The Use of Remote Magnetic Navigation in Catheter Ablation of Atrial Arrhythmias

Jeffrey A. Tice, MD

Disclosures

CTAF 

In This Article

Background

Catheter Ablation of Atrial Arrhythmias

In the past, surgery was sometimes performed to treat arrhythmias such as atrioventricular nodal reentrant tachycardia (AVNRT), atrioventricular reentrant tachycardia (AVRT), atrial tachycardia (AT), atrial flutter (AFlut) and atrial fibrillation (AFib).[1–5] The key insight that allowed for surgical treatment of these arrhythmias was that they have a focal site of origin in the heart. The surgeon destroyed the tissue at that critical site in order to prevent recurrence of the arrhythmia.

In the early 1980's, catheter ablation was introduced as a less invasive approach to produce the same effect.[6,7] Catheter ablation is performed in the electrophysiology laboratory. Multiple percutaneous catheters are positioned in the heart, most commonly using the femoral vein for access. The catheters are used for electrical pacing and recording at multiple sites within the heart in order to map the electrical pathway responsible for the arrhythmia. The initial catheter technique used direct current energy to cause tissue damage, but this approach rapidly fell out of favor because it required general anesthesia and had a high incidence of significant complications including death.[8] Radiofrequency (RF) energy produces a more controlled heating of tissue and has become the standard technique for the past two decades. Tissue temperatures above approximately 50° C are sufficient to cause irreversible damage to the heart.[9,10] However, the temperature recorded at the ablation catheter tip may not accurately reflect the underlying tissue temperature.[11] The catheter tip temperature is affected by cooling provided by surrounding blood flow, the orientation of the catheter tip, and the contact force of the catheter.[12] The catheter tip temperature becomes even less relevant when using irrigated ablation catheters, a tool that is becoming more common. Temperatures above 90°C can cause clotting and tissue char that can cause embolic strokes or pulmonary emboli.[9,13] The radiofrequency power is adjusted to maintain temperatures near 65° CPEV[9,11,14] and more recent catheter designs include cooling devices[15] or saline irrigation[16,17] to avoid the risks associated with overheating. The lesions produced by typical radiofrequency catheters are about 7 mm in diameter and penetrate about 3 mm into the myocardium. This is sufficient to block electrical conduction through that small region of the heart. Multiple lesions are commonly needed to eradicate the arrhythmia.

The high single procedure success (90–98%) and low complication (0–3%) rates observed for ablation of supraventricular tachycardias (SVT) and typical atrial flutter have established radiofrequency catheter ablation as a first line therapy for these arrhythmias.[18–31] However, similar success rates have not yet been achieved for atrial fibrillation (AF) ablation,[32] and thus AF ablation results must be analyzed separately from results for SVT and atrial flutter ablation.

As noted above, a wide variety of arrhythmias can be treated with catheter ablation. The most common are the so-called supraventricular arrhythmias: AVRT, AVNRT, and AT. In AVRT, there is an accessory pathway allowing electrical conduction between the atria and ventricles through tissue other than the usual atrioventricular (AV) node. The arrhythmia is caused by a circuit loop with electrical conduction down one pathway from the atria to the ventricles and back up through the other pathway. Using sophisticated mapping techniques, electrophysiologists can identify the location of the accessory pathway and use radiofrequency energy to destroy tissue along the accessory pathway, thus preventing a looping circuit from being established. The pathophysiology of AVNRT is similar, except that there is both a slow and a fast pathway within the AV node itself. Thus, the reentrant circuit is within the AV node. The slow pathway is usually ablated in AVNRT because of greater long term success rate and a lower risk of causing complete AV nodal block. In atrial tachycardia, there is often a single focus of tissue that is the source of the electrical impulse causing the arrhythmia. By ablating that tissue, the arrhythmia can be cured.

Catheter ablation can also treat two other common atrial arrhythmias: atrial flutter and atrial fibrillation. In typical atrial flutter, there is a reentrant circuit within the right atrium that is amenable to treatment by destroying tissue along the pathway of the circuit. Atrial fibrillation is more difficult to treat because there are usually multiple regions of the heart that can trigger the arrhythmia. However, in most cases they are localized near the pulmonary veins. By creating a ring of ablated tissue encircling the pulmonary veins (pulmonary vein isolation), electrical signals from this area can be isolated and atrial fibrillation can be prevented from recurring.

The majority of these arrhythmias present with episodic symptoms caused by a rapid heart rate. The most common symptoms include palpitations, shortness of breath, fatigue, lightheadedness and chest pressure. Less commonly, the patient may present with syncope or sudden death. Catheter ablation using radiofrequency energy is now considered as the initial therapy for most patients who are symptomatic from these arrhythmias.

The harms of catheter ablation are relatively uncommon, but can be serious including strokes, heart attacks, heart block requiring a pacemaker, and emergency heart surgery. One early summary of 1,205 patients with accessory pathways who were treated with RF ablation at multiple institutions reported that the most common serious complication was perforation of the heart causing non-fatal tamponade (0.5%).[33] The other relatively common complications included atrioventricular block (0.5%), femoral access complications (0.5%), coronary artery spasm (0.2%), coronary artery thrombosis (0.1%), transient ischemic attack (0.1%), and bacteremia (0.1%).[33] The frequency of complications varies by the complexity of the ablation procedure being performed. A recent case series of 1,676 consecutive catheter ablations at a single site reported that the major complication rate ranged from 0.8% for the treatment of supraventricular tachycardias to 5.2% for atrial fibrillation.[34]

For AF ablation, the complexity of the procedure contributes to the incidence of major complications including femoral access, transient ischemic attack/stroke, and effusion/tamponade. A large number of ablation lesions is created for pulmonary vein isolation. With non-irrigated ablation, the electrode-tissue interface temperature and changes in electrical impedance are important indicators of charring and thrombus formation on the catheter that can pose a risk of embolic events.[13] Additional ablation in this setting can contribute to the formation of steam pops and perforation.[13] If ablation is performed without adequate tissue contact with the tip floating in the blood pool, denaturation and aggregation of proteins can lead to the formation of soft thrombus on the catheter tip.[35] As a result, irrigated catheters have become essential to achieving greater RF lesion depths while minimizing the incidence of thrombus formation.[36]

The other, more subtle harm associated with catheter ablation is radiation exposure. Fluoroscopy is used to guide the catheters into the heart and to help position them within the heart. Depending on the procedure, the fluoroscopy time can range from 20 minutes to over an hour.[37–40] This can cause radiation damage to the skin,[41–43] but more importantly it can increase the risk for future cancers.[37,44] Studies have estimated that 50 to 60 minutes of fluoroscopy will cause one or two excess deaths from cancer per 1000 treated individuals.[37,44]

Remote Magnetic Navigation

Electrophysiologists require extensive training and great skill to be able to precisely position the catheters within the heart through manual manipulation of catheter sheaths inserted through the femoral vein. Remote magnetic navigation was developed to facilitate the positioning of catheters within the heart. The system uses two computer-controlled external magnets to create and adjust an external magnetic field (0.08 Tesla) in order to guide the magnetic tip of the catheter used for ablation. The catheter is advanced or retracted from a remote workstation using a computer console that controls both the magnets and a motor-driven catheter at the bedside. The catheter tip is more flexible than a traditional ablation catheter and it moves parallel to the lines of the magnetic field established by the external magnet. By adjusting the external magnetic field, the operator can direct the catheter to the desired location within the heart. Because of the strong magnetic field, the device must be used in a specially built electrophysiology laboratory with equipment designed specifically for magnetic guidance.

Potential benefits of remote magnetic navigation include more precise control of the catheter facilitating more rapid and accurate guidance of the catheter to the desired location in the heart. This may reduce the time that the patient is exposed to radiation during fluoroscopy as well as the total time of the procedure. In addition, once initial fluoroscopic images are obtained, they can be viewed on the remote console throughout the procedure to guide navigation without additional fluoroscopy. The risk for cardiac puncture and tamponade may be lower because the catheter tip is softer and more flexible that that of a traditional catheter. While this may lower the risk of complications such as perforation and tamponade, studies have shown that a reduced force-time integral is associated with smaller lesion volumes.[45] This has the potential to make ablation less effective. Thus, long-term outcome studies with sufficient power to demonstrate both equivalent efficacy and improved safety are needed. Because the device is controlled remotely, the operator no longer is exposed to radiation from bedside fluoroscopy during much of the procedure. Thus the operator no longer has to wear a lead apron for long periods of time while performing catheter ablation, reducing operator fatigue in addition to their lifetime exposure to ionizing radiation.

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