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©2013 UpToDate ® Print Email AV conduction with a concealed accessory pathway Schematic representation of atrioventricular 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 and bundle 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.

©2013 UpToDate ® Print Email AV conduction with a concealed accessory pathway Schematic representation of atrioventricular 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 and bundle 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. AV conduction through an open accessory pathway Compared to normal conduction in the preceding diagram, the accessory pathway (AP) is now open. As a result, ventricular activation results from both early activation (preexcitation) of the free wall of the left ventricle (shown in blue) and from normal activation (shown in yellow). The degree of unopposed preexcitation depends upon the time required to conduct through the right and left atria, the AP, and the ventricular myocardium as compared to conduction through the normal pathways. The inset on the right shows the ECG timing of these events. The net effect is a QRS complex that is a fusion of ventricular preexcitation (blue) and normal excitation (yellow). Early activation throught the AP (V P ) occurs at about the same time as His bundle depolarization (H). This leads to a shorter PR interval, a small delta wave (arrow), and some prolongation of the QRS duration.

©2013 UpToDate ® Print Email AV conduction with a concealed accessory pathway Schematic representation of atrioventricular 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 and bundle 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. AV conduction through an open accessory pathway Compared to normal conduction in the preceding diagram, the accessory pathway (AP) is now open. As a result, ventricular activation results from both early activation (preexcitation) of the free wall of the left ventricle (shown in blue) and from normal activation (shown in yellow). The degree of unopposed preexcitation depends upon the time required to conduct through the right and left atria, the AP, and the ventricular myocardium as compared to conduction through the normal pathways. The inset on the right shows the ECG timing of these events. The net effect is a QRS complex that is a fusion of ventricular preexcitation (blue) and normal excitation (yellow). Early activation throught the AP (V P ) occurs at about the same time as His bundle depolarization (H). This leads to a shorter PR interval, a small delta wave (arrow), and some prolongation of the QRS duration. Conduction through an accessory pathway with AV nodal delay

Compared to normal conduction in the preceding diagram, the accessory pathway (AP) is now open. As a result, ventricular activation results from both early activation (preexcitation) of the free wall of the left ventricle (shown in blue) and from normal activation (shown in yellow). The degree of unopposed preexcitation depends upon the time required to conduct through the right and left atria, the AP, and the ventricular myocardium as compared to conduction through the normal pathways. The inset on the right shows the ECG timing of these events. The net effect is a QRS complex that is a fusion of ventricular preexcitation (blue) and normal excitation (yellow). Early activation throught the AP (V P ) occurs at about the same time as His bundle depolarization (H). This leads to a shorter PR interval, a small delta wave (arrow), and some prolongation of the QRS duration. Conduction through an accessory pathway with AV nodal delay Compared to conduction through an AP with normal AV node conduction, delayed conduction through the AV node allows more of the ventricular myocardium to be activated by preexcitation (shown in blue). The inset on the right shows the ECG timing of these events. The atrial to His interval is increased due to the AV nodal delay (RA to H); His activation is so delayed that is follows activation caused by the AP (V P ). The PR interval is short due to the preexcitation, the delta wave (arrow) is more pronounced due to the greater and unopposed early forces (blue), and the QRS duration is prolonged due to the later than normal ventricular activation caused by the AV nodal delay (yellow).