before the second ablation procedure was lower than the amplitude before the first ablation, and the decreased CMAP amplitude before the second freeze was explained by an initial injury to the nerve.9 Mondesert’s study showed that the CMAP amplitude decreased by an average of 13.8 ± 13.8% at the end of the ablation.10 These data suggested that the CMAP amplitude could be used for a quantitative evaluation of the phrenic nerve function and potential PNI. In our data, a > 30% decrease in the CMAP amplitude was observed in 2 patients after the SVC isolation in addition to a case with apparent PNI. Although no abnormal findings could be recognized either on fluoroscopy or on chest X-ray, it was possible that the decrease in the amplitude reflected a potential PNI. It is likely that (1) detecting mild PNI is difficult on fluoroscopy in the supine position during the procedure in clinical practice, and (2) transient PNI usually recovers over a short period as shown with cryoballoon ablation. These might be reasons why no PNI was detected in these cases. Also there was a mean reduction of close to 10% in the CMAPs post ablation with both techniques; however, the average decrease in the CMAP amplitude was small. The decreased CMAP amplitude could indicate an initial injury to the nerve. Once the change in the CMAP is appreciated, measures can be taken to prevent further effects on the nerve. The operator can be more vigilant in monitoring for PNI. Further studies are required to examine the significance of the small amplitude reduction. Study Limitations The study was a single center study and the population was relatively small. It is possible that different races and physical size might have influenced the absolute value of the CMAP amplitude. The clinical significance of a minor reduction in the amplitude of the CMAP was not clarified in the present study and a further prospective study is necessary to clarify this point. Given the higher prevalence of right PNI, CMAP monitoring seems to be more helpful during cryoballoon ablation. However, we believe that a monitoring technique of the PN function should be introduced in RF ablation considering the high prevalence of PNI during SVC17 and PV isolation.18 Diaphragmatic CMAP recordings have variability in the amplitude of the CMAPs induced by
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respirations. To minimize this issue, we measured the amplitude during shallow respirations.
7.
Conclusions A stable diaphragmatic CMAP amplitude could be easily obtained from the standard surface electrodes in the vast majority of patients undergoing AF ablation regardless of the patient’s BMI. Recording the CMAP from the hepatic vein could be an alternative technique and complementary method for monitoring the CMAP. The CMAP amplitude decreased after the SVC isolation in a part of the patients, but not after the PVAI using RF energy. Acknowledgment: We would like to thank Mr. John Martin for his help in the preparation of the manuscript.
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after atrial fibrillation catheter ablation: Characterization and outcome in a multicenter study. J Am Coll Cardiol 2006;47:2498-2503. Franceschi F, Dubuc M, Guerra PG, Khairy P: Phrenic nerve monitoring with diaphragmatic electromyography during cryoballoon ablation for atrial fibrillation: The first human application. Heart Rhythm 2011;8:1068-1071. Franceschi F, Koutbi L, Mancini J, Attarian S, Prevˆot S, Deharo JC: Novel electromyographic monitoring technique for prevention of right phrenic nerve palsy during cryoballoon ablation. Circ Arrhythm Electrophysiol 2013;6:1109-1114. Lakhani M, Saiful F, Parikh V, Goyal N, Bekheit S, Kowalski M: Recordings of diaphragmatic electromyograms during cryoballoon ablation for atrial fibrillation accurately predict phrenic nerve injury. Heart Rhythm 2014;11:369-374. Mond´esert B, Andrade JG, Khairy P, Guerra PG, Dyrda K, Macle L, Rivard L, Thibault B, Talajic M, Roy D, Dubuc M: Clinical experience with a novel electromyographic approach to preventing phrenic nerve injury during cryoballoon ablation in atrial fibrillation. Circ Arrhythm Electrophysiol. 2014;7:605–611. Arruda M, Mlcochova H, Prasad SK, Kilicaslan F, Saliba W, Patel D, Fahmy T, Morales LS, Schweikert R, Martin D, Burkhardt D, Cummings J, Bhargava M, Dresing T, Wazni O, Kanj M, Natale A: Electrical isolation of the superior vena cava: An adjunctive strategy to pulmonary vein antrum isolation improving the outcome of AF ablation. J Cardiovasc Electrophysiol 2007;18:1261-1266. Miyazaki S, Kuwahara T, Kobori A, Takahashi Y, Takei A, Sato A, Isobe M, Takahashi A: Long-term clinical outcome of extensive pulmonary vein isolation-based catheter ablation therapy in patients with paroxysmal and persistent atrial fibrillation. Heart 2011;97:668673. Miller MA, d’Avila A, Dukkipati SR, Koruth JS, Viles-Gonzalez J, Napolitano C, Eggert C, Fischer A, Gomes JA, Reddy VY: Acute electrical isolation is a necessary but insufficient endpoint for achieving durable PV isolation: The importance of closing the visual gap. Europace 2012;14:653-660. Hachiya H, Hirao K, Takahashi A, Nagata Y, Suzuki K, Maeda S, Sasaki T, Kawabata M, Isobe M, Iesaka Y: Clinical implications of reconnection between the left atrium and isolated pulmonary veins provoked by adenosine triphosphate after extensive encircling pulmonary vein isolation. J Cardiovasc Electrophysiol 2007;18:1-7. Miyazaki S, Taniguchi H, Kusa S, Uchiyama T, Hirao K, Iesaka Y: Conduction recovery after electrical isolation of superior vena cava– prevalence and electrophysiological properties. Circ J 2013;77:352358. Dionne A, Parkes A, Engler B, Watson BV, Nicolle MW: Determination of the best electrode position for recording of the diaphragm compound muscle action potential. Muscle Nerve 2009;40:37-41. Miyazaki S, Usui E, Kusa S, Taniguchi H, Ichihara N, Takagi T, Iwasawa J, Kuroi A, Nakamura H, Hachiya H, Hirao K, Iesaka Y: Prevalence and clinical outcome of phrenic nerve injury during superior vena cava isolation and circumferential pulmonary vein antrum isolation using radiofrequency energy. Am Heart J 2014;168:846-853. Yong Ji S, Dewire J, Barcelon B, Philips B, Catanzaro J, Nazarian S, Cheng A, Spragg D, Tandri H, Bansal S, Ashikaga H, Rickard J, Kolandaivelu A, Sinha S, Marine JE, Calkins H, Berger R: Phrenic nerve injury: An underrecognized and potentially preventable complication of pulmonary vein isolation using a wide-area circumferential ablation approach. J Cardiovasc Electrophysiol 2013;24:1086-1091.
Evaluation of Diaphragmatic Electromyograms in Radiofrequency Ablation of Atrial Fibrillation: Prospective Study Comparing Different Monitoring Techniques SHINSUKE MIYAZAKI, M.D.,∗ NOBORU ICHIHARA, M.D.,∗ HIROSHI TANIGUCHI, M.D.,∗ HITOSHI HACHIYA, M.D.,∗ HIROAKI NAKAMURA, M.D.,∗ EISUKE USUI, M.D.,∗ YOSHIHISA KANAJI, M.D.,∗ TAKAMITSU TAKAGI, M.D.,∗ JIN IWASAWA, M.D.,∗ AKIO KUROI, M.D.,∗ KENZO HIRAO, M.D.,† and YOSHITO IESAKA, M.D.∗ From the ∗ Cardiovascular Center, Tsuchiura Kyodo Hospital, Tsuchiura, Ibaraki, Japan; and †Heart Rhythm Center, Tokyo Medical and Dental University, Tokyo, Japan
Diaphragmatic Electromyograms During AF Ablation. Background: The utility of compound motor action potential (CMAP) monitoring for anticipating phrenic nerve injury (PNI) during cryoballoon ablation has been reported. We sought to compare two different CMAP recording techniques and evaluated the feasibility during pulmonary vein antrum isolation (PVAI) and superior vena cava isolation (SVCI) using radiofrequency energy. Methods and Results: Forty-two patients undergoing paroxysmal atrial fibrillation ablation were prospectively enrolled. SVCI was performed following PVAI if SVC potentials were observed. CMAPs were recorded 3 times (before and after PVAI, and after SVCI) simultaneously from surface electrodes (CMAPsuf) and a decapolar catheter in the subdiaphragmatic hepatic vein (CMAPabd). The baseline CMAPsuf and CMAPabd were 0.92 ± 0.36 and 0.65 ± 0.43 mV except in one case with catheter inaccessibility. The CMAPsuf did not correlate with the body mass index, or CMAPabd. In 2 and 9 patients, the CMAPsuf and CMAPabd amplitudes were < 0.5 and < 0.3 mV, respectively. The diaphragm to catheter distance was significantly longer in cases with a CMAPabd < 0.3 mV than one > 0.3 mV (39.2 ± 10.8 vs. 21.5 ± 6.6 mm, P < 0.0001). Two cases with a CMAPsuf < 0.5 mV had larger amplitudes on the CMAPabd. In 1 patient, apparent PNI occurred during the SVCI, and the CMAP disappeared after the SVCI in both techniques. The CMAPs did not significantly decrease after the PVAI and SVCI; however, a >30% decrease was observed in 2 patients in both techniques. In both, no PNI was apparent on fluoroscopy or chest X-ray. Conclusions: Stable evaluable CMAPs were obtained with the CMAPsuf in most patients. Monitoring with the CMAPabd could be an alternative and complementary method. (J Cardiovasc Electrophysiol, Vol. 26, pp. 260-265, March 2015) phrenic nerve injury, diaphragmatic electromyogram, superior vena cava, catheter ablation, atrial fibrillation, pulmonary vein isolation Introduction Electrical isolation of thoracic veins has become a widely accepted strategy in atrial fibrillation (AF) ablation.1-5 However, phrenic nerve injury (PNI)6 is one of the important complications during right pulmonary vein (PV) isolation, and a specific concern during superior vena cava (SVC) isolation because of the anatomical proximity to the course of the right phrenic nerve. Recently, some groups reported the utility of phrenic compound motor action potential (CMAP) monitoring for anticipating PNI during right PV isolation using a cryoballoon.7-10 Two different monitoring techniques have
been introduced to monitor the CMAP. One is recording from the abdominal surface electrodes, which is technically easy but potentially seems to be difficult in obese patients.7,9,10 The other is recording from a multipolar catheter positioned in the subdiaphragmatic hepatic vein.8 However, these monitoring techniques have not been applied to either radiofrequency (RF) ablation or clinical SVC isolation. The objective of this prospective study was to compare the two different monitoring techniques for CMAP recording and to evaluate the feasibility of CMAP recording during PV antrum isolation (PVAI) and SVC isolation using RF energy. Methods
No disclosures.
Study Population Address for correspondence: Shinsuke Miyazaki, M.D., Cardiology Division, Cardiovascular Center, Tsuchiura Kyodo Hospital, 11–7 Manabeshinmachi, Tsuchiura, Ibaraki 300-0053, Japan. Fax: +81-29-826-2411; E-mail: [email protected] Manuscript received 25 August 2014; Revised manuscript received 6 October 2014; Accepted for publication 15 October 2014. doi: 10.1111/jce.12571
This prospective study was comprised of 42 patients who underwent either a first or second catheter ablation procedure for paroxysmal AF using RF energy. In 34 (80.9%) patients who underwent a first procedure, an SVC isolation was performed following the PVAI if SVC potentials were recorded.11 The CMAP was recorded 3 times: before the PVAI, after the PVAI (before the SVC isolation), and after
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the SVC isolation. In the remaining 8 (19.1%) patients who underwent a second procedure, an SVC isolation was performed following the consolidation of the previous PVAI. CMAPs were recorded twice: before and after the SVC isolation. AF was classified according to the HRS/EHRA/ECAS 2012 Consensus Statement on Catheter and Surgical Ablation of AF.5 All patients gave their written informed consent. Electrophysiological Study and Catheter Ablation All antiarrhythmic medications were discontinued for at least 5 half-lives prior to the ablation if they were prescribed before the procedure. Transesophageal echocardiography was performed within 24 hours preprocedurally to exclude any left atrial (LA) thrombi. Enhanced cardiac computer tomography was performed for the evaluation of any relevant cardiac anatomy before the procedure. The surface electrocardiogram (ECG) and bipolar intracardiac electrograms were continuously monitored and stored on a computer-based digital recording system (LabSystem PRO, Bard Electrophysiology, Lowell, MA, USA). Bipolar electrograms were filtered from 30 to 500 Hz. A 7Fr 20-pole three site mapping catheter (BeeAT, Japan Life Line, Tokyo, Japan) was inserted through the right jugular vein for pacing, recording, and internal cardioversion. The 4 proximal electrodes, middle 8 electrodes, and distal 8 electrodes were positioned in the SVC, right atrium, and coronary sinus, respectively. The electrophysiological study was performed under minimal sedation with pentazocine and hydroxyzine pamoate. The details of the ablation protocol have been described previously.12 After one transseptal puncture, two long sheaths (SL0, AF Division, St. Jude Medical, Minneapolis, MN, USA) were introduced into both superior PVs. Pulmonary venography during ventricular pacing and contrast esophagography were performed to obtain the relative locations of the PV ostia vis-a-vis esophagus. A 100 IU/kg body weight of heparin was administered following the transseptal puncture, and heparinized saline was additionally infused to maintain the activated clotting time at 300–350 seconds. Two decapolar circular mapping catheters (Lasso, Biosense Webster, Diamond Bar, CA, USA) were placed in the superior and inferior PVs, and the left- and right-sided ipsilateral PVs were circumferentially ablated under the guidance of a 3-D mapping system (CARTO3, Biosense Webster). Posteriorly, ablation was performed anatomically in the LA, 1–3 cm from the PV ostia. Anteriorly, ablation was performed on the edge of the left PVs guided by the earliest PV potentials. The electrophysiological endpoint of the PVAI was the achievement of bidirectional conduction block between the LA and PVs and the anatomic endpoint was the creation of a complete continuous circumferential lesion around the ipsilateral veins.13 RF current was delivered point-by-point for 30 seconds with a 3.5 mm externally irrigated-tip ablation catheter (Thermocool, Biosense-Webster) with a power of up to 35 W, target temperature 38◦ C and irrigation rate of 30 mL/min. The power was limited to 20 W on the posterior wall close to the esophagus. After completing the PVAI, a 30 mg bolus of adenosine triphosphate was injected to unmask any dormant PV conduction, and any gap responsible for dormant conduction was eliminated by additional RF applications.14 Electrical SVC isolation was added during pacing from the high right atrium. The circular mapping catheter was placed at the level of the lower border of the pulmonary artery above
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the SVC-right atrial junction guided by SVC angiography.15 Before the RF delivery, high output pacing (10 V, 2 ms) was performed at every site, and if diaphragmatic stimulation was observed, ablation was performed with a maximal power of 10 W. The endpoint of the ablation was the elimination of all SVC potentials on the mapping catheter. Recording of the Phrenic CMAP with Different Techniques CMAPs were obtained simultaneously with the different recording techniques described below. First, two additional standard surface ECG electrodes were positioned 5 cm above the xiphoid process and at 16 cm along the right costal margin.7,9,10,16 The CMAP signals were amplified using a bandpass filter between 1 and 2 kHz and recorded on a Bard recording system (Fig. 1). Second, a deflectable decapolar catheter (1.2 mm electrodes spaced 5 mm apart, Snake, Japan Life Line) was placed in a right-sided subdiaphragmatic hepatic vein8 and connected to the Bard recording system. Bipolar CMAP signals were recorded between the distal electrode and one of the proximal electrodes on the decapolar catheter so as to record the highest CMAP amplitude. The signals were amplified and band-pass filtered between 1 and 250 Hz. The distance between the right diaphragm and mapping catheter on the right mammillary line was measured on the fluoroscopic image after the procedure (Fig. 1). The phrenic nerve was paced from the ablation catheter at 60 bpm, using 10-V output for a duration of 2.0 ms. The appropriate pacing site was carefully identified inside the SVC to obtain the highest stable CMAP amplitude, and recording was obtained during shallow respirations according to the operator’s command. It was possible because all procedures were undertaken under minimal sedation and any paralytic agents that inhibit phrenic nerve capture were not administered. Special attention was paid to avoid atrial capture. The phrenic capture was confirmed by abdominal palpation and dynamic diaphragmatic movement on fluoroscopy. The maximum CMAP amplitude was measured from peak to peak. Each measurement involved averaging 4 consecutive phrenic CMAP amplitude values. All measurements and analyses were performed offline after the procedure. Evaluation of PNI The diaphragmatic movement was evaluated throughout the procedure in the supine position, especially at the time of obtaining the CMAP recording. In addition, a chest X-ray (standard PA view) was undertaken in a standing position 1 day before and the next day after the ablation procedure in all patients. If the level of the right diaphragm was significantly elevated on the next day of the procedure relative to that before the procedure, we diagnosed it as PNI. In cases with PNI, the PNI was evaluated by chest X-ray with provocative maneuvers (inspiration and exhalation) during the follow-up. Statistical Analysis Continuous data are expressed as the mean ± standard deviation for normally distributed variables or as median (25th, 75th percentiles) for nonnormally distributed variables, and were compared using a Student’s t-test or Mann-Whitney U-test, respectively. Categorical variables were compared using the chi-square test. A probability value of P < 0.05 indicated statistical significance.
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Figure 1. (A) Representative diaphragmatic compound motor action potential (CMAP) electrograms recorded from surface electrodes (red arrow) and those recorded from a mapping catheter placed in the hepatic vein (green arrow) during phrenic nerve pacing. (B) Decapolar catheter positioned in the subsiaphragmatic vein. The distance between the diaphragm and mapping catheter was 26 mm. CMAPsuf : CMAP recorded from the surface electrodes, CMAPabd : CMAP recorded from a mapping catheter placed in the hepatic vein. For a high quality, full color version of this figure, please see Journal of Cardiovascular Electrophysiology’s website: www.wileyonlinelibrary.com/journal/jce
Results Clinical Characteristics The patient characteristics are summarized in Table 1. The CMAP was recorded during the initial and second ablation procedures in 34 and 8 patients, respectively. Among the 34 patients who underwent an initial procedure, an SVC isolation was not performed in 2 patients because no SVC potentials were recorded in 1 patient and cardiac tamponade occurred during the LA ablation unrelated to the CMAP recording in the other patient. Therefore, a successful PVAI and SVC isolation was achieved in 33 and 32 patients, respectively. As a result, the CMAP was recorded at baseline, after the PVAI (before SVC isolation), and after the SVC isolation in 34, 33, and 32 patients, respectively. In one patient, right PNI occurred during the SVC isolation, which was recognized on fluoroscopy during the procedure. The PNI completely recovered 8 months after the procedure. Among 8 patients who underwent a second procedure, successful SVC isolation was achieved following consolidation of the PVAI in all. Therefore, the CMAP was recorded before and after the SVC isolation in all 8 patients. No other complications were observed. CMAP Recorded from the Surface Electrodes In all 42 patients, a stable diaphragmatic CMAP could be obtained from abdominal surface electrodes with a mean amplitude of 0.92 ± 0.36 mV. There was no significant difference in the CMAP amplitude between the patients with a normal weight and those that were overweight/obese (0.92 ± 0.35 vs. 0.91 ± 0.39 mV, P = 0.89). No correlation was observed between the body mass index (BMI) and baseline CMAP amplitude (P = 0.96). In 2 (4.8%) cases, the baseline CMAP amplitude was < 0.5 mV, whereas it was > 0.5
mV in the remaining 40 (95.2%) patients. The mean CMAP amplitude after achievement of the PVAI was 0.89 ± 0.39 mV, which was similar to the baseline value (P = 0.79). No patients had a >30% reduction in the CMAP amplitude from baseline after the PVAI. The mean CMAP amplitude after the SVC isolation was 0.84 ± 0.30 mV except for in 1 patient with apparent PNI. In that patient, the CMAP could not be recorded after the SVC isolation, and diaphragmatic paralysis was observed on chest X-ray on the next day (Fig. 2). A >30% reduction in the CMAP amplitude after, relative to before, the SVC isolation was observed in 2 patients, whereas no PNI could be recognized either on fluoroscopy during the procedure, nor on chest X-ray on the next day. CMAP Recorded from a Catheter Placed in the Subdiaphragmatic Hepatic Vein In 1 (2.4%) patient, a deflectable mapping catheter could not be inserted into the subdiaphragmatic hepatic vein. Among the remaining 41 patients, the catheter position was stable without the need for repositioning, and the mean baseline CMAP amplitude was 0.65 ± 0.43 mV, including 9 patients with an amplitude < 0.3 mV. The distance between the diaphragm and mapping catheter on fluoroscopy was significantly longer in the cases with an amplitude of < 0.3 mV than in those with an amplitude > 0.3 mV (39.2 ± 10.8 vs. 21.5 ± 6.6 mm, P < 0.0001). The CMAP amplitude before and after the SVC isolation was 0.60 ± 0.42 and 0.53 ± 0.36 mV (P = 0.14), respectively. Among 32 patients with a baseline CMAP amplitude of > 0.3 mV, a >30% reduction in the amplitude was observed after the SVC isolation in 2 patients. In both patients, a >30% reduction was also observed in the surface electrode recordings. In a case with apparent PNI, the CMAP could not be obtained after the SVC isolation (Fig. 2).
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Figure 2. A case with clinically apparent PNI. (A) CMAP recorded before the SVC isolation. The CMAP recorded from the surface electrodes (red arrow) was smaller than that recorded from a mapping catheter positioned in the hepatic vein (green arrow) at baseline. (B) CMAP recorded after the SVC isolation. The CMAP could not be recorded in either technique. (C) The distance between the diaphragm and mapping catheter placed in the hepatic vein was 19 mm. (D) A chest X-ray 1 day before the procedure. (E) A chest X-ray 1 day after the procedure. Elevation of the right diaphragm was observed. For a high quality, full color version of this figure, please see Journal of Cardiovascular Electrophysiology’s website: www.wileyonlinelibrary.com/journal/jce
Comparison of the CMAP Amplitude Recorded by Different Techniques There was no correlation of the CMAP amplitude obtained by the 2 different techniques (P = 0.67). Among a total of 42 patients, the baseline CMAP amplitude was considerably higher in the recordings from the surface electrodes and those from the hepatic vein in 27 (64.3%) and 8 (19.0%) patients, and was almost the same in the recordings obtained by the 2 different techniques in the remaining 7 (16.7%) patients. In 2 patients in whom the CMAP amplitude recorded from the surface electrodes was < 0.5 mV (0.26 and 0.36 mV), a much higher CMAP amplitude was obtained from the hepatic vein in both (0.74 and 0.74 mV). One of those 2 patients was the patient with apparent PNI during the SVC isolation (Fig. 2). Discussion This study compared 2 different CMAP recording techniques during AF ablation. To the best of our knowledge, the present study is one of the first to evaluate the utility of the CMAP in RF ablation, and during clinical SVC isolation. We found that (1) a stable evaluable CMAP could be easily obtained from the surface electrodes in the vast majority of patients undergoing AF ablation, (2) the amplitude of the CMAP recorded from the surface electrodes did not correlate with the BMI, (3) the amplitude of the CMAP amplitude measured from the subdiaphragmatic hepatic vein highly depended on the anatomical location of the vein, (4) there was no correlation between the CMAP amplitude recorded from
the 2 different techniques, and (5) the CMAP amplitude decreased after the SVC isolation without any clinically apparent PNI in a part of patients undergoing an SVC isolation. Recording the Phrenic CMAP During Catheter Ablation In patients with neuromuscular disorders affecting phrenic nerve conduction, CMAP recordings have provided useful functional information.16 Franceschi et al. initially applied this technique to monitoring the phrenic nerve function during ablation procedures, and demonstrated that the CMAP amplitude could be reliably recorded on modified surface leads during ablation.7 Subsequently, Lakhani et al.9 and Mondesert et al.10 demonstrated that monitoring the CMAP could be an early and reliable method for predicting PNI using cut offs of a 35% and 30% decrease in the CMAP amplitude from the baseline in patients undergoing cryoballoon ablation for AF. This technique is simple and easily applicable; however, it seemed to be limited in obese patients. More recently, Franceschi et al. reported the clinical use of CMAP recordings using a multipolar catheter in the subdiaphragmatic hepatic vein in an effort to overcome the drawbacks of the measurement using the surface electrodes.8 They showed that a stable and reliable CMAP amplitude could be recorded safely in the majority of patients, and no PNI was observed using a cut off value of a >30% drop in the CMAP amplitude. This technique has theoretically some advantages such as (1) a electrode location close to the diaphragm allows recording of high CMAP amplitude values, and (2) the distance between the muscle and electrode does not vary during respirations because the liver moves with the diaphragm.
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TABLE 1 Clinical Characteristics N Age (y) Paroxysmal AF, n (%) Initial procedure, n (%) Female, n (%) Structural heart disease, n (%) Hypertension, n (%) Diabetes mellitus, n (%) Body mass index (kg/m2 ) Normal weight (BMI < 25) Overweight (30 > BMI 25) Obesity (BMI 30) LA diameter (mm) LV ejection fraction (%)
42 60.3 ± 12.0 42 (100%) 34 (80.9%) 10 (23.8%) 0 (0%) 13 (31.0%) 5 (11.9%) 24.5 ± 3.4 24 (57.1 %) 15 (35.7%) 3 (7.1%) 36.8 ± 4.7 66.3 ± 3.7
in whom the amplitude recorded from the surface electrodes was less than 0.5 mV. While recording from the hepatic vein provides reliable signals in the majority of cases, it necessitates an additional venous puncture and electrophysiology catheter, thus incurring additional cost and the risk of complications. In contrast, the use of the surface electrodes is relatively simple, easy, and noninvasive. It is likely that the reproducibility of CMAPs would be higher for surface CMAPs because the best position of the surface electrodes is established,16 but abdominal CMAPs are influenced by the catheter position. Based on our study results, we believe that monitoring the CMAP recorded from the hepatic vein is recommended in patients in whom evaluable large baseline CMAPs could not be obtained from the surface electrodes. Change in the CMAP Amplitude During the PVAI and SVC Isolation
Baseline Amplitude of the CMAP On the surface electrodes, an initial average CMAP amplitude of 0.6 mV in healthy volunteers was previously reported in the neurologic literature,16 and 0.34 and 0.87 mV in Lakhani’s and Mondesert’s studies.9,10 The difference might be explained by the different medications used during the procedure. Patients received paralytics during intubation in the former study, and the procedure was performed under conscious sedation in the latter study. CMAP recordings were unable to be acquired in 7% because there was an initial amplitude of < 0.15 mV in the former study.9 In the present study, the mean amplitude was 0.92 mV under minimal sedation, which was similar to the result of the latter study. Clear CMAP recordings could be obtained in the vast majority of the cases; however, in 2 patients the amplitude was < 0.5 mV. Because the CMAP usually fluctuates with respirations, it is important to obtain a high CMAP amplitude at baseline to evaluate it during the procedure. Although it is likely that effective placement of the surface electrodes can be a challenge in obese patients, no correlation between the CMAP amplitude and BMI was observed. Our study has shown that a stable CMAP was easily recorded in the vast majority of the patients regardless of the patient’s BMI. In Franceschi’s study, a stable baseline phrenic CMAP amplitude could not be obtained in 12% of the patients, because the pacing catheter could not be positioned properly, and the mean CMAP amplitude was 0.64 mV except in those patients.8 In the present study, the mean CMAP amplitude was 0.65 mV except for in one case in whom the catheter could not be placed in the vein. In addition, the amplitude was less than 0.3 mV in 21.4% of the patients, and it seemed to be difficult to use the CMAP to monitor the phrenic nerve function. In such cases, the distance between the catheter and diaphragm was significantly longer than that in the others, which suggested that the CMAP amplitude was highly influenced by the anatomical location of the vein. The main limitation of this technique is the difficulty in predicting the venous anatomy before the procedure. On the other hand, a > 0.3 mV amplitude could be obtained in 76.2% of the patients without any correlation to the amplitude obtained from the surface electrodes. In fact, a significantly higher amplitude could be obtained from the hepatic vein than from the surface electrodes in 19% of the patients including 2 patients
Lakhani’s study presented that, in 38% of cryoballoon applications, the amplitude of the diaphragmatic CMAP before the second ablation procedure was lower than the amplitude before the first ablation, and the decreased CMAP amplitude before the second freeze was explained by an initial injury to the nerve.9 Mondesert’s study showed that the CMAP amplitude decreased by an average of 13.8 ± 13.8% at the end of the ablation.10 These data suggested that the CMAP amplitude could be used for a quantitative evaluation of the phrenic nerve function and potential PNI. In our data, a > 30% decrease in the CMAP amplitude was observed in 2 patients after the SVC isolation in addition to a case with apparent PNI. Although no abnormal findings could be recognized either on fluoroscopy or on chest X-ray, it was possible that the decrease in the amplitude reflected a potential PNI. It is likely that (1) detecting mild PNI is difficult on fluoroscopy in the supine position during the procedure in clinical practice, and (2) transient PNI usually recovers over a short period as shown with cryoballoon ablation. These might be reasons why no PNI was detected in these cases. Also there was a mean reduction of close to 10% in the CMAPs post ablation with both techniques; however, the average decrease in the CMAP amplitude was small. The decreased CMAP amplitude could indicate an initial injury to the nerve. Once the change in the CMAP is appreciated, measures can be taken to prevent further effects on the nerve. The operator can be more vigilant in monitoring for PNI. Further studies are required to examine the significance of the small amplitude reduction. Study Limitations The study was a single center study and the population was relatively small. It is possible that different races and physical size might have influenced the absolute value of the CMAP amplitude. The clinical significance of a minor reduction in the amplitude of the CMAP was not clarified in the present study and a further prospective study is necessary to clarify this point. Given the higher prevalence of right PNI, CMAP monitoring seems to be more helpful during cryoballoon ablation. However, we believe that a monitoring technique of the PN function should be introduced in RF ablation considering the high prevalence of PNI during SVC17 and PV isolation.18 Diaphragmatic CMAP recordings have variability in the amplitude of the CMAPs induced by
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respirations. To minimize this issue, we measured the amplitude during shallow respirations.
7.
Conclusions A stable diaphragmatic CMAP amplitude could be easily obtained from the standard surface electrodes in the vast majority of patients undergoing AF ablation regardless of the patient’s BMI. Recording the CMAP from the hepatic vein could be an alternative technique and complementary method for monitoring the CMAP. The CMAP amplitude decreased after the SVC isolation in a part of the patients, but not after the PVAI using RF energy. Acknowledgment: We would like to thank Mr. John Martin for his help in the preparation of the manuscript.
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