Electrophysiology (EP) study showing surface and intracardiac electrocardiogram tracings of an atrial premature beat (APB)

Simultaneous surface electrocardiographic leads I, II, V1 and intracardiac recordings from the high right atrium (HRA), His bundle (HBE), proximal (CSp) and distal (CSd) coronary sinus, and right ventricular apex (RVA). During sinus rhythm, the impulse originates in the HRA and is then conducted through the HBE, CSp, CSd, and ultimately the RVA (blue line 1). An atrial premature beat (APB), denoted by the red asterisk, originates in the CSp (blue line 2) and is conducted retrogradely to the HRA and antegradely to the CSd and RVA. The APB has a longer AH interval compared with sinus rhythm and is associated with a full compensatory pause.