Electrophysiology Unit (EPU) 26.19 Practice Test

What method is used during pace mapping to identify the exit site of VT?

Comparison of paced QRS morphology with VT QRS morphology on the 12-lead ECG. Measuring stimulus duration only. Only map unipolar signals. Wait for spontaneous QRS morph.

Correct Answer:

Comparison of paced QRS morphology with VT QRS morphology on the 12-lead ECG.

Explanation:

During pace mapping, the exit site of VT is identified by pacing from different endocardial sites and comparing the resulting QRS morphology to the VT QRS on the 12-lead ECG. When the paced QRS closely matches the VT QRS, the pacing site is likely at or near the VT exit, because the activation pattern sent from that site drives the same ventricular activation sequence seen in the VT. A good match across multiple leads indicates proximity to the exit path and helps target the ablation. This approach relies on morphological correlation, not on measuring stimulus duration, focusing solely on unipolar signals, or waiting for spontaneous QRS morphs, which do not localize the exit site in the same way.

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