Define long-stimulus to QRS interval and what it indicates.

• A. The time from stimulus to QRS onset is longer than what has been measured, indicating regions of slow conduction in the isthmus and identifying exit sites for ablation.
• B. The time from stimulus to QRS onset is longer than normal and confirms exit sites precisely.
• C. The stimulus-to-QRS time is shorter and indicates fast conduction.
• D. The stimulus-to-QRS time is unaffected by scar tissue.

Answer: (B)

The main idea is that a prolonged stimulus-to-QRS interval after pacing signals slow conduction in diseased tissue. When the heart has scar or fibrotic areas, electrical impulses move more slowly through those regions, particularly in channels like an isthmus that connect scar to normal tissue. Pacing from within or near the scar yields a delayed onset of the ventricular depolarization (the QRS), so the stimulus-to-QRS time becomes longer than normal. This slowing helps map where the impulse exits the scar into healthy tissue, pointing to exit sites that are often targeted for ablation. So a longer-than-normal stimulus-to-QRS interval indicates slow conduction through scar and helps identify ablation targets at the exit points.