Atrial Tachycardia With Variable Block

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Atrial Tachycardia

Fig. 1. ECGs in three patients with atrial tachycardia. (A) Atrial activity is most prominent in lead Vi. Note that the P wave is closer to the next QRS complex than the previous QRS complex; a so-called "long-RP tachycardia." There is 1:1 conduction until the end of the tracing, when AV nodal block reveals low amplitude P waves. (B) Atrial tachycardia with 2:1 A:V conduction, and left bundle-branch block (LBBB) aberrancy in a patient with dilated cardiomyopathy. P-wave activity is most evident in lead V1. (C) Atrial tachycardia with variable A:V conduction. Once again, P-wave activity is most obvious in lead V1. The atrial rate is 180 BPM; 1:1 conduction occurred with minimal exertion. This tachycardia was incessant, and the patient was referred for cardiac transplantation because of intractable class IV heart failure (left ventricular ejection fraction 9%). Catheter ablation of this left atrial focus led to prompt symptomatic improvement, with normalization of left ventricular function over a six month period.

Fig. 1. ECGs in three patients with atrial tachycardia. (A) Atrial activity is most prominent in lead Vi. Note that the P wave is closer to the next QRS complex than the previous QRS complex; a so-called "long-RP tachycardia." There is 1:1 conduction until the end of the tracing, when AV nodal block reveals low amplitude P waves. (B) Atrial tachycardia with 2:1 A:V conduction, and left bundle-branch block (LBBB) aberrancy in a patient with dilated cardiomyopathy. P-wave activity is most evident in lead V1. (C) Atrial tachycardia with variable A:V conduction. Once again, P-wave activity is most obvious in lead V1. The atrial rate is 180 BPM; 1:1 conduction occurred with minimal exertion. This tachycardia was incessant, and the patient was referred for cardiac transplantation because of intractable class IV heart failure (left ventricular ejection fraction 9%). Catheter ablation of this left atrial focus led to prompt symptomatic improvement, with normalization of left ventricular function over a six month period.

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Multifocal Atrial Tachycardia
Fig. 1. (Continued.)

Multifocal Atrial Tachycardia

This arrhythmia is diagnosed when there are at least three P-wave morphologies, and an irregular atrial rate of at least 100 beats per minute (BPM) (see Fig. 2) (5). Variability in the PR interval is common. This arrhythmia tends to occur in acutely ill, elderly patients, particularly those with active pulmonary disease. Discrete P waves, with intervening isoelectric segments, distinguish this arrhythmia from atrial fibrillation (AF).

Atrial Fibrillation (AF)

The most common sustained tachyarrhythmia results from multiple reentrant circuits in the right and left atria. Atrial fibrillation (AF) is discussed in great detail in Chapters 5-8, and thus will not be considered further in this chapter.

Atrial Flutter

A macroreentrant arrhythmia, atrial flutter may involve a stereotypical right atrial circuit (typical counterclockwise and clockwise isthmus-dependent flutter) or other circuits (atypical flutters). Atypical flutters commonly occur in patients with prior surgical repair of congenital heart disease. Atrial flutter is considered in detail in Chapter 9, and will therefore not be discussed further in this chapter.

A-V Junctional Tachyarrhythmias

AV Nodal Reentrant Tachycardia (AVNRT)

AVNRT is the most common form of paroxysmal supraventricular tachycardia (PSVT) (6). The typical form of AVNRT is a short RP tachycardia; P waves are either buried entirely within the QRS complex, or visible just as a tiny deflection at the

Atrial Tachycardia With Variable Block
Fig. 2. ECG in a patient with advanced COPD. Multiple P-wave morphologies, most evident in the lead V1 and lead II rhythm strips, and rapid rate are diagnostic of multifocal atrial tachycardia (MAT). Note the variability in PR interval and occasional blocked P wave.

junction of the QRS complex and ST segment (see Fig. 3). Atypical AVNRT is a long RP tachycardia, and electrocardiographically may resemble unifocal atrial tachycardia with 1:1 A:V conduction. AVNRT occurs in patients with dual AV nodal pathways, usually called the fast and slow pathways (see Fig. 4). During sinus rhythm, anterograde conduction occurs over the fast pathway. An atrial premature depolarization (APD) may find the fast pathway still refractory from the previous depolarization, and may thus conduct down the slow pathway (see Fig. 5). If this conduction is slow enough, the fast pathway may recover excitability, so the impulse can turn around propagate retrogradely up to the atrium over the fast pathway, creating a reentry circuit. Atypical AVNRT is usually initiated by a ventricular premature depolarization (VPD), which conducts retrogradely up the slow pathway, and then turns around and proceeds antero-gradely over the fast pathway. Patients tend to have one of these two forms of AVNRT clinically, but occasionally both types are inducible during electrophysiology study (EPS). Because infranodal tissue is not part of the reentry circuit itself, AVNRT occasionally occurs with 2:1 infranodal block, resembling atrial tachycardia with 2:1 A:V conduction.

A-V Reentrant Tachycardia (AVRT)

Normally, the AV node and His-Purkinje system constitute the only electrical connection between the atria and ventricles. An accessory pathway or bypass tract consists of strands of working myocardium that bridge the mitral or tricuspid annulus, creating an additional electrical connection between atria and ventricles. When an accessory pathway capable of anterograde conduction is present, ventricular pre-excitation and the Wolff-Parkinson-White Syndrome (WPW) are apparent during sinus rhythm (see Figs. 6, 7) (7,8). The most common arrhythmia in patients with WPW is orthodromic

Short Avnrt
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Fig. 3. ECG in a patient with typical AVNRT. Retrograde P waves are evident as deflections at the junction of the QRS complex and ST segment. Pseudo S waves are evident in leads II, III, and aVF, while pseudo R' waves are seen in aVR and V!. Since the P waves are much closer to the previous QRS complex compared with the next complex, this is a short RP tachycardia.

AVRT; in this macroreentrant circuit, anterograde conduction is over the AV node, and retrograde conduction is up the accessory pathway. Like AVNRT, this orthodromic AVRT is a short RP tachycardia, and the P wave is sometimes visible superimposed on the ST segment (see Figs. 7, 8). Less commonly in patients with WPW, the opposite circuit occurs, inscribing a tachycardia with a wide and bizarre QRS complex called antidromic AVRT (see Figs. 7, 9) In WPW patients, atrial fibrillation and atrial flutter are of great concern (see Figs. 10, 11). Unlike the AV node, many accessory pathways are capable of rapid, nondecremental conduction. Ventricular rates of 250-300 BPM or higher can occur, with the risk of degeneration into ventricular fibrillation (VF). Thus, there is a risk of sudden death associated with the WPW syndrome.

Some accessory pathways are capable of only retrograde conduction; since there is no evidence of pre-excitation in sinus rhythm, these pathways are said to be concealed. Patients with concealed accessory pathways can have orthodromic but not antidromic AVRT, and are not at risk for sudden death caused by extremely rapid, pre-excited AF or atrial flutter. Some concealed pathways display slow retrograde conduction. These patients have an orthodromic AVRT characterized by a long RP interval called permanent junctional reciprocating tachycardia (PJRT) (see Fig. 12) (9). As this tachycardia tends to be incessant, it can lead to tachycardia-induced cardiomyopathy (10).

Nonparoxysmal Junctional Tachycardia (NPJT)

This tachycardia is extremely rare. Unlike AVNRT and AVRT, which are reentrant in nature, NPJT is believed to be caused by enhanced automaticity or triggered activity within the AV junction (11). Electrocardiographically, this may resemble AVNRT, or there may be A-V dissociation during the narrow complex tachycardia (see Fig. 13).

Fig. 4. Mechanism of AVNRT. Each panel shows a diagram of the AV node, a Lewis (ladder) diagram, and surface ECG lead II. Continuous lines represent anterograde conduction; broken lines represent retrograde conduction. Straight lines signify fast pathway conduction; wavy lines slow pathway. P represents sinus P waves, P' A-V nodal echoes, APD-atrial premature depolarization, VPD-ventricular premature depolarization. The left panel represents sinus rhythm, in which anterograde conduction is over the fast pathway. In the middle panel, an APD finds the fast pathway refractory and travels anterograde over the slow pathway. When the impulse has traversed the slow pathway, the fast pathway has recovered; thus the impulse can turn around and travel retrograde up the fast pathway. Note the nearly simultaneous inscription of P' and the QRS complex during common AVNRT. In the right panel, a VPD finds the retrograde fast pathway refractory, and travels up the slow pathway, initiating uncommon AVNRT. Note that in common AVNRT RP' < P'R, while in uncommon AVNRT, RP' > P'R. (Reproduced from Ganz LI, Friedman PL. Supraventricular tachycardia. N Engl J Med 1995;332:162-173. Copyright © 1995 Massachusetts Medical Society.)

Uncommon Avnrt
Fig. 5. Event monitor in patient with PSVT. An APD (arrow), conducted with a long PR interval, initiates tachycardia. This is characteristic of typical AVNRT.

This arrhythmia tends to occur in the setting of digitalis toxicity or early after valve surgery; it is also seen rarely in the setting of acute myocardial infarction (MI).

Junctional Ectopic Tachycardia (JET)

This is also a nonparoxysmal, nonreentrant tachycardia, frequently described in the pediatric literature (12). This arrhythmia may have the same mechanism as NPJT. JET tends to occur following surgical repair of congenital heart disease, has an extremely rapid ventricular response, and is associated with a very high mortality.

Ecg Ventricular Pre Excitation
Fig. 6. ECG in a patient with WPW. Note the short PR interval and delta wave reflective of ventricular pre-excitation. In this case, the pathway is right septal.

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Responses

  • kristian
    What is the rate of atrial tach with block?
    12 months ago
  • thomas eisenhower
    Why do you call it atrial tachycardia with block when the ventricular response is not over 100?
    3 months ago

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