A slightly earlier premature beat –as illustrated in the upper panel of this figure-will find both the AV node and the bypass tract refractory and therefore unable to propagate to the ventricles. A slightly later premature beat (lower panel) will be able to conduct to the ventricles via both pathways causing the WPW type of QRS complex because the bypass tract will no longer be refractory. Thus, there will be no unidirectional block and re-entry will not occur.