AV conduction with a concealed accessory pathway

Schematic representation of AV conduction. The normal pacemaker is in the sinoatrial (SA) node at the junction of the superior vena cava and the right atrium. The SA node activates the right and left atria (shown in green). In the absence of an accessory pathway (AP) or, as in this case, if the AP is concealed, ventricular activation results from the impulse traversing the AV node, the specialized infranodal conducting system (His bundle, bundle branches, and fascicular branches, shown in red), thereby activating the ventricular myocardium (shown in yellow). The ECG shows a normal PR interval and a narrow QRS complex. The inset on the right shows the timing of SA node (SAN), right (RA) and left atrial (LA), His bundle (H), and the beginning of normal ventricular activation (V_N). All of ventricular activation (shown in yellow) is due to normal AV nodal and infranodal conduction.