Electrocardiograms taken from 11 patients in sinus rhythm with ventricular 
ectopic rhythms from two different foci were analyzed to find the number of 
sinus beats, S, between the ectopic rhythms (S values). Three out of 11 patients 
had the S values typical for concealed ectopic rhythms. One of them had 
concealed bigeminy of 2n-1 form that occasionally shifted to 2n form. Following 
the shift, S values of 2n-1 form were always achieved by the occurrence of 
double ventricular ectopic rhythms in succession. Concealed trigeminy of 3n and 
3n-2 form was seen in the other two patients. Double ventricular ectopic rhythms 
had bizarre abnormal QRS complexes of two different morphologies and were 
inscribed in opposite directions. Ectopic rhythms in each case had parasystolic 
characteristics. These observations suggest bifocal automaticity as a mechanism 
for bidirectional ventricular tachycardia.