Cardiac Ablation for Atrial Fibrillation
Normal atrial contraction also known as the atrial kick can increase left ventricular filling and cardiac output by 20 to 30%. This is lacking in atrial fibrillation and can negatively impact exercise tolerance and quality of life. Should the practicing clinician recommend cardiac ablation for atrial fibrillation?
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by Stuart M. Caplen, MD
Normally, electrical signals from sinoatrial node cardiac pacemaker cells cause the heart to contract in an efficient manner pushing blood from the atria into the ventricles and then out to the lungs and the rest of the body. In atrial fibrillation (AF) the electrical signals become rapid, irregular and disorganized overriding the normal pacemaker cells, and the heart pumps less efficiently.
In the United States it is projected that 12.1 million people will have AF by 2030. More than 454,000 people are hospitalized each year with AF as the primary diagnosis, and this is a contributing factor to an estimated 158,000 deaths a year. The estimated mortality rate of new AF varies in the literature. In one large study of over 185,000 Medicare, patients with an AF diagnosis there was a 48.8% 5-year risk of mortality, a 13.7% risk for heart failure and a 7.1 % risk of stroke.[2,3] Other sources note that the presence of AF is associated with a 1.5 to 2-fold increase in death and heart failure (HF) and 5-fold increase in the rates of stroke and systemic thromboembolism. Normal atrial contraction also known as the atrial kick can increase left ventricular filling and cardiac output by 20 to 30%. This is lacking in AF and can negatively impact exercise tolerance and quality of life.
Ablation therapy for AF is considered a standard of care and may be used regardless of previous medication history. It can be performed by radiofrequency, cryoenergy, or laser to destroy the abnormal tissue that precipitates AF.
Myocardial muscle fibers from the left atrium extend like sleeves around all the pulmonary veins (PV). It is known that areas near the pulmonary veins may frequently contain triggers for paroxysmal AF due to the presence of large numbers of autonomic nerves that reside there. Conducting fibers in that area also have shorter refractory times than those in the left atrium. For those reasons, the most common ablation technique is to create an electrical isolation of the pulmonary veins (PV) with circumferential lesions around the PV. In addition, a “roof line” connecting the mitral valve to the ablations around the left PV may be added. An encircling lesion of the superior vena cava (SVC) may also be performed if focal firing from the SVC can be demonstrated. Individual AF triggers are found in about 10-33% of patients during electrophysiologic mapping and may be also targets for ablation. Some cardiologists will also try to ablate any areas of fibrotic scar tissue found there which can also serve as AF triggers.
Rapid electrical reconduction from the PV after an initial attempt at ablation can be as high as 33% in 30 minutes and 50% in 60 minutes after the procedure. An observational waiting period of at least 20 to 30 minutes during the procedure is recommended to see if any acute reconduction does occur and if additional targeted ablation might be needed. Intraprocedural adenosine infusion may also be used to help differentiate a permanently blocked conduction path from one that is dormant and could become active, however studies on efficacy have been mixed.[6,7,8,9,10]
Isoproterenol infusion is the most commonly used agent to provoke and locate non-PV triggers to be ablated and is sometimes also used with adenosine.
Electrical pacing techniques may also be used intra-procedure to try to ensure that the PV are electrically isolated from the left atrium.
Open cardiac ablation may be performed at times by surgeons on patients receiving concomitant cardiac valve replacements or coronary artery bypass grafts.
Complications of Ablation
Listed complications of ablation include: death (<0.1% to 0.4%), stroke or TIA (0% to 2%), asymptomatic cerebral emboli (2% to 15%), pericarditis (0% to 50%), cardiac tamponade (0.2% to 5%). Other potential complications include: air embolism, atrial esophageal fistula, esophageal perforation, coronary artery stenosis/occlusion, gastric hypomotility from vagus nerve injury, permanent phrenic nerve paralysis, pulmonary vein stenosis, stiff left atrial syndrome (dyspnea, heart failure, pulmonary hypertension and atrial dysfunction, typically from atrial scarring if due to ablation), mitral valve entrapment of the catheter, as well as other vascular complications.
Atrial Fibrillation Recurrence After Ablation
Due to the fairly frequent reoccurrence rate, reablation is a commonly needed procedure.
Early recurrence within the first 3 months after ablation is observed in 50% or more of patients and may be due to tissue inflammation from the procedure. A “blanking period” of 3 months is recommended after the procedure, during which reintervention should be avoided, because up to half of the patients with early recurrence remain AF- free during long-term follow-up. Patients who experience multiple early recurrences are more likely at one-year to be back in AF and may be candidates for early reablation.
Late recurrence, from 3 to 12 months post-procedure occurs in approximately 25%–40% of cases. The mechanism for late-term recurrence is predominantly linked to the recovery of electrical conduction between the PVs and the left atrium. Scar tissue from a previous ablation procedure affecting electrical conduction may also lead to AF recurrence.
In one study only 26.4% of subjects who had ablations were still in sinus rhythm after 4.8 years. Another trial found after 1, 2, and 5 years, 40%, 37%, and 29% of subjects respectively were arrhythmia-free post-ablation.
Obesity, obstructive sleep apnea, and uncontrolled HTN, have been found to put patients at risk for recurrent AF after ablation, although use of CPAP and controlling HTN reverses that higher risk.
Ablation Versus Drug Therapy
In the literature, ablation trials had a 59%–89% success rate in terminating paroxysmal AF after 12 months compared to drug therapy’s reported success rate of 5%–23%. Trials which included patients with persistent AF or combined paroxysmal and persistent AF, reported success rates with ablation ranging from 59%–80% at 6 or 12 months, compared to success rates with drug therapy only ranging from 9%–58%.
There have been mixed results in trials with respect to ablation for AF and its effect on mortality, although some other quality of life measures favor ablation.
A study of 411 patients with persistent or frequent AF and heart failure compared ablation to drug therapy. The primary outcome was a composite of all-cause mortality and heart failure events, with at least 2 years follow-up. There was no significant difference in the primary outcome between the ablation and drug therapy groups. Secondary endpoints favored ablation. Left ventricular ejection fraction, 6-minute walk distance and the AF Effect on QualiTy-of-Life questionnaire scores were statistically higher in the ablation group. However, the trial was prematurely terminated at 19 months after an interim review revealed a lack of benefit in mortality or heart failure symptom improvement with ablation compared to drug therapy.
A larger prospective trial with more than 2,000 subjects, The Catheter Ablation vs Antiarrhythmic Drug Therapy for Atrial Fibrillation (CABANA) trial found that catheter ablation, compared with drug therapy, did not significantly reduce the primary composite end point of death, disabling stroke, serious bleeding, or cardiac arrest. Catheter ablation was associated with a lower AF recurrence rate than drug therapy (50% vs 69% at 3 years). One secondary endpoint, mortality or CV hospitalization revealed a significant 17% lower event rate for the catheter ablation group.
Another study of 203 heart failure patients with AF and an implanted pacemaker or defibrillator, compared ablation to amiodarone therapy. At two years, 70% of patients in the ablation group were AF-free vs only 34% in the amiodarone group. The ablation group also had significantly less hospitalizations and mortalities.
A large study of over 37,000 patients followed for at least 3 years found that AF subjects who had ablations had lower rates of death, dementia, and stroke than those who did not have ablations. A meta-analysis comparing ablation to drug therapy for AF found that catheter ablation is associated with a 32.3% rate of recurrent atrial arrhythmias (AF, atrial flutter, or atrial tachycardia) compared to a higher 53% in the medication group. The number needed to treat (NNT) with ablation to prevent 1 arrhythmia was estimated to be 5 patients. Hospitalizations were 5.6% in the ablation group compared to 18.7% in the drug therapy group.
Other trials have found that ablation for AF significantly improved patients’ quality of life. In one of the trials AF patients had significantly reduced cardioversions, emergency department visits and hospitalizations compared to the pre-ablation period.[19,20]
In competitive athletes, it is suggested that ablation may be a first line therapy to avoid the adverse effects of medication on performance.
Summary: There are mixed results for improvement of mortality from ablation versus drug therapy and the true impact is unknown. There are some preliminary studies that have shown a decrease in strokes and dementia in patients who have received ablation.[6,20] It is clear from the literature that ablation prevents recurrent AF more effectively than drug therapy, and a primary benefit from AF ablation is an improvement in quality of life, with a decrease in fatigue and an improvement in exercise tolerance.
Time From Atrial Fibrillation Diagnosis to Ablation
A study of the relationship between time of AF diagnosis to the performance of an ablation procedure found that the time frame affected efficacy. Four time periods from diagnosis to ablation were studied, ≤1, 1.1–3.0, 3.1–6.5, and >6.5 years, and any atrial arrhythmia recurrence (not just AF) rates at two years were 33.6%, 52.6%, 57.1%, and 54.6% respectively.
A meta-analysis and systematic review found that when AF diagnosis to ablation time was less than one year there were 27% less recurrences than if the time frame for ablation was greater than one year.
Anticoagulation After Ablation
Patients in AF are typically anticoagulated to prevent blood clots form forming in the left atrium which can dislodge and potentially cause a stroke. One advantage of ablation used to terminate AF is the possibility of eliminating the need for anticoagulation, and the potential adverse events associated with that therapy. Patients are at increased risk for thromboembolism during the ablation procedure and also up to several months after. A consensus recommendation is that post-ablation anticoagulation should be continued for at least 2 months. After that, the need for anticoagulation should be based on the patient's stroke risk profile rather than the success or failure of the ablation procedure. If anticoagulation is discontinued after ablation termination of AF, continuous or frequent ECG monitoring to screen for AF recurrence is recommended.
Ablation therapy for AF is a recognized treatment option. Traditionally, medication was recommended as the first therapy for AF and only if that failed was ablation recommended. However, some studies have found that the sooner the procedure is done, the higher the likelihood of success and earlier first-line ablation is sometimes recommended. Ablation procedures do carry some risk of adverse events.
Some studies are inconclusive whether ablation significantly reduces mortality over drug therapy, but compared to medical treatment ablation can reduce symptomatic atrial arrythmias, hospitalizations and improve exercise tolerance and quality of life. There are also some preliminary data that ablation may reduce stroke and dementia risk. Some of the varying results in trials may be due to the different ablation techniques or the primary and secondary endpoints employed in those studies.
In some patients, returning to sinus rhythm can eliminate the need for anticoagulation, but the need for anticoagulation should be based on the patient's stroke risk profile and not solely on a positive result of ablation therapy.
Due to recovery of electrical pathways, recurrence of AF may necessitate repeat ablation procedures. While short term results are good, according to some studies, less than 30% of patients who received ablation are still in sinus rhythm after 5 years.
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