Patterns of human atrioventricular nodal accommodation to a sudden acceleration of atrial rate

Atrioventricular nodal (AVN) accommodation to an abrupt increase in atrial rate was systematically studied in 10 patients using a pacing protocol incorporating a programmable pause (S1S2) between the last beat of basic atrial drive (S1S1) and the onset of an 18-beat paced atrial train (S2S2) of shorter constant cycle length (CL) than that of S1S1. Pacing was repeated, varying S1S2 while keeping S1S1 and S2S2 CLs fixed. In all patients there existed a zone of 1 or more critical S1S2 intervals for which the new steady-state AVN conduction time (S2H2) was attained "instantaneously," that is, with the first beat, and maintained for subsequent beats of the S2S2 train. At S1S2 intervals that exceeded or were less than critical values, S2H2 progressively increased (crescendo pattern) or decreased (decrescendo pattern), respectively, until the steady-state value was achieved. The zone of S1S2 intervals that resulted in decrescendo or instantaneous AVN accommodation contracted when either the S1S1 CL was increased or the S2S2 CL was shortened. These findings have relevance to the interpretation of electrophysiologic studies and explain the spectrum of AVN accommodation patterns observed at the onset of supraventricular tachycardia.