Proposed mechanism of a presystolic murmur during 2:1 atrioventricular block with hyperthyroidism

A patient who developed 2:1 AV block ten weeks after the initiation of therapy for Graves' disease demonstrated a presystolic murmur related to atrial systole of the conducted beat. The murmur mimicked a pericardial friction rub. With the return of normal conduction as the thyrotoxicosis was finally controlled, the murmur disappeared. We postulate that the murmur resulted frm atrial contraction with rapid late-diastolic inflow into an increased right ventricular volume, causing premature opening of the pulmonic valve. This phenomenon was attainable only during 2:1 AV block with long diastolic filling periods and a hyperkinetic circulatory system. The murmur may be indicative of severe hyperthyroidism. Murmurs found in other disease entities may share similar physiologic principles.