Catheter ablation of SVT has emerged as a curative treatment with high success rates and plays a central role in the management of supraventricular tachycardia (SVT).This short review aims to give an update on the outcomes of catheter ablation and provide a guide to appropriate decision making in patients with SVT.
Mechanisms of SVT
Tachycardias can be categorized as ventricular (involving the ventricle ± the His-Purkinje system) and supraventricular (all the others arrhythmias). Atrial fibrillation is the most common SVT,but will not covered in this review, as its mechanisms and treatment differ from the mechanism and treatment of the other SVTs. Regular SVTs usually have a reentry mechanism and may be classified in different categories.The main four forms are:
- atrioventricular nodal re-entry tachycardia (AVNRT), where the re-entry circuit is within the AV node and is due to the presence of two AV nodal pathways with different electrical properties;
- atrioventricular re-entry tachycardias (AVRT), with one limb of the circuit using an accessory atrioventricular pathway (AP);
- atrial tachycardia, which may have a focal origin or may be due to a reentry; and
- atrial flutter, which has a macro reentrant circuit in the right or, less often, left atrium.
Common symptoms of SVT include palpitations, anxiety, light-headedness, neck and chest discomfort, and dyspnea. Syncope is uncommon, but some patients experience serious psychologic distress. SVTs are often recurrent, occasionally persistent, and are a frequent cause of emergency room (ER) and primary care physician (PCP) visits. The usual electrocardiographic presentation of SVT is a narrow complex tachycardia (QRS less than 120ms), but in some cases (less than 10%), SVT presents as wide complex tachycardia. After restoration of sinus rhythm, the twelve-lead ECG should be examined for the presence of delta waves, which indicate an accessory pathway.
Most SVTs depend on the AV node for maintenance of the re-entry circuit and can be interrupted by (vagal) maneuvers or pharmacological agents, which slow AV nodal conduction, the first-line pharmacological agent being adenosine. For patients in whom SVT recurs, preventive therapy is generally warranted if there are frequent, prolonged, or highly symptomatic episodes that cannot easily be terminated by the patient using vagal maneuvers. Options for long-term treatment include medication and ablation therapy. Referral to an electrophysiologist is warranted for patients with syncope, severe symptoms, or pre-excitation syndrome.
Antiarrhythmic Drug Therapy
Patients with recurrent episodes of SVT without pre-excitation may be treated with AV node blocking agents such as verapamil, β-receptor blockers or digoxin. These agents decrease the frequency of the episodes and the severity of symptoms in a number of patients, but complete suppression of SVT is uncommon. Long-term therapy with class IC or III drugs is generally not recommended because of their potential adverse effects. For patients whose recurrences are infrequent but prolonged, 'pill in the pocket' treatment (for example, flecainide 100-200mg) to take at the onset of SVT is a possible approach. Verapamil and digoxin are contraindicated in patients with WPW syndrome, unless the AP has been shown to have a long refractory period (300msec or more), because these drugs may increase the risk of rapid ventricular response causing ventricular fibrillation in case of atrial fibrillation.Although catheter ablation is considered the treatment of choice in these patients, both flecainide and propafenone are effective and have been approved for the prevention of paroxysmal SVTs mediated by an accessory pathway.
Catheter Ablation of SVT
Medical treatment of SVT with antiarrhythmic drugs has limited efficacy. In addition, these drugs can produce significant side effects or may be inconvenient for a number of patients. Catheter ablation has been increasingly used in the management of SVT, based on its observed efficacy and overall safety in experienced centers. In most SVT, success rates of catheter ablation are greater than 95%. Generally, radiofrequency ablation is recommended as primary therapy for patients with pre-excitation syndrome or hemodynamic instability during their arrhythmia. Moreover, catheter ablation is recommended for the patients with arrhythmias refractory to AV node blocking agents. Finally, catheter ablation can also be proposed for patients who experience adverse effects or find it inconvenient to take medication. Indeed, many patients, informed of the risks and benefits of catheter ablation, prefer this approach than a long-term antiarrhythmic drug therapy.
Catheter ablation of SVT can be performed as a one-day out-patient procedure, or it may require overnight hospitalization. One to four catheter electrodes are introduced into the heart cavities through femoral (or, alternatively, internal jugular or subclavian) venous access after local anesthesia is administered. For left-sided accessory pathways, a retrograde approach through the femoral artery and the aortic valve can be used, or a transseptal puncture can be performed to gain access to the left atrium.
Radiofrequency current—a low-voltage, high-frequency (500kHz) form of electrical energy used for electrocautery in surgery—is delivered through a catheter electrode to create small lesions through thermal injury in the myocardial tissue, the conduction system, or both, which have been identified as critical for mediating the cardiac arrhythmia. Cryothermal ablation is an interesting approach in patients with AV nodal re-entrant tachycardia or an accessory pathway close to a His bundle because of the reversibility of the initial effect and the negligible risk of AV block. Most ablation procedures take one to three hours.
Efficacy of Catheter Ablation
In patients with AV nodal re-entry tachycardia, the AV nodal slow pathway is targeted by catheter ablation in the postero-septal region of the tricuspid annulus. Serious complications are uncommon, including pulmonary embolism (up to 0.2%) and the development of AV block requiring pacemaker therapy (up to 1%). Recurrence occurs in 3-7% of cases. Catheter ablation of tachycardia mediated by an accessory pathway has success rates of more than 95% in experienced centers and recurrence rates lower than 5% over the first few months. Late recurrences are exceptional. Acute and long-term success rates are slightly lower in patients with right-sided and septal accessory pathways, or multiple accessory pathways. In cases where the accessory pathway is close to the His bundle, radiofrequency current application can be complicated by AV block requiring pacemaker therapy. In this situation, cryothermal ablation appears effective and reduces the potential for AV block. Other complications of accessory pathway ablation, occur in less than 2-3% of cases and include damage to an artery, bleeding, arterio-venous fistula, venous thrombosis, pulmonary embolism, myocardial perforation, valvular damage, systemic embolism (in case of left-sided accessory pathway), and death (rare).
Catheter ablation of focal atrial tachycardias has slightly lower success rates (about 85%) and higher recurrence rates (about 8%). Procedural risks are slightly higher for treatment of left atrial tachycardia, which requires a trans-septal puncture.The arrhythmia is probably due to focal mechanism that has not been clearly defined; indeed, it may be that the cause is a very small re-entry circuit, which can be targeted with focal ablation.
Atrial Flutter and Re-entry Atrial Tachycardias
Most atrial flutter is caused by re-entrant excitation in the right atrium; the cavotricuspid isthmus being an essential part of the circuit. Recurrent atrial flutter episodes are common despite anti-arrhythmic drug therapy. Currently, radiofrequency catheter ablation treatment provides a cure in more than 95% of cases with very low complication rates. However, atrial fibrillation may subsequently occur.
In patients late after catheter ablation of atrial fibrillation or mitral valve surgery, left atrial flutter may occur. In patients with congenital heart disease, atrial tachycardias most often have a re-entrant mechanism, related to regions of scar in the right atrium and are called incisional atrial tachycardia. Some patients may have more than one circuit and often have isthmus-dependent atrial flutter. In these patients, radiofrequency ablation has high success rates and is often used as first line.
Medical treatment of SVT with antiarrhythmic drugs has limited efficacy and may cause adverse effects or be inconvenient to young patients.The safety and efficacy of radiofrequency catheter ablation for treatment of most types of SVT is well established. Ablation for most SVT has such a low complication rate and high success rate that it is more cost-effective and may be safer than antiarrhythmic drugs. Referral of the patients should be considered in patients with SVT to assist in decision-making regarding therapy.