During an EP study, when a VT is unstable, where are pacing maneuvers typically performed first, and what mapping method is used?

• A. Right ventricle and then right ventricular outflow tract; pace mapping with 12-lead ECG comparison.
• B. Left ventricle first; pace mapping alone.
• C. Atria first; activation mapping only.
• D. Coronary sinus pacing; unipolar mapping only.

Answer: (A)

When VT is unstable, you map by pacing rather than trying to sustain the tachycardia. The safest and fastest approach is to start by pacing from the right ventricle, and then proceed to the right ventricular outflow tract. This endocardial approach is practical and often reveals pacing-induced QRS patterns that resemble the VT, helping localize the exit site without needing to maintain the unstable tachycardia.

The mapping method used is pace mapping with comparison of the 12-lead ECG morphology. You pace from various endocardial sites and compare the resulting QRS shape across all 12 leads to the VT morphology. The site whose paced QRS most closely matches the VT QRS is the best estimate of the VT origin, guiding subsequent ablation.

Starting in the left ventricle, atria, or coronary sinus with unipolar mapping isn’t typically the first choice when VT is unstable, because these approaches are less rapid or reliable for identifying the ventricular exit, and certain mapping techniques (like activation mapping during unstable VT) aren’t feasible.