Atrial tachycardia and other ectopic rhythms, which originate in the atria, occur outside the sinus nodes, and they create action potentials faster than the sinus node, which enables them to become the primary pacemaker of the heart. When any atrial rate is greater than 100 beats per minute, the term for the rhythm is atrial tachycardia. However, if the rate is less than 100 beats per minute, the term is ectopic atrial rhythm.
When the origin of electrical activity is not from the sinus node, the P wave does not have its normal appearance, which is upright in lead II and biphasic in V1. Depending upon where it originates, the morphology will differ. These P waves are called “ectopic P waves.” They are seen in multifocal atrial tachycardia, wandering atrial pacemaker, and premature atrial contractions.
Atrial tachycardia itself is common, and is caused by chronic hypertension, valvular disease, congestive heart failure, and the degenerative changes of age. Often, on 24 hour Holter monitoring of the elderly, there are brief but asymptomatic runs of atrial tachycardia.
The symptoms of atrial tachycardia depend upon the response of the ventricles – does each impulse lead to a ventricular beat? The symptoms also depend upon the duration of the tachycardia. Palpitations may occur, and hypotension may cause dizziness and weakness. Because there is a shortened diastolic filling time during tachycardic periods, decreased cardiac output occurs and there are potentially symptoms of congestive heart failure.
Treatment of atrial tachycardia is normally treated with medications that block the AV node, including beta-blockers, or non-dihydropyridine calcium channel blockers. Sometimes adenosine can terminate the rhythm, but it is not always successful. When medical treatment fails, ablation of the foci of atrial activation is an option.
In atrial tachycardia with a 2:1 block, digoxin toxicity should be a consideration.
A junctional rhythm occurs when the activating impulse originates near the AV node or within the AV node. The normal ventricular His-Purkinje system is used, so the QRS is usually narrow. Junctional rhythms are normally less than 60 beats per minute. A faster rhythm is known as accelerated junctional rhythm.
Since the origin of electrical activation is not at the SA node, but at or near the AV node, the P wave may be buried within the QRS complex, or it can occur slightly before or after. For the same reason, the morphology of the P wave may be different from the sinus P wave, which, again, is normally upright in lead II and biphasic in lead V1 .
The strip above shows a junctional rhythm with retrograde P waves before the QRS complex. The strip below shows retrograde P waves after the QRS complex.
Since the AV node limits the impulse conduction to the ventricles, a rhythm like atrial flutter at a rate of 300 beats per minute might be conducted at 2:1 intervals.
In junctional ectopic tachycardia, conduction bypasses the AV node, and this occurs usually after surgery or as a neonate, when it is known as His bundle tachycardia.
Junctional escape beats originate from subsidiary pacemaker cells in the AV junction. The pacemaker’s firing rate is between 40 and 60 beats per minute. Whenever the primary pacemaker, the SA node, fails to fire, or the AV node blocks the atrial impulse, junctional escape beats will occur. A junctional escape rhythm is a sequence of at least three or more junctional escape beats which occur at a rate of 40-60 beats per minute. In the ECG below, an athlete has a resting sinus rate which is slower than his junctional rate, and you will notice retrograde P waves hidden in the ST-T waves.
Accelerated junctional rhythms are caused by events that activate the junctional pacemaker cells, including electrolyte abnormalities, medications, or ischemia, and the rate is 60 to 100 beats per minute.
Nonparoxysmal junctional tachycardia begins as an accelerated junctional rhythm with the rate gradually increasing to greater than 100 beats per minute. Retrograde atrial capture may occur, or there may be AV dissociation with the ventricular rate faster than the atrial rate.
Junctional tachycardia more often has an irregular rhythm and rate variability than AVNRT or AVRT. It doesn’t respond well to vagal maneuvers, and it will not revert to sinus rhythm.
When you see a retrograde P wave, it is not necessarily a junctional P wave. It may be an ectopic pacemaker located near the bottom of the atria. If you don’t see a P wave, it may be a junctional rhythm, but a retrograde P wave may also be a junctional rhythm. Sometimes the rate is the only clue you will have to the origin of the electrical impulse driving the heart. In junctional rhythms, the retrograde P waves are usually inverted in the inferior leads and upright in aVR and VI.
Just some food for thought. I will do a post soon on AVNRT and AVRT, which are sometimes difficult to distinguish from reentrant junctional tachycardias.