How many milliseconds before the P wave do early signals occur in atrial tachycardia?

• A. 10-20 ms
• B. 20-30 ms
• C. 30-50 ms
• D. 60-80 ms

Answer: (C)

In atrial tachycardia the impulse originates in an ectopic site within the atria, so atrial activation begins inside the heart before the surface ECG shows the P wave. Intracardiac electrograms will detect this early atrial activity a short time before the P wave appears on the surface tracing. That pre-P interval is typically around 30 to 50 milliseconds, reflecting the time needed for the initial atrial activation to spread to the rest of the atria so the P wave becomes visible on the surface.

Choices that are much shorter or longer than this range don’t fit the usual timing of how early atrial activation can be seen before the surface P wave in atrial tachycardia.