Concealed ventricular extrasystoles due to interference and due to exit block.

Two cases of sinus rhythm with ventricular extrasystoles are reported in which extrasystoles arising from the same focus obey the rule of "comcealed bigeminy" and the "rule of bigeminy". In a comparatively rapid sinus rhythm, shortening of the sinus cycle favors the appearance of extrasystoles, and the extrasystoles obey the fule of "concealed bigeminy"; namely, sinus impulses intervene between extrasystoles in even numbers. The sinus impulses here include those both conducted and not conducted to the ventricles. Conversely, in a comparatively slow sinus rhythm, interectopic sinus impulses appear in odd numbers only, and the extrasystoles obey the "rule of bigeminy"; namely, lengthening of the sinus cycle favors the appearance of extrasystoles. From these observations, a new mechanism governing both of the rules is suggested as follows. Ectopic impulses arise following all the interctopic sinus beats, but they become concealed extrasystoles of two types. One of them is the "concealed extrasystole due to interference at the external end of the ventricular-ectopic (V-E) junction."The other is the "concealed extrasystole due to exit block within the V-E junction" because of refractoriness following stimulation. They alternate with each other. In the of "concealed begeminy", the last concealed extrasystole intervening between manifest extrasystoles is due to exit block, whereas in the "rule of bigeminy", it is due to interference.