Background

Radiofrequency (RF) treatment is the first-choice therapy for simple arrhythmias, considering the numerous secondary effects and low efficacy of antiarrhythmic drugs in this indication . Over the past few years, RF treatment has taken a decisive place in the treatment of complex arrhythmias and, more particularly, in the treatment of atrial fibrillation (AF) . For these particular indications, operators must be experienced, especially in manipulating catheters in difficult clinical situations, which may lead to long, tedious and potentially risky procedures . A major limitation of this manual method is caused by the catheter technology, as catheter mobility is limited by the transmission of the torque, depending on vessel tortuosity, orientation of the catheter in the heart and its rigidity or instability. During these procedures, the operator is exposed not only to X-rays but also to abnormal fatigue, which may lead to a loss of concentration. This decreased concentration may result in delayed analysis and, thus, a lengthened procedure or greater risk of complications. In addition, AF treatment is increasingly used in electrophysiological laboratories, owing to the prevalence of AF (2–3% of the population aged more than 60 years) and the low benefit/risk ratio of antiarrhythmic drugs compared with RF techniques, as shown in randomized studies .

The future is therefore in favour of a technology that is at least as effective as the manual RF technique but has an improved safety profile regarding potential complications and other variables, such as X-ray exposure for the patient and operator. Such technology should eventually allow for the management of more patients without affecting operator health. The magnetic navigation system (MNS) appears to be a futuristic technology that benefits from a very favourable benefit/risk ratio for both the patient and operator . This article aims to present an overview of this modern technology, which seems particularly adapted to the treatment of complex arrhythmias using RF.

Description of the system

The MNS (Niobe II; Stereotaxis, Inc., St. Louis, MO, USA) is a technological platform using a steerable magnetic field, which remotely guides a supple catheter inside the heart . The steerable magnetic field contains two giant computer-controlled 1.8-ton magnets positioned on opposite sides of the fluoroscopy table ( Fig. 1 ). A magnetic field of 0.08–0.1 Tesla is generated (according to the initial choice), such that the three small magnets incorporated in parallel with the tip of the RF catheter permit three-dimensional (3D) navigation ( Fig. 2 ). The magnetic field is applied to a theoretical cardiac volume of 20 × 20 cm. The catheter tip may be directed very precisely by using a vector-based computer system (Navigant; Stereotaxis, Inc., St. Louis, MO, USA) ( Fig. 3 ). This system operates by aligning the catheter relative to the magnetic field generated, whereby the movements of the catheter depend on changes in the direction of the two magnets in relation to each other. Advancing or retracting the catheter is controlled by a computerized motor drive system (Cardiodrive; Stereotaxis, Inc., St. Louis, MO, USA) ( Fig. 4 ), while its orientation in space requires a computerized workstation (Navigant 2.1; Stereotaxis, Inc., St. Louis, MO, USA) ( Figs. 3 and 4 ). Using a keypad (arrows) or joystick, the catheter may be continuously advanced or retracted, or even adjusted (from 1–9 mm). The second-generation Niobe II allows the magnets to be tilted in directions ranging from 40° left anterior oblique to 30° right anterior oblique. The constant application of the magnetic field during the ablation procedure keeps the catheter tip in permanent contact with the endocardial tissue throughout the cardiac cycle, thus improving delivery of the RF current. The flexibility and weak force (15–20 g) exerted by the magnetic field result in reliable navigation inside the heart, with a near-zero risk of perforation . The system is able to memorize certain data, such as the position of veins, and reutilize these vectors during the examination to facilitate the navigation of the catheter or improve procedure times. In addition, automatic navigation is possible using NaviLine (Stereotaxis, Inc., St. Louis, MO, USA), which allows for automatic processing by producing a line or surrounding veins.

Simulation of the examination table, with the active position of the magnets in the presence of the patient.

Irrigated magnetic catheter with three magnets incorporated into the catheter tip. Note the extreme flexibility of the magnetic catheter.

‘QuickCAS’ Cardiodrive system positioned in the patient’s groin, permitting the catheter to be advanced by means of a keypad or joystick, as shown above.

Screenshot of the Navigant system, which allows for real-time navigation in the different parts of the heart using simple vector orientation from the keyboard, where the physician works remotely on the patient.

The main progress made over the past months has been to integrate a platform with the 3D mapping systems Carto XP, Cartomerge and Carto 3 (Biosense Webster, Diamond Bar, CA, USA) ( Fig. 5 ). With the development of the 3.5-mm tip irrigated magnetic catheter, this technology is being used for the treatment of complex left atrial arrhythmias. In this platform, the advantage of Carto 3 is the visualization of all the catheters and the possibility of rapid reconstruction of the anatomical structures using the magnetic lasso tool (Lasso NAV; Biosense Webster, Diamond Bar, CA, USA).

Integration of the Carto 3 RMT with the Navigant system, permitting instantaneous navigation, with visualization of both the Navistar RMT catheter and Lasso NAV. The virtual vector in green is followed by the catheter vector in yellow.

Experimental studies

The first experimental studies using a magnetic field to test the displacement of catheters were carried out by Tillander et al. and then by Ram et al., but the attempts of these authors were limited by the weak force of the magnetic field, the size of the catheters and the absence of precise control in three dimensions . Subsequent developments incorporated the use of stereotactic localization and vector control using dedicated software .

The first feasibility study carried out in animals was published by Faddis et al. in 2002. The authors studied MNS technology in the laboratory . The precision of the navigation system was tested and shown to be viable in six animals undergoing an RF procedure, for which 51 anatomical targets were tested, including the pulmonary veins . Several variables were evaluated, including navigation, deflection force of the catheter, interference of the electrocardiogram signal analysed during the procedure and RF efficacy . The authors showed that the maximal force applied on the catheter was 26.8 g versus 31.4 g using the manual method (limits of 19.7–45.4 g), while navigation precision was possible in 46/51 of the targets tested (90%). The only failure occurred in an animal in which the aortic arch could not be crossed. To overcome this difficulty, the magnetic catheter was modified by improving the profile of the two catheter segments (flexible section and section more rigid). For the 30 targets tested following modification of the catheter, the success rate of lesion completion was 29/30 (i.e. 97% of the cases tested). During the course of the same procedures, the variation measured for the position of the catheter with respect to the target was only 0.73 mm. Navigation at the level of the pulmonary veins was tested in five animals, including 30 veins, and was found to be possible in 100% of cases versus 70% using the manual method . The feasibility of the technology in terms of obtaining an efficacious lesion was validated as excellent. Interference of the signals studied via the signal/noise ratio was very small, despite the distortion exerted on the catheter and, above all, was non-significant compared with the manual method. A certain number of limitations exist for this study, such as the lack of a randomized study design as well as the absence of evaluation of microscopic lesions caused by RF treatment .

Greenberg et al. were the first researchers to test the MNS on the electrical disconnection of the pulmonary veins in animals (seven healthy dogs) . They showed that electrical disconnection of the veins was possible in 100% of the cases without the risk of pulmonary vein stenosis at 80 days, as analysed via necropsy and computed tomography scan .

Faddis et al. subsequently conducted a clinical feasibility and safety study in 31 patients requiring ablation of complex arrhythmias, including three patients with AF . In this study, blinded analysis showed that there was no qualitative difference in the signal compared with the manual method . Navigation in the cardiac cavities was possible in 213/215 sites tested on the right (99%) and in 13/13 sites tested on the left, while the levels of pacing thresholds were not significantly different between the manual method and the MNS . No complication was noted in this study and arrhythmia ablation was possible in the seven patients tested (100% of cases) .

Ernst et al. subsequently investigated the feasibility and efficacy of the MNS in patients with intranodal re-entry reciprocating tachycardia . In the 42 patients tested, analysis of the slow potentials was possible in 100% of cases, as was ablation of the slow path in 100% of cases . The advantages of this technique were underlined by the authors, who highlighted the absence of the risk of perforation, the excellent stability of the catheter and the possibility of navigating in complex anatomical structures, such as was described for a patient with persistence of the superior vena cava associated with a giant ostium of the coronary sinus and a junctional tachycardia, who was efficaciously treated with the MNS .