Until electrophysiologic testing became widely available, focal AT was always a bit of a dilemma for clinicians. There was some spirited debate between two camps of clinicians as to the cause of the arrhythmia. One side believed that the arrhythmia was triggered by a microreentry loop (less than 2 cm in diameter) in either the left or right atria that did or did not involve the sinoatrial node. The other side believed that the rhythm was due to increased automaticity of an ectopic focus. The result of this debate was that there were a lot of terms used to describe this arrhythmia. These terms included ectopic atrial tachycardia, paroxysmal atrial tachycardia, intra-atrial reentrant supraventricular tachycardia, sinus nodal reentry SVT, and automatic atrial tachycardia, to name a few. This has led to continued confusion about the rhythm and its nomenclature.
It turned out that both camps were right. The rhythm is sometimes caused by a microreentry mechanism, and sometimes caused by increased automaticity of a single ectopic pacemaker. Electrocardiographically, however, it is impossible to distinguish between these two possibilities on a surface 12-lead ECG. That is the reason focal atrial tachycardia has become the accepted term used to label this arrhythmia.
Traditionally, focal AT accounts for 5% to 15% of all regular, narrow supraventricular tachycardias in adults, and the numbers are a bit higher in children. If the causative mechanism is known, reentry beats automaticity as the main cause. It is found equally among males and females.
Basically, a focal AT is an ectopic atrial rhythm that is over 100 BPM. It usually occurs paroxysmally (rapid onset and termination) and is self-limiting. All of the characteristics that we covered in the chapter on ectopic atrial rhythms apply to these tachycardias (Figure 15-1). A quick summary of the important points is as follows:
Figure 15-1 The focal atrial tachycardia shown in this strip demonstrates all of the typical characteristics of the rhythm: an ectopic P-wave morphology, rate greater than or equal to 100 BPM, normal or slightly prolonged PR interval, and normal QRS morphology. In addition, a slight alteration in the QRS amplitude is noted (electrical alternans), which can normally be found in many tachycardias.
From Arrhythmia Recognition: The Art of Interpretation, courtesy of Tomas B. Garcia, MD.
The rhythm can be transient, sustained, or incessant. Transient episodes occur when three or more complexes are noted for a very brief period of time. If the rhythm lasts for longer than 30 seconds, it is considered to be sustained. If the rhythm does not terminate, or is present most of the time, it is considered incessant.
Transient episodes are fairly common in the general population, usually involve heart rates about the 150 BPM range, and are relatively benign. Incessant rhythms are rare, and can lead to serious congestive heart failure or cardiomyopathies if left untreated. As usual, all abnormal rhythms should be referred for further evaluation and possible treatment. Any hemodynamically significant rhythm should be emergently treated with medication and/or cardioversion/defibrillation.
The tachycardia is usually regular. We say “usually” because the onset of most tachycardias has some irregularity associated with it. This is found in many, many types of tachycardias. Think of it like an engine that is just starting up. It usually needs a slight warm-up period before it can really clip along at its normal pace. The heart, and the ectopic focus in particular, need the same type of warm-up period. At the onset of a focal AT, however, the warm-up period is quite short and is usually triggered by a premature complex. Due to the short warm-up period, the heart rate ramps up quickly in these patients until it reaches the “cruising speed” of the tachycardia.
As mentioned in the first paragraph, the P-wave morphology will always be different from that normally seen with sinus rhythm. In the case of an inverted P wave in leads I, II, III, and aVF, the diagnosis will be clear and easy to make (inverted P waves in leads II, III, and aVF that are associated with normal or prolonged PR intervals are considered ectopic atrial until proven otherwise). However, sometimes the morphologic differences between an ectopic and a sinus P wave may be very slight. Indeed, there will be many times that a skilled clinician will find it difficult, if not impossible, to distinguish the morphology of a P wave on an isolated strip as ectopic, unless an old strip or ECG is available for comparison. When an old strip is available, you will always notice that the ectopic P waves will be either wider, taller, notched, or biphasic, or somehow different in many leads when compared to the sinus P.
A final word about P-wave morphology. If you are lucky enough to catch the onset of the tachycardia, you will note that it is usually initiated by a premature atrial contraction (PAC). The morphology of the P wave that triggered the PAC, and the ones found within the tachycardia itself, are usually the same or very, very similar (Figure 15-2). This occurs because the irritable ectopic focus in both cases is the same. The morphology of the P wave in the initiating complex is a very useful tool in helping you to identify the correct rhythm in a case of supraventricular tachycardia.
Figure 15-2 This strip starts off with a normal sinus rhythm. The premature atrial contraction (PAC) labeled in the diagram shows an inverted P-wave morphology. This PAC triggers a focal atrial tachycardia that has the same P-wave morphology as the PAC. The similarity of the P-wave morphology between the triggering PAC and those present in the tachycardia itself is typical for this arrhythmia.
From Arrhythmia Recognition: The Art of Interpretation, courtesy of Tomas B. Garcia, MD.
Conduction through the AV node will be either normal or prolonged during the tachycardia. But note that this does not directly translate into a normal or prolonged PR interval in all cases. As we discussed in Chapter 13, Premature Atrial Contraction, and Chapter 14, Ectopic Atrial Rhythm, the location of the ectopic focus, or circus loop, also affects the length of the PR interval. If the focus or loop is near the AV node, then the PR interval would be shorter than expected, and vice versa. Therefore, depending on the combination of various factors, the PR interval in the ectopic complexes may be either shortened, the same, or prolonged when compared to the sinus complexes in any one patient.
Occasionally, the rate of the tachycardia will exceed the rate at which the AV node can transmit the impulse. When this occurs, the AV node may block some of the impulses from reaching the ventricles. When this occurs, we say that an AV block exists. (We will discuss AV blocks in great detail later.) In the case of focal AT, we can have a rhythm with an atrial tachycardia but the ventricular rate may be within the normal range. Traditionally, a focal AT with AV nodal block was called a paroxysmal atrial tachycardia with block. We will review this particular entity in depth in the next chapter. For now, just be aware that it exists.
The QRS complex is normal in both appearance and duration. The only exception is when a rate-related aberrancy develops because of the tachycardia or there is a preexisting bundle branch block. Electrolyte abnormalities can often cause widening of the QRS complexes as well. Electrical alternans may also be seen occasionally. This is typically due to the tachycardia and not to some other underlying pathologic problem.
The ST segment may frequently be depressed, sometimes up to 2 mm, and may appear ischemic. Resolution of the ST segments back to baseline should occur on termination of the tachycardia. Remember to always have a high index of suspicion when approaching any ST-segment changes. As always, the clinical presentation and hemodynamic status of the patient are critical elements to keep in mind as you evaluate the ECG.
Clinically, the rhythm is usually well tolerated. This is because the ventricular rates are usually slower than many tachycardias. AV block is often present at very high atrial rates, which helps to maintain tolerable hemodynamic parameters. Recurrences tend to occur in adults and may need to be referred for catheter ablation for permanent control.
ARRHYTHMIA RECOGNITION
Focal Atrial Tachycardia
| Rate: | Between 100 and 200 BPM, although rates up to 250 BPM are possible |
| Regularity | Usually regular |
| P wave:
Morphology: Upright in II, III, and aVF: |
Present
Different Sometimes |
| P:QRS ratio: | 1:1 AV block can occur |
| PR interval: | Normal, prolonged |
| QRS width: | Normal or wide |
| Grouping: | None |
| Dropped beats: | None |
DIFFERENTIAL DIAGNOSIS
Focal Atrial Tachycardia
This list represents the common causes of focal AT, but is not complete.