Bidirectional ventricular tachycardia, defined as the rapid alternation of the 
QRS complexes with successive opposing axial deviation, is a rare arrhythmia. In 
the rare cases which have undergone endocavitary investigations, an infrahisian 
origin has generally been proved. However, the mechanism of these tachycardias 
remains poorly understood and is discussed with respect to a new case. 
Bidirectional tachycardia occurred in a 79 year old woman with previous 
diaphragmatic and anterior wall infarction. It was a wide QRS tachycardia at 
180/min with a succession of ventriculogrammes of opposing axis in the frontal 
plane and permanent right bundle branch block over the right precordium. The two 
types of tachycardia were observed, monomorphic type A or Type B or a 
combination of the two realising an A-B bidirectional tachycardia. The origin of 
these episodes, which occurred on a background of atrial tachycardia at about 
100/min, was ventricular as shown by the absence of a His potential before the 
ventricular complexes in tachycardia. The presence of ventricular extrasystoles 
with relatively fixed coupling intervals, and the results of endocavitary 
investigation were suggestive of a reentry phenomenon ventricular extrastimuli 
were capable of transforming the bidirectional into monomorphic tachycardia and 
vice versa; this suggests that A was at times the origin of a reentry B, but 
protected by A, tachycardia B could be sustained. In the light of previously 
reported cases with documented endocavitary investigation and this new case, it 
seems possible to talk in terms of true "bidirectional ventricular tachycardia", 
a tachycardia whose mechanism is obscure but certainly not univocal.