Effects of stable and changing rates and premature ventricular beats on transient tachycardia-, pseudobradycardia-, and bradycardia-dependent bundle branch block alternans.

New circumstances under which bundle block (BBB) alternans may appear or disappear are described. 1) Tachycardia-dependent as well as bradycardia-dependent BBB alternans may begin after constant BBB is interrupted by a premature ventricular beat. Tachycardia- and bradycardia- dependence may be differentiated by the shape of the first beat after the pause. 2) When BBB alternans disappears during a constant ventricular rate, tachycardia-dependent BBB alternans changes to persistent normal or more normal intraventricular conduction, whereas bradycardia-dependent BBB alternans changes to a persistently greater degree of BBB. 3) BBB alternans appears to be tachycardia- or pseudobradycardia-dependent in relation to the cycle length and antegrade and retrograde refractory periods in the involved bundle branch. 4) BBB alternans may be recognized during persistent irregular ventricular action in atrial fibrillation. Here the recognition of BBB alternans depends upon the sequence of contours as well as upon the cycle lengths.