![]() ![]() The location of the P wave depends on the relative speeds of retrograde activation of the atria and anterograde activation of the ventricles via the His-Purkinje system. Finally, in about 10% of cases, the P wave is observed in the initial portion of the QRS complex. In addition, a terminal negative deflection (pseudo-S wave) is seen in the inferior leads (II, III, and aVF). The easiest place to see the retrograde P wave is in lead V 1, where a low-amplitude terminal positive deflection (pseudo-R′ wave) is seen ( Figure 7–2). In approximately 40% of cases, the retrograde P wave is observed in the terminal portion of the QRS complex. Most commonly (in approximately 50% of cases), the P wave is buried in the QRS complex and is not seen. Regardless of the mechanism, because the tachycardia originates within the AV junction, the atria and ventricles are activated simultaneously. In rare circumstances, a site within the AV node fires rapidly as a result of increased automaticity. In some cases, a premature atrial contraction can block one of the pathways (usually the fast pathway), conduct down the slow pathway, and activate the fast pathway retrogradely, initiating a reentrant circuit. Usually, one of the pathways has relatively rapid conduction properties but a long refractory period (“fast pathway”), and the other has slow conduction and a short refractory period (“slow pathway”). In AVNRT, two separate parallel pathways of conduction are present within junctional and perijunctional tissue.
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