We retrospectively selected 139 consecutive patients undergoing AVNRT ablation guided by an MNS by a single operator (MNS group) between January 2009 and June 2016 (procedures before 2009, the learning period, were excluded) and compared them to a group of 101 consecutive patients undergoing manual AVNRT ablation in the same period by the same operator in another hospital (MAN group). The characteristics of both patient groups are displayed in Table 1. Patients who had already undergone ablation were excluded.

Patients were studied in a fasting non-sedated state under local anesthesia. All antiarrhythmic drugs were discontinued at least five half-lives before the electrophysiological study. Patients gave their written informed consent.

We used the standard number of catheters inserted via the right femoral vein under fluoroscopic guidance: a quadripolar catheter in the right ventricle recording the His bundle electrogram in the proximal dipole and a decapolar catheter in the coronary sinus. The presence of dual AV node conduction was assessed, and accessory pathways were excluded. When AVNRT was not inducible in the resting state, an isoproterenol infusion was administered intravenously, as needed.

All procedures were performed using the Niobe II MNS (Stereotaxis) working with the single-plane AXIOM Artis fluoroscopy system (Siemens).

The MNS, previously described,5 consists of two computer-controlled permanent magnets positioned on opposite sides of the fluoroscopy table. These magnets create a magnetic field of 0.1 T. The position of the magnets is remotely controlled by a console, the Navigant workstation, which changes the orientation of the magnetic field according to the vectors chosen by the operator (Figure 1). The ablation catheter has three magnets in its distal portion that keeps it parallel to the magnetic field. Changes in the orientation of the magnetic field deflect the catheter, which is remotely advanced or retracted with the aid of a motor drive, Cardiodrive (Stereotaxis). Magnetic field vectors can be stored in order to automatically navigate the ablation catheter to previous sites.

Figure 1.

Navigant workstation screen: junctional rhythm during radiofrequency delivery (top panel); reference X-ray image in left anterior oblique view displaying the real-time position of the ablation catheter, His bundle catheter and His bundle location (white dots); the magnetic navigation system's automatic vector to slow pathway location (vector).

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The procedures in the MAN group took place in an electrophysiology laboratory equipped with a Philips BV Pulsera fluoroscopy system. The methodology of mapping and ablation was the same in both groups. Koch's triangle was mapped by bending and pulling the ablation catheter from the His position and rotating to the coronary sinus in order to obtain a slow pathway potential with a small atrial and large ventricular electrogram in the distal bipole. RF energy was applied at this site in order to obtain JR during RF delivery. The catheter was moved to another site whenever JR did not appear within seconds of RF application. A 4-mm tip Navistar RMT magnetic catheter (Biosense Webster) was used in the MNS group and a 4-mm tip catheter (Medtronic Marinr®, Biosense Webster Celsius® or St. Jude Medical Therapy®) was used in the MAN group. RF was applied under fluoroscopic guidance in order to check catheter position, for up to 120 s in the MNS group and 90 s in the MAN group, to a maximum temperature of 55°C and a power output limit of 55 W. RF was applied for longer in the MNS group due to the lower force applied by the magnetic catheter to the endocardium, in an empiric attempt to improve lesion formation without increasing risk in view of the greater softness and stability of this catheter. During ablation, light sedation with midazolam (bolus) was administered when needed.

Total procedural and fluoroscopy times were recorded. Procedure time was defined as the time between the beginning of venous puncture until removal of the sheaths. Fluoroscopy times were measured for the patient and separately for the operator.

Acute success was defined as failure to induce AVNRT, including after isoproterenol infusion. The presence of a nodal echo beat was not perceived as failure.

All patients were monitored in the hospital for 24 hours after the procedure.

Follow-up was performed at outpatient clinical visits or by telephone. Recurrence was defined as the presence of symptoms (palpitations) and electrocardiographic documentation of AVNRT.

Continuous variables were presented as mean and standard deviation and compared with the Student's t test for independent samples. Categorical data were expressed as percentages and compared with the chi-square test. Outcomes were analyzed with time-to-event methods. Kaplan-Meier plots were calculated using the log-rank test for AVNRT recurrence. Statistical analysis was performed using IBM SPSS 23.0 (IBM SPSS Inc., Chicago, IL, USA). Statistical significance was defined as p<0.05.

The characteristics of the two patient groups are displayed in Table 1. Patients in the MNS group were younger (45±14 vs. 52±16 years, p=0.01) and fewer had hypertension (nine vs. 17 patients, p=0.02), but the prevalence of structural heart disease was not significantly different.

Typical slow-fast AVNRT was induced in all patients in both groups. Procedure time and total fluoroscopy time were not significantly different (Table 1) but operator fluoroscopy time was significantly shorter in the MNS group (2.4±1.5 min vs. 7.2±4 min, p<0.001). The mean duration of RF applications was higher for the MNS group (229±208 vs. 181±131, p=0.03). JR appeared during RF application in all patients in both groups. Ablation was successful in all patients in both groups. RF delivery was interrupted in one patient in the MNS group and in one patient in the MAN group due to transient second-degree AV block, which disappeared within seconds of RF interruption. No charring was found on the catheter tip in either group. There were no other procedure-related complications.

The duration of follow-up was similar in the two groups (44.8±24 months in MNS and 47.3±25 in MAN). During this period three patients died in the MAN group, two of cancer and one of heart failure. No patients were lost to follow-up. Five patients had recurrence of AVNRT, four in the MAN group (two in the first month, one after six months and one after eight months) and one in the MNS group (after nine months). Although the recurrence rate was higher in the MAN group, this was not statistically significant (0.7% vs. 4%, p=0.097 by the log-rank test). All these patients underwent a second successful procedure and there were no further recurrences.

In our study the procedure time was similar in the two groups, unlike in previous studies8,9 which report a longer procedure time with MNS. The fact that most such studies report an initial experience may be responsible for their longer procedure time.

The RF settings used in previous studies were not uniform, each group using its own settings. In the first reports of MNS for AVNRT, Ernst et al.5 used a maximum of 40 W, Davis et al.9 used 30 W and others, including Kerzner et al., 10 up to 50 W. Some state that the use of higher power is associated with clot formation at the catheter tip8 and accordingly use less power. We use the same settings with MNS as with manual ablation, namely power up to 50 W to a maximum temperature of 55°C, and have never observed charring or popping. The high temperature (65°C) programmed by Moreno et al.’s group8 was probably responsible for the charring they observed at the catheter tip.

Ricard et al.11 reported a lower incidence of JR during RF application, which according to the authors was due to the greater catheter stability, which minimizes micro-dislodgement toward the AV node. By contrast, Davis et al.9 found that JR appeared sooner with lower maximum temperature and explained this as resulting from greater catheter stability.

JR was achieved in all patients in both MNS and MAN groups, either at the initial position or after remapping and re-ablating at another site. We are convinced that the presence of JR is as important in MNS as in MAN ablation and is essential to obtain successful slow pathway ablation. In our series, RF application time was longer in the MNS group, which is not in agreement with previous studies. This is probably related to the longer duration of RF for each lesion that was routine in the MNS group compared to the MAN group.

The acute success rate with the MNS was high and comparable to MAN ablation.

Although not statistically significant, there was a trend towards a lower recurrence rate in the MNS group. This finding was also observed by our group in the ablation of other arrhythmias12,13 and may be explained by better catheter contact and less edema formation, leading to longer-lasting lesions. However, we cannot exclude the possibility that the longer duration of RF application could affect long-term results.

The overall complication rate was similar in both groups, with one transient second-degree AV block in one patient in each group. This is a known complication in 1-2% of AVNRT ablation procedures.14

The most important difference in terms of radiation exposure is the reduction of operator fluoroscopy time with MNS to one third that with manual ablation. This finding is consistent in all studies, and is undoubtedly due to the fact that after positioning the catheters the operator can leave the patient's side and perform the ablation remotely in another room. The cumulative radiation dose during a lifetime of exposure is a concern for healthcare professionals involved in fluoroscopically driven procedures, especially electrophysiologists and interventional cardiologists. This significantly decreased operator fluoroscopy time reduces the risk of malignancy and other potentially deleterious effects of radiation.15,16

There is less agreement concerning reduction of total fluoroscopy time. In some studies MNS enabled reductions in radiation for both patients and medical staff. Kim et al.17 found a significant reduction in total fluoroscopy time with MNS, although the authors did not discuss the reason for this finding. Some authors8,11,18 found no significant reduction in total fluoroscopy time, as was the case in our series, while others, like Ricard et al.,11 recorded longer total fluoroscopy time. In our opinion there is no reason that total fluoroscopy time would be reduced in AVNRT ablation. The main reason that fluoroscopy time is reduced with the use of MNS is that the catheter can be manipulated more safely without fluoroscopy due to the softness of the catheter tip. This may make a difference in complex cases with a greater need for catheter manipulation such as ablation of atrial fibrillation, atrial flutter or ventricular tachycardia.12,13,19 In the case of AVNRT the amount of fluoroscopy needed to place the ablation catheter over the slow pathway is low with both manual and magnetic navigation. Most of the radiation exposure occurs during RF application, since we always apply RF energy under fluoroscopic guidance, even with the MNS. In fact, although the MNS catheter itself is very stable, and some authors2 state that due to the stability of the catheter the RF may be applied without fluoroscopy, we believe that in situations such as deep breathing, intense cough or inadvertent body movements we cannot totally rely on the stability of the catheter, given its proximity to the AV node.