Traditionally, a junctional impulse is considered to be one that arises from the AV node or bundle of His above the bifurcation. Pacemaker tissue cannot be found in the AV node of experimental animals. The question is not settled in humans. From its source, probably in the junction, the impulse spreads retrograde toward the atria and antegrade toward the ventricles. Depending on the site of origin, conduction velocity, and refractory periods, the atria may be activated before, during, or after ventricular depolarization. Atrial depolarization may not be visible if retrograde conduction is blocked or atrial activation occurs simultaneously with ventricular activation and the QRS complex obscures the P waves. AV dissociation may occur if the rate of discharge from the junctional pacemaker is faster than the sinus node rate and the junctional impulse is blocked from retrograde conduction toward the atria.
JUNCTIONAL PREMATURE CONTRACTIONS (JPCs) JPCs are due to an ectopic pacemaker within the AV node or common AV bundle. The ECG characteristics of JPCs are as follows:
1. The ectopic QRS complex is premature.
2. The ectopic P' wave has a different shape and direction (usually inverted in leads II, III, and aV F).
3. The ectopic P' wave may occur before or after the QRS complex.
4. The PR interval of the ectopic beat is shorter than normal.
5. The QRS complex is usually of normal shape unless there is aberrant conduction.
6. The sinus node is usually affected and the postectopic pause is not fully compensatory ( Fig. 24:13). Some JPCs do not conduct retrograde; therefore a compensatory pause may be seen.
JPCs may be isolated, multiple (as in bigeminy or trigeminy), or multifocal.
Clinical Significance JPCs are uncommon in healthy hearts. They occur in congestive heart failure, digoxin toxicity, ischemic heart disease, and acute myocardial infarction (especially of the inferior wall).
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