Paradoxical Effects of Interatrial Conduction Delay. We present a unique case where early proarrhythmic and late antiarrhythmic characteristics of interatrial conduction delay were observed during the long-term progression of HCM. Occurrence of AT constantly increased as the interatrial conduction delay became more prominent, while the P-wave width in sinus rhythm and the AT cycle length both showed an instantaneous increase in parallel. As the interatrial delay reached a critical point, the right and left atrial P-wave became virtually separated, as demonstrated by the findings of ECGs and echocardiography. This phenomenon resulted in the complete cessation of tachycardias. (J Cardiovasc Electrophysiol, Vol. 22, pp. 587-589 May 2011).