Mechanisms in T-wave alternans caused by intraventricular block

It is recognized that 2:1 intraventricular (IV) block can result in T-wave alternans but is usually assumed that it would also affect QRS waveform. Bl ock in a local region is not, however, varied activation sequence of the sa me muscle mass because the blocked region is not activated and is not part of the mass that is activated in cycles without block. Also, the block regi on may have electrocardiogram (ECG) effects when its state differs from oth er regions. In view of those considerations, the ECG effects of IV block we re evaluated by using a computer model of excitation and recovery. ECGs wer e calculated from differences between the excited state and various degrees of recovery. Results provided evidence that boundaries associated with reg ions of block rather than regions having varied activation sequence were th e major factors in T-wave alternans caused by IV block. Effects of the boun daries included cancellation of the effects of IV block on QRS complexes. F indings suggest that IV block cannot be excluded as a mechanism of T-wave a lternans in the absence of QRS alternans.