In asynchronous pacing modes (DOO/VOO), what phenomenon may occur if the native heart rate exceeds the programmed rate?

In asynchronous pacing modes (DOO/VOO), what phenomenon may occur if the native heart rate exceeds the programmed rate?

• A. Pacemaker resetting
• B. R-on-T phenomenon
• C. Fusion beats
• D. Ventricular tachycardia

Answer: (B)

In asynchronous pacing modes like DOO/VOO, the device paces at a fixed rate without sensing the heart’s own activity. If the native heart rate becomes faster than the programmed rate, an intrinsic ventricular beat can occur before the next paced beat. The next pacing stimulus may then land on the T wave of that preceding beat—the heart’s vulnerable repolarization period. Stimulating during this R-on-T moment can trigger dangerous ventricular arrhythmias, which is the phenomenon described. Since there’s no sensing in this mode, the pacemaker can’t coordinate with intrinsic beats to avoid this risk, and the other options don’t describe this specific interaction: resetting requires sensing, fusion beats rely on paced and intrinsic waves being coordinated, and ventricular tachycardia is a possible consequence rather than the defined phenomenon.

In asynchronous pacing modes like DOO/VOO, the device paces at a fixed rate without sensing the heart’s own activity. If the native heart rate becomes faster than the programmed rate, an intrinsic ventricular beat can occur before the next paced beat. The next pacing stimulus may then land on the T wave of that preceding beat—the heart’s vulnerable repolarization period. Stimulating during this R-on-T moment can trigger dangerous ventricular arrhythmias, which is the phenomenon described. Since there’s no sensing in this mode, the pacemaker can’t coordinate with intrinsic beats to avoid this risk, and the other options don’t describe this specific interaction: resetting requires sensing, fusion beats rely on paced and intrinsic waves being coordinated, and ventricular tachycardia is a possible consequence rather than the defined phenomenon.