A 52-year-old man with no prior cardiac history presents with palpitations. His 12-lead ECG shows a regular rhythm with alternating sinus beats and wide QRS complexes of bizarre morphology. The rhythm strip labeled **B** demonstrates ventricular bigeminy. Which of the following ECG findings is MOST characteristic of the premature ventricular complexes (PVCs) in this pattern and helps distinguish them from premature atrial complexes (PACs)?
A. Non-compensatory pause — the interval between the two sinus beats flanking the PVC is less than twice the baseline R-R interval, because the PVC conducts retrogradely to the SA node
B. Presence of a preceding P wave that is clearly related to the premature complex
C. Narrow QRS complex with normal morphology identical to the patient's sinus QRS
D. Fully compensatory pause — the interval between the two sinus beats flanking the PVC equals twice the baseline R-R interval, because the PVC does not retrograde-conduct to reset SA nodal timing
Explanation
Why "Fully compensatory pause" is right
The hallmark ECG feature that distinguishes PVCs (including those in ventricular bigeminy marked B) from PACs is the fully compensatory pause. Because a PVC arises from ventricular myocardium and does not conduct retrogradely to depolarize the SA node, the SA node continues its intrinsic timing uninterrupted. Therefore, the interval between the two sinus beats that bracket the PVC equals exactly twice the baseline R-R interval. This is the most characteristic and diagnostically useful feature of ventricular bigeminy. (Harrison 21e Ch 248; Braunwald 12e)
Why each distractor is wrong
Non-compensatory pause: This describes PACs, not PVCs. PACs typically conduct retrogradely to reset SA nodal timing, producing a pause that is less than fully compensatory. This is the opposite of what occurs in ventricular bigeminy.
Narrow QRS complex with normal morphology: PVCs in bigeminy (structure B) are defined by a wide QRS complex (≥120 ms in adults) with bizarre morphology dissimilar from the sinus QRS. Narrow complexes would indicate a supraventricular origin, not a ventricular one.
Presence of a preceding P wave clearly related to the premature complex: PVCs are characterized by the ABSENCE of a preceding related P wave. If a P wave precedes the complex, it would indicate a supraventricular origin (PAC or conducted sinus beat), not a PVC.
High-YieldNEET PG
Fully compensatory pause = PVC; non-compensatory pause = PAC. The SA node is not reset by retrograde conduction from a PVC.
Harrison 21e Ch 248; Braunwald 12e
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