Between January 2008 and October 2012, 38 consecutive patients underwent electrophysiological study and catheter ablation of typical AFL with the Niobe MNS at our institution. AFL was documented by 12-lead ECG and the patients were symptomatic despite medication. No patient was contraindicated for magnetic navigation, and all gave their written informed consent.

Patients were assessed after six hours’ fasting and antiarrhythmic medication was suspended for five half-lives; amiodarone was suspended at least one month before the procedure. Patients with persistent AFL who were not under oral anticoagulation underwent transesophageal echocardiography to exclude the presence of left atrial thrombi.

Surface electrocardiograms and intracavitary electrograms were recorded on an AXIOM Sensis system (Siemens Healthcare). Programmed stimulation was performed using a UHS 3000 heart stimulator (Biotronik). The catheters were inserted via the femoral vein and positioned under fluoroscopic guidance, a quadripolar catheter (Navistar, Biosense Webster) in the His bundle and an octapolar catheter (Dynamic XT, Bard) in the coronary sinus (CS).

When ablation was carried out in AFL rhythm, entrainment pacing was used to demonstrate the presence of isthmus-dependent atrial flutter. Patients in sinus rhythm underwent CS pacing to measure conduction time across the CTI before ablation.

All procedures were carried out using the Niobe II system. The MNS has been previously described9; it basically consists of two computer-controlled magnets positioned on either side of the fluoroscopy table, which create a magnetic field (0.1 T). The position of the magnets is controlled from a console, the Navigant workstation, which orientates the magnetic field according to vectors selected by the operator. The ablation catheter has three magnets at its distal end, which orientate it parallel to the magnetic field. Changes in the orientation of the magnetic field deflect the catheter tip, which is advanced or withdrawn remotely by a motor at the proximal end of the catheter (Cardiodrive, Stereotaxis). Magnetic field vectors can be stored, enabling subsequent automatic navigation to previous sites.

The MNS is integrated with a CARTO XP RMT (Biosense Webster) electroanatomical mapping system and provides real-time information on the position and orientation of the mapping catheter tip. This information is overlaid on the fluoroscopic images on the Navigant workstation, providing real-time monitoring of the catheter position without the need for further fluoroscopy (Figure 1). The location of the His bundle is marked on the screen of the CARTO workstation.

Figure 1.

Reference fluoroscopic images on the Navigant workstation in right and left anterior oblique views showing the real-time position of the ablation catheter, without further fluoroscopy, in the cavotricuspid isthmus at the 5 o’clock position, the automatic vector of the magnetic navigation system (yellow arrow) and the His bundle (yellow point). OAD: right anterior oblique; OAE: left anterior oblique.

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A Navistar RMT (Biosense Webster) catheter with an 8-mm tip was used for the first 17 procedures and a Navistar RMT Thermocool (Biosense Webster) irrigated-tip catheter for the subsequent 21 procedures. The ablation catheter was introduced via the femoral vein and advanced manually to the right atrium without the need for long sheaths, then remotely to the tricuspid annulus at the 6 o’clock position in left anterior oblique view and subsequently to the right ventricle, after which it was gradually withdrawn until an atrial potential was detected on the distal bipole of the ablation catheter. RF energy was then applied at this point and the ablation catheter was progressively withdrawn under remote control to the inferior vena cava (IVC).

The MNS vector was directed downward in the initial portion of the CTI and more anteriorly in the final portion, closer to the IVC (Figure 2). Using an EP Shuttle RF generator (Stockert), RF was applied between the distal electrode of the ablation catheter and a cutaneous patch electrode, with a power limit of 70 W and maximum temperature of 55°C in the case of the 8-mm catheter, and a power limit of 50 W and maximum temperature of 45°C with the irrigated catheter. The duration of RF application depended on the disappearance of the atrial potential on the distal electrogram, but was never less than 30 s. Light sedation with midazolam (bolus) or remifentanil (perfusion) was administered during RF application when necessary. Procedure and fluoroscopy times were recorded, procedure time being defined as the interval between venous puncture and removal of the introducer. Acute success was defined as demonstration of bidirectional block in the CTI and non-inducibility of AFL after 30 min. Bidirectional block was demonstrated by documentation of downward activation in the right atrial lateral wall during CS pacing and downward activation in the septum during low-lateral right atrial pacing, as well as a conduction delay across the isthmus. As previously described,10 the minimum conduction delay across the isthmus during CS pacing that demonstrates conduction block in the CTI was taken to be 110 ms (Figure 3).

Figure 2.

Radiofrequency applications (red circles) on the CARTO mapping system. On the right, fluoroscopic image showing the ablation catheter and the magnetic navigation vector. OAD: right anterior oblique; OAE: left anterior oblique.

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Figure 3.

Pacing by the mid-proximal bipole in the coronary sinus with the mapping catheter on the lateral side of the ablation line, showing a conduction delay of 192 ms in the cavotricuspid isthmus.

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If the initial ablation line did not result in CTI block, the line was mapped to detect possible gaps, and if CTI conduction persisted after any gaps were eliminated, additional lines were created in more lateral or septal positions. If bidirectional CTI block could not be achieved, the procedure was considered unsuccessful. All patients remained under surveillance in hospital for 24 hours after the procedure.

The results obtained with an 8-mm tip catheter were compared with those using an irrigated catheter.

Follow-up consultations included clinical assessment between the first and third month after the procedure and then every six months. Holter 24-hour monitoring was performed between the third and sixth month and then at least once a year. Clinical success was defined as absence of palpitations and/or absence of AFL on ECG.

Data are presented as means ± standard deviation for continuous variables and as frequencies for categorical variables. Categorical variables were analyzed with the chi-square test and continuous variables with the Student's t test.

Thirty-eight consecutive patients, mean age 61±15 years, 28 male, underwent ablation of typical AFL. Twenty-six had persistent AFL. Nineteen had hypertension, four had coronary disease and had undergone coronary artery bypass grafting, two had non-ischemic dilated cardiomyopathy, one had Ebstein's anomaly and 12 had no structural heart disease. The baseline characteristics of the two groups (those treated by 8-mm tip catheter and by irrigated catheter) are presented in Table 1. There were no significant differences between the groups.

Ablation was carried out in AFL rhythm in 31 patients and in sinus rhythm in seven. The procedure was successful in 37 patients (97%). FLA was terminated during the procedure in 30 cases.

Procedure time was 167±59 min and fluoroscopy time was 7.8±5.3 min. The mean number of RF applications was 22±15, with a mean application time of 24±13 min.

The ablation procedure characteristics in the study population are presented in Table 2. The success rate was similar in both groups, 94% in the 8-mm tip catheter group and 100% in the irrigated catheter group, as was procedure time, but fluoroscopy time was significantly shorter in the irrigated catheter group (13.4±3.7 vs. 6±4.4 min; p<0.01). The number of RF applications and total RF application time did not differ significantly between the groups.

There were no procedure-related complications.

During a follow-up of 32±19 months (6–60) two patients had relapse of AFL, one in each group (Table 3). One patient refused reablation and in the other the procedure was repeated with an irrigated catheter, which was successful. Six patients developed atrial fibrillation (AF) during follow-up that was treated by ablation. After pulmonary vein isolation, conduction across the CTI was assessed and continuing bidirectional block was confirmed in all patients.

There were no deaths during follow-up.

The authors declare that no experiments were performed on humans or animals for this study.

The authors declare that they have followed the protocols of their work center on the publication of patient data.

The authors have obtained the written informed consent of the patients or subjects mentioned in the article. The corresponding author is in possession of this document.