After atrial flutter ablation, how is bidirectional block confirmed?

• A. By demonstrating bidirectional block across the ablation line with pacing from both sides.
• B. By imaging alone.
• C. By symptoms.
• D. By noninvasive testing.

Answer: (A)

Bidirectional block means impulses cannot cross the ablation line in either direction. To prove this, you pace from one side of the line and look for activation on the opposite side; then you pace from the other side and look for activation on the first side. If pacing from both sides fails to produce conduction across the line, you have confirmed bidirectional block.

This is the most reliable way to verify that the flutter circuit cannot re-enter across the ablation line, ensuring the ablation has effectively interrupted the circuit in both directions. Imaging, symptoms, or noninvasive testing cannot directly prove this conduction block across the line, so they are insufficient on their own.