Atrial bigeminy with block associated with bradycardia and paroxysmal atrial fibrillation—an important variant of the tachycardia‐bradycardia syndrome

Abstract

We wish to add one similar case to Dr. Washington's series of seven patients of the so-called variant sick sinus syndrome and suggest one probable mechanism for this particular syndrome.Case Report. A 60-year-old white female was admitted to the special care unit following the sudden onset of abdominal pain associated with dizziness, nausea, and diarrhea I d prior to admission. She complained about persistent palpitations and an irregular rhythm with "skipping" of her heart beats.Previous medical history included a known history of primary hypothyroidism, presently treated with 0. I mg Synthroide every day. The patient denied any history of dizziness, syncope, chest pain, or other cardiac symptoms prior to this acute event.Physical examination revealed a slightly obese white female in no acute distress. Cardiac examination was essentially negative. Lab results indicated a borderline hypothyroid condition with a T4 of 5.6pg/dl (normal = 5.4-10.9) and a thyroid-stimulating hormone of 10 U (normal = less than 10 U). Other lab results, including cardiac enzymes, were all within normal limits. Monitor strips taken in the special care unit revealed persistent sinus bradycardia at 35 beats/ min. The admission ECG showed no evidence of an acute injury. A pacemaker insertion was considered, but subsequent ECGs revealed the true underlying etiology of this bradycardia to be blocked atrial bigeminy. The PACs were either conducted normally, with a right bundle branch block pattern, with first-degree A-V block and a left bundle branch block pattern, or blocked, depending on the critical R-P interval of the PAC. Several short runs of PAT were also observed.To evaluate sinus and A-V node function, sino-atrial conduction time was measured and found to be normal by both the Narula and Strauss methods. The sinus node recovery time was within normal limits. A negative response to carotid sinus massage ruled out carotid sinus hypersensitivity. Atrio-ventricular nodal conduction appeared to be within normal limits with an A-V Wenckebach type of block seen only at pacing rates of I50 beats/min. Administration of 0.5 mg atropine i.v. did not suppress atrial ectopy nor alter thesinus node recovery time. Intravenous administration of lnderale achieved abolition of the atrial bigeminy for approximately 45 s with a sinus bradycardia at 55 beats/min. This effect was only transient and atrial bigeminy recurred immediately.On the basis of the above study, the patient was restarted on Synthroid' replacement, 0.2 mg every day, and started on lnderala therapy, 20 mg per 0s every 6 h. Over the next 2 d, the patient demonstrated almost constant atrial bigeminy with little reduction in the episodes of blocked PACs. lnderal therapy was stopped and Pronestyle was started at 250 mg per 0s every 4 h and increased to 500 mg every 4 h. Atrial bigeminy continued, but the PACs occurred later in the cycle and the episodes of block became rare. Upon discharge, the patient was on a regimen of Synthroido, 0.2 mg every day, and Pronestyle, 500 mg per 0s every ...