Intraventricular Conduction Delay as a Determinant of Atrial Echo Beats

Re-entry within the atrioventricular (A-V) node is a common cause of atrial echo beats and paroxysmal atrial tachycardia. This report deals with observations made in four patients in whom the determining factor for atrial echo beats appeared to be a critical conduction delay occurring within the His-Purkinje system and not within the A-V node. The evidence in support of this conclusion is as follows: 1) at a constant A-V nodal delay, echo beats only occurred when a critical His-Purkinje delay was reached; 2) in the presence of different intraventricular conduction times, atrial echoes were dependent on a requisite His-Purkinje conduction delay; 3) at variable A-V nodal conduction times, echo beats appeared only when a requisite delay in His-Purkinje conduction developed; 4) at a constant premature coupling interval atrial echo beats could be abolished when refractoriness within the His-Purkinje system was decreased by decreasing the cycle length of the basic drive; and 5) at constant A-V nodal conduction times, a reciprocal relationship existed between the magnitude of the His-Purkinje delay and the "echo interval" defined as the interval between the onset of ventricular depolarization and the echo beat.