Apparent Sinus Pause and Premature Atrial Complexes Attributable to an Accessory Pathway

BACKGROUND: A concealed accessory pathway is typically not characterized on a sinus rhythm electrocardiogram (ECG). However, in exceptionally rare cases, it is misdiagnosed as sinus pause because retrograde conduction via an accessory pathway generates a P' wave that is concealed within the T wave and not conducted to the ventricles; such cases are rarely reported. CASE SUMMARY: A 54-year-old man presented to a local hospital with recurrent chest tightness and palpitations in 2023. Holter ECG results at another hospital suggested sinus pause with junctional escape beats, and pacemaker implantation was planned. At our hospital, an electrophysiologic study led to a definitive diagnosis of a right-sided accessory pathway, and the patient successfully underwent radiofrequency ablation. He did not experience any further arrest or premature beats postoperatively. DISCUSSION: Differentiating sinus arrest suggested on an ECG from nonconducted premature beats is necessary. Meanwhile, when the RP' interval remains constant and P wave morphology following right ventricular pacing is identical to that of the P' wave, a concealed accessory pathway should be suspected. TAKE-HOME MESSAGE: In exceptionally rare cases, a concealed accessory pathway may manifest as apparent sinus pause on ECG.