Tachyarrhythmias arising from the AV junction are common. When impulses are propagated through the AV node, the atrial activation occurs retrogradely, and the P wave is therefore inverted in leads II, III and aVF. However, the P wave is often inscribed simultaneously with or immediately after the QRS complex, and determination of its presence and polarity may be difficult. Ventricular activation during these arrhythmias proceeds normally; thus QRS morphology is similar to sinus beats. Like atrial arrhythmias, aberrant conduction may also occur during AV junctional arrhythmias. All arrhythmias arising from the AV junction or atria can also be collectively called supraventricular.
Atrioventricular junctional beats
Isolated premature beats from this site usually originate in the region of the His bundle rather than the AV node, as frequently assumed. From the His bundle, less time is needed to activate the ventricle than the atria; therefore, the QRS complex is expected to precede the P wave. However, in many cases, retrograde conduction across the AV node is absent, and the junctional beat blocks in the AV node. In turn, the dissociated sinus impulse may block in the AV node or conduct with a longer PR interval due to the effect of retrograde concealed conduction of the preceding junctional beat in the AV node, which can result in the ECG pattern of interpolated junctional beat.
Premature junctional complexes
A premature junctional complex is an electrical impulse that originates in the AV junction and occurs before the next expected sinus impulse. This often results in retrograde atrial depolarization (hence the P wave in leads II, III and aVF will be negative). The retrograde P wave may precede, coincide with or follow the QRS. The relation of a retrograde P wave to QRS complex depends on the relative conduction times from the site of origin within the junction to the atria and ventricles. It is therefore likely that an impulse arising in the higher portion of the junction, above the AV node, would result in a P wave occurring before or during the QRS complex, whereas one arising at a lower level would result in a P wave that occurs after the QRS complex. Conduction from the junction to the ventricles usually occurs along normal pathways. Thus the QRS complex is usually normal, although it can be wide owing to either a bundle-branch block or aberrant conduction. The pause following a premature junctional complex may be noncompensatory (if the sinus node is depolarized by the premature beat) or fully compensatory if the sinus node has discharged before it is reached by the premature beat.
Treatment
Specific suppressive treatment is rarely needed.
Summary of ECG criteria
- QRS: Usually normal. The interval may sometimes be widened (aberrant ventricular conduction), usually indicating right bundle branch block.
- Rhythm: Irregular.
- P waves: Because atrial depolarization is usually retrograde, P waves are generally negative in leads II, III and aVF. P waves can precede, coincide with, or follow the QRS. Either a noncompensatory or a fully compensatory pause may occur.
- PR interval: If the P wave precedes the QRS, the PR interval is usually less than 0.12 second. However, the PR interval may be prolonged. Complete AV block may occur.
Nonparoxysmal junctional tachycardias
Enhanced automaticity in the region of the bundle of His is the most probable underlying mechanism or nonparoxysmal junctional tachycardia. Depending on retrograde conduction across the AV node, the atrial may be dissociated or a 1:1 or 2:1 retrograde response may be noted. The arrhythmia generally occurs in the setting of acute myocardial infarction, after open-heart surgery and sometimes with digitalis intoxication.
AV junctional escape complexes and rhythms
The AV junction can function as a pacemaker. It initiates impulses at a rate of 40 to 60 beats/min., equivalent to an RR interval between 1.0 and 1.5 seconds. Under normal circumstances, the sinus node pacemaker, which is faster, predominates. If the AV node is not depolarized by the arrival of a sinus impulse within approximately 1.0 to 1.5 seconds, it will initiate an impulse. This is called a junctional escape complex. It occurs because of failure of the sinus node to initiate an appropriately timid impulse or because of a conduction problem between the sinus node and the AV junction. A repeated series of such impulses is referred to as junctional escape rhythm.
Treatment
Most commonly, junctional escape complexes and rhythms are benign events of automaticity. They may also be due to digitalis intoxication. In nonparoxysmal junctional tachycardia due to digitalis intoxication, digitalis should be withheld. The serum potassium level should be checked, and if low, potassium should be given to raise the serum potassium to the normal range. If the patient is severely compromised, antibodies to digitalis may be used.
Summary of ECG criteria
- There is a normal-looking QRS.
- Rate: A junctional escape rhythm has a rate of 40 to 60 beats/min.
- Rhythm: The presence of some junctional escape complexes may lead to an irregular rhythm. Junctional escape complexes occur approximately 1.0 second or more following the last depolarization. A junctional escape rhythm is usually regular.
- P waves: Retrograde P waves (negative) may be seen in leads II, III and aVF. P waves may precede, coincide with or follow the QRS. Sinus P waves, at a rate equal to or slower than the junctional rhythm, may occur. This may result in AV dissociation.
- PR interval: This interval is variable but is usually less than the PR interval of the normally conducted beat from the sinus node.
- QRS interval: Ventricular conduction is usually normal unless a ventricular conduction problem is present or aberrant conduction occurs.
Figure 6. ECG of AV Junctional Tachycardia
Ventricular preexcitation
Normally, the atrial impulses conduct to the ventricles by the AV node-His-Purkinje system. Most conduction delay accounting for the normal PR interval is located in the AV node. On occasion, however, additional pathways connecting the atria with the ventricles may exist, and these are called accessory pathways. The most common of these, the Kent bundle, is a direct muscle-to-muscle bridge, anatomically separate from the normal conduction system. This type of accessory pathway forms the anatomic basis for WPW syndrome. The Kent bundle can be located anywhere around the AV junction, connecting the right atrium and right ventricle or the left atrium and left ventricle. Sometimes these bypass tracts are located within the septum. Conduction velocity is often faster in these tracts compared with the normal AV node; therefore, sinus impulses usually activate the ventricle through these pathways, resulting in a short PR interval. Initial ventricular activation by the accessory pathway is muscle to muscle; thus, the first part of the QRS complex, the delta wave, is slurred. These patients also have an intact normal pathway through which sinus impulses reach and activate the remainder of the myocardium beyond what has been depolarized via the accessory pathway. Thus, the resulting QRS is a fusion complex, which is initially activated through the accessory pathway and finally via the normal pathway.
This content is reviewed regularly. Last updated 12/4/08.
