What is the treatment for a left lateral accessory pathway, specifically how to ablate it?

Left Lateral Accessory Pathway Ablation

Catheter ablation of left lateral accessory pathways achieves success rates exceeding 95% and should be performed using radiofrequency energy via either a retrograde aortic or transseptal approach to access the mitral annulus, with the coronary sinus approach reserved for failed endocardial attempts. 1

Anatomical Considerations

Left lateral accessory pathways connect the left atrium to the left ventricle along the mitral annulus, typically traversing the atrioventricular groove. The key anatomical challenge is that approximately 10% of left lateral pathways are subepicardial, meaning they course along the epicardial surface rather than the endocardium, which explains some ablation failures with standard endocardial approaches. 2

Primary Ablation Approach

Access Techniques

Two primary approaches exist for accessing left lateral pathways 1:

• Retrograde aortic approach: The ablation catheter is advanced retrograde through the femoral artery, across the aortic valve, and positioned on the ventricular aspect of the mitral annulus
• Transseptal approach: Increasingly used in many laboratories, requiring transseptal puncture to access the left atrium directly 1

Mapping Criteria for Target Site

The optimal ablation site is identified using multiple electrogram characteristics 3:

• Accessory pathway potential: A sharp, high-frequency spike between atrial and ventricular electrograms indicating direct pathway activation
• Timing: Earliest ventricular activation during orthodromic AVRT or earliest atrial activation during antidromic AVRT or ventricular pacing
• Unipolar morphology: QS pattern on unipolar recording indicates proximity to the pathway
• Local electrogram amplitude ratios: Higher pathway potential amplitude relative to atrial/ventricular signals

Energy Application

Radiofrequency energy of 20-30 watts is applied for 10-60 seconds at the target site using a steerable ablation catheter with a 4-mm distal electrode. 4 Success is typically achieved with a median of 3-8 radiofrequency applications. 3

Alternative Approach: Coronary Sinus Ablation

When to Consider

If endocardial ablation fails after multiple attempts (typically >10-18 applications), coronary sinus ablation should be attempted as it successfully eliminates 71-83% of pathways refractory to endocardial approaches. 5, 2

Identifying Subepicardial Pathways

Key markers suggesting a subepicardial pathway requiring coronary sinus ablation 2:

• Absence or small amplitude of accessory pathway potentials during endocardial mapping
• Large accessory pathway potential within the coronary sinus (amplitude ratio ≥1 relative to atrial/ventricular electrograms) 5
• Earlier activation timing in coronary sinus compared to endocardial sites 4

Coronary Sinus Technique

The ablation catheter is positioned in the mid or distal coronary sinus using 4:

• Unipolar configuration for energy delivery through the coronary sinus wall to the epicardium
• Slight catheter deflection toward the annulus to increase ventricular electrogram component and improve epicardial contact
• 20-30 watts for 10-60 seconds 4

Critical safety consideration: Marked nonspecific pain occurs during energy application. The catheter may adhere to the coronary sinus wall, requiring withdrawal during a brief radiofrequency application to prevent coronary sinus rupture. 4

Success Rates and Outcomes

• Initial success rate: >95% for left lateral pathways (slightly higher than other locations) 1
• Recurrence rate: ~5% due to resolution of inflammation/edema from initial injury 1
• Recurrent pathways can usually be successfully re-ablated in a second session 1

Complications Specific to Left Lateral Pathways

Procedure-related mortality ranges from 0% to 0.2%, with one reported death from left main coronary artery dissection during left free-wall pathway ablation. 1 This highlights the proximity of left lateral pathways to coronary arteries.

Major Complications (Overall Rate 1.82-4.4%) 1:

• Vascular access: Hematomas, arteriovenous fistula, arterial perforation
• Catheter manipulation: Coronary artery dissection, coronary sinus perforation, valvular damage, thromboembolism
• RF energy delivery: Coronary artery spasm/occlusion, myocardial perforation, stroke/TIA
• Cardiac tamponade: 0.13-1.1% incidence 1
• Complete AV block: 0.17-1.0% (rare with lateral pathways, more common with septal pathways) 1

Coronary Sinus-Specific Complications 4:

• Mural thrombus formation in coronary sinus (observed in 2 of 8 patients in one series)
• Coronary sinus rupture risk during catheter withdrawal
• Severe pain during energy application

Common Pitfalls and How to Avoid Them

Pitfall 1: Assuming all left lateral pathways are endocardial. Solution: If multiple endocardial attempts fail and no clear pathway potential is seen, actively look for larger signals in the coronary sinus indicating a subepicardial pathway. 2

Pitfall 2: Inadequate catheter contact at the mitral annulus. Solution: Use catheter deflection and monitor contact force; ensure stable electrograms before energy delivery. 4

Pitfall 3: Targeting sites with small pathway potentials. Solution: Prioritize sites with pathway potential amplitude ratios ≥1 for coronary sinus ablation. 5

Pitfall 4: Premature abandonment of coronary sinus approach due to pain. Solution: Warn patients about expected severe pain; use adequate sedation; recognize pain is expected and does not indicate complication. 4