Ivabradine for inappropriate sinus tachycardia-induced symptomatic Mobitz type I atrio-ventricular block: A case summary

Abstract

Background: Inappropriate sinus tachycardia (IST) is characterized by a continuum of symptoms, and the aetiology of IST is imprecise. IST-induced autonomic dysfunction is well known, but IST-induced atrio-ventricular block is not reported to our knowledge. Case summary: A 67-year-old female presented with a 4-day history of random intermittent difficulty in breathing, chest tightness, palpitations, and dizziness, with a recorded heart rate of 30 beats per minute (BPM) on home monitoring equipment. The initial electrocardiogram (ECG) demonstrated sinus rhythm with intermittent Mobitz type I second degree atrio-ventricular (AV) block, with continuous cardiac monitoring demonstrating frequent episodes of Wenckebach phenomenon throughout the day, with a sinus rate of 100-120 BPM. Echocardiogram showed no significant structural abnormalities. The patient was on bisoprolol, and hence, it was suspected Wenckebach may be due to that and so stopped. However, there was no tangible effect on rhythm 48 hours after stopping bisoprolol, leading to a suspicion of IST-induced Mobitz type I second degree AV block; and so decided to introduce ivabradine 2.5 mg twice daily. After 24hours of Ivabradine, the patient remained in sinus rhythm with no documented episodes of Wenckebach phenomenon on cardiac monitor, a finding subsequently confirmed by 24-hour Holter monitoring. During a recent follow-up visit in clinic, the patient remained symptom-free, with an ECG demonstrating sinus rhythm at a physiological rate. Discussion: Mobitz type I second degree AV block is usually due to reversible conduction block at the level of the AV node whereby malfunctioning AV nodal cells tend to progressively fatigue until they fail to conduct an impulse. Under conditions of increased vagal tone and autonomic dysfunction, Wenckebach occurrence will be increased. Thus, selective impulse conduction within the sinoatrial (SA) node by ivabradine to reduce beat conduction to the AV node in patients with IST/dysautonomia-induced Mobitz type I AV will reduce the occurrence of Wenckebach.