A 42-year-old man presents with palpitations and a sense of "extra beats" in his chest. His ECG shows an early P wave with abnormal morphology followed by a narrow QRS complex. The interval between the ectopic beat and the next sinus beat is shorter than the normal RR interval. The feature marked **C** in the diagram represents this shortened interval. What is the mechanism responsible for this finding?

Explanation

## Why retrograde conduction resetting the sinus node is right The incomplete compensatory pause (marked **C**) in a PAC occurs because the ectopic atrial impulse conducts retrogradely up the AV node to reset the sinus node. This resets the sinus node's timing, so the next sinus beat occurs sooner than expected — hence the pause is "incomplete" (shorter than a full compensatory pause seen in PVCs). This is the defining ECG feature of PAC and directly reflects the clinical anchor: the sinus node is reset by retrograde conduction from the ectopic focus. (Harrison 21e Ch 247) ## Why each distractor is wrong - **Complete block of the ectopic impulse at the AV node**: This would produce a blocked PAC with no QRS after the early P wave — not the scenario described, which shows a conducted beat with an incomplete pause. - **Prolonged refractoriness of the ventricular conduction system**: This would delay the QRS or cause aberrant conduction (wide QRS), not explain the shortened interval to the next sinus beat. - **Failure of the ectopic impulse to depolarize the atrium**: This contradicts the ECG finding of an early abnormal P wave, which proves the ectopic focus did depolarize the atrium. **High-Yield:** Incomplete compensatory pause in PAC = retrograde sinus node reset; complete compensatory pause in PVC = sinus node unaffected (marches through). [cite:Harrison 21e Ch 247]