Atrial Tachycardia

Atrial tachycardia is defined as a supraventricular tachycardia (SVT) that does not require the atrioventricular (AV) junction, accessory pathways, or ventricular tissue for its initiation and maintenance. Atrial tachycardia can be observed in persons with normal hearts and in those with structurally abnormal hearts, including those with congenital heart disease and particularly after surgery for repair or correction of congenital or valvular heart disease.

In adults, tachycardia is usually defined as a heart rate of more than 100 beats per minute (bpm). In children, the definition of tachycardia varies because the normal heart rate is age dependent, as follows ^{[4, 5]} :

• Age 1-2 days: 123-159 bpm

• Age 3-6 days: 129-166 bpm

• Age 1-3 weeks: 107-182 bpm

• Age 1-2 months: 121-179 bpm

• Age 3-5 months: 106-186 bpm

• Age 6-11 months: 109-169 bpm

• Age 1-2 years: 89-151 bpm

• Age 3-4 years: 73-137 bpm

• Age 5-7 years: 65-133 bpm

• Age 8-11 years: 62-130 bpm

• Age 12-15 years: 60-119 bpm

As in most SVTs, the electrocardiogram (ECG) typically shows a narrow QRS complex tachycardia (unless bundle branch block aberration occurs). Heart rates are highly variable, with a range of 100-250 bpm. The atrial rhythm is usually regular. (See the image below.)

Atrial tachycardia. This 12-lead electrocardiogram demonstrates an atrial tachycardia at a rate of approximately 150 beats per minute. Note that the negative P waves in leads III and aVF (upright arrows) are different from the sinus beats (downward arrows). The RP interval exceeds the PR interval during the tachycardia. Note also that the tachycardia persists despite the atrioventricular block.

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The conducted ventricular rhythm is also usually regular. It may become irregular, however, especially at higher atrial rates, because of variable conduction through the AV node, thus producing conduction patterns such as 2:1, 4:1, a combination of those, or Wenckebach AV block.

The P wave morphology on the ECG may give clues to the site of origin and mechanism of the atrial tachycardia. In the case of a focal tachycardia, the P wave morphology and axis depend on the location in the atrium from which the tachycardia originates. In the case of macroreentrant circuits, the P wave morphology and axis depend on activation patterns (see Workup).

Multifocal atrial tachycardia (MAT) is an arrhythmia with an irregular atrial rate greater than 100 bpm. Atrial activity is well organized, with at least three morphologically distinct P waves, irregular P-P intervals, and an isoelectric baseline between the P waves. ^[6] Multifocal atrial tachycardia has previously been described by names such as chaotic atrial rhythm or tachycardia, chaotic atrial mechanism, and repetitive paroxysmal MAT. Go to Multifocal Atrial Tachycardia for more complete information on this topic.

Classification methods

A number of methods are used to classify atrial tachycardia. Classification in terms of origin can be based on endocardial activation mapping data, pathophysiologic mechanisms, and anatomy.

On the basis of endocardial activation, atrial tachycardia may be divided into the following two groups (see Presentation):

• Focal atrial tachycardia: Arises from a localized area in the atria such as the crista terminalis, pulmonary veins, ostium of the coronary sinus, or intra-atrial septum.

• Reentrant atrial tachycardias: Usually macroreentrant; reentrant atrial tachycardias most commonly occur in persons with either structural or complex heart disease, particularly after surgery involving atrial incisions or scarring

Other methods of classification are as follows:

• Pathophysiologic mechanisms: Atrial tachycardia can be classified as the result of enhanced automaticity, triggered activity, or reentry (see Pathophysiology)

• Anatomy: Classification of atrial tachycardia can be based on the location of the arrhythmogenic focus (see Anatomy)

Diagnosis and treatment

A 12-lead ECG with rhythm strip is an important tool to help identify, locate, and differentiate atrial tachycardia. Laboratory studies may be indicated to exclude systemic disorders that may be causing the tachycardia. Invasive electrophysiologic study (EPS) may be required. (See Workup.) The morphology of the P wave may provide valuable clues to the origin of the tachycardia, and this is the reason why a 12-lead ECG is of special value.

The primary treatment during an episode of atrial tachycardia is considered to be rate control using AV nodal blocking agents, such as beta-blockers or calcium channel blockers (see Treatment and Medication). Cardioversion should be considered for any patient in whom the rhythm is not tolerated well hemodynamically and/or in whom rate control drugs are ineffective or contraindicated.

Catheter ablation for atrial tachycardia has become a highly successful and effective treatment option for symptomatic patients whose condition is refractory to medical therapy or who do not desire long-term antiarrhythmic therapy. It can cure macroreentrant and focal forms of atrial tachycardia. (See Treatment.) ^{[7, 8]}

Atrial tachycardia can have a right or left atrial origin. Some atrial tachycardias actually originate outside the usual anatomic boundaries of the atria, in areas such as the superior vena cava, pulmonary veins, and vein of Marshall, where fingers of atrial myocardium extend into these locations. Rare locations, such as the noncoronary aortic cusp ^[1] and hepatic veins, have been described, as well. (See the video below.)

A number of aspects of the atrial anatomy can contribute to the substrate for arrhythmia. The orifices of the vena cava, pulmonary veins, coronary sinus, atrial septum, and mitral and tricuspid annuli are potential anatomic boundaries for reentrant circuits.

Anisotropic conduction in the atria due to complex fiber orientation may create the zone of slow conduction. Certain atrial tissues, such as the crista terminalis and pulmonary veins, are common sites for automaticity or triggered activity. Additionally, disease processes or age-related degeneration of the atria may give rise to the arrhythmogenic substrate.

Abnormalities that have been reported at the sites of atrial tachycardia origin include the following ^[9] :

• Extensive myocardial fibrosis

• Myocyte hypertrophy

• Endocardial fibrosis

• Mononuclear cell infiltration

• Mesenchymal cell proliferation

• Islets of fatty tissue

• Thinning

• Blebs

Several pathophysiologic mechanisms have been ascribed to atrial tachycardia. These mechanisms can be differentiated on the basis of the pattern of onset and termination and the response to drugs and atrial pacing.

Enhanced automaticity

Automatic atrial tachycardia arises due to enhanced tissue automaticity and is observed in patients with structurally normal hearts and in those with organic heart disease. The tachycardia typically exhibits a warm-up phenomenon, during which the atrial rate gradually accelerates after its initiation and slows prior to its termination.

Automatic atrial tachycardia is rarely initiated or terminated by a single atrial stimulation or rapid atrial pacing, but it may be transiently suppressed by overdrive pacing. It almost always requires isoproterenol infusion to facilitate induction and is predictably terminated by propranolol. ^[10] Carotid sinus massage and adenosine do not terminate the tachycardia even if they produce a transient AV nodal block. Electrical cardioversion is ineffective (being equivalent to attempting electrical cardioversion in a sinus tachycardia).

Triggered activity

Atrial tachycardia caused by triggered activity is due to delayed after-depolarizations, which are low-amplitude oscillations occurring at the end of the action potential. ^[11] These delayed after-depolarization–related oscillations are triggered by the preceding action potential and are the result of calcium ion influxes into the myocardium. If these oscillations are of sufficient amplitude to reach the threshold potential, depolarization occurs again and a spontaneous action potential is generated.

If single, this is recognized as an atrial ectopic beat (an extra or premature beat). If it recurs and spontaneous depolarization continues, a sustained tachycardia may result.

Most commonly, atrial tachycardia due to triggered activity occurs in patients with digitalis intoxication ^[2] or conditions associated with excess catecholamines. Characteristically, the arrhythmia can be initiated, accelerated, and terminated by rapid atrial pacing. It may be sensitive to physiologic maneuvers and drugs such as adenosine, verapamil, and beta-blockers, all of which can terminate the tachycardia.

Occasionally, this atrial tachycardia may arise from multiple sites in the atria, producing a multifocal or multiform atrial tachycardia. This may be recognized by varying P wave morphology and irregularity in the atrial rhythm.

Pulmonary vein tachycardias

Pulmonary vein tachycardias originate from the os of the pulmonary vein or even deeper localized atrial fibers. These strands of atrial tissue are generally believed to gain electrical independence, since they are partially isolated from the atrial myocardium. These tachycardias are typically very rapid (heart rate of 200-220 bpm or more)

Although pulmonary vein tachycardias frequently trigger episodes of atrial fibrillation, the associated atrial tachycardias may be the clinically dominant or exclusive manifestation. The latter typically involves only a single pulmonary vein as opposed to the multiple pulmonary vein involvement seen in atrial fibrillation.

Reentrant tachycardia

Intra-atrial reentry tachycardias may have either a macroreentrant or a microreentrant circuit. Macroreentry is the usual mechanism in atrial flutter and in scar- and incision-related (postsurgical) atrial tachycardia.

The more common and recognized form of atrial tachycardia, seen as a result of the advent of pulmonary vein isolation and linear ablation procedures, is left atrial tachycardia. In this situation, gaps in the ablation lines allow for slow conduction, providing the requisite anatomic substrate for reentry. These tachycardias may be self-limiting but if they persist, mapping and a repeat ablative procedure should be considered.

Microreentry can arise in a small focal area, such as in sinus node reentrant tachycardia. Typically, episodes of reentrant atrial tachycardia arise suddenly, terminate suddenly, and are paroxysmal. Carotid sinus massage and adenosine are ineffective in terminating macroreentrant tachycardias, even if they produce a transient AV nodal block. During electrophysiologic study, it can be induced and terminated by programmed extrastimulation. As is typical of other reentry tachycardias, electrical cardioversion terminates this type of atrial tachycardia.

Classification of atrial tachycardia

Based on endocardial activation, atrial tachycardia may be divided into two groups: focal and reentrant. Focal atrial tachycardia arises from a localized area in the atria such as the crista terminalis, pulmonary veins, ostium of the coronary sinus, or intra-atrial septum. If it originates from the pulmonary veins, it may trigger atrial fibrillation and often forms a continuum of arrhythmias.

Reentrant (usually macroreentrant) atrial tachycardias most commonly occur in persons with structural heart disease or complex congenital heart disease, particularly after surgery involving incisions or scarring in the atria. Electrophysiologically, these atrial tachycardias are similar to atrial flutters, typical or atypical. Often, the distinction is semantic, typically based on arbitrary cutoffs of atrial rate.

Some tachycardias cannot be easily classified. Reentrant sinoatrial tachycardia (or sinus node reentry) is a subset of focal atrial tachycardia due to reentry arising within the sinus node situated at the superior aspect of the crista terminalis. The P wave morphology and atrial activation sequence are identical or very similar to those of sinus tachycardia.

Atrial tachycardia can occur in individuals with structurally normal hearts or in patients with organic heart disease. When it arises in patients with congenital heart disease who have undergone corrective or palliative cardiac surgery, such as a Fontan procedure, an atrial tachycardia can have potentially life-threatening consequences. ^[12]

The atrial tachycardia that manifests in association with exercise, acute illness with excessive catecholamine release, alcohol ingestion, altered fluid states, hypoxia, metabolic disturbance, or drug use (eg, caffeine, albuterol, theophylline, cocaine) is associated with automaticity or triggered activity. Digitalis intoxication is an important cause of atrial tachycardia, with triggered activity being the underlying mechanism.

Reentrant atrial tachycardia tends to occur in patients with structural heart disease, including ischemic, congenital, postoperative, and valvular disorders. Iatrogenic atrial tachycardias have become more common and typically result from ablative procedures in the left atrium. Several typical origination sites for these tachycardias have been identified, including the mitral isthmus (between the left lower pulmonary vein and mitral annulus), the roof of the left atrium, and, for reentry, around the pulmonary veins.

The most common reason for postablation tachycardias is gaps in the ablation lines, allowing for slow conduction and initiation reentry circuits or circuits excluded by the set of ablation lines. Typically, these patients have undergone an atrial fibrillation ablation procedure. This is true for catheter ablation and surgical epicardial ablation. Similarly, patients with prior surgical procedures involving the left atrium may have surgical incision lines and, hence, the potential for macroreentrant circuits.

MAT is often related to underlying illnesses, frequently occurring in patients experiencing an exacerbation of chronic obstructive pulmonary disease (COPD), ^[3] a pulmonary thromboembolism, an exacerbation of heart failure, or severe illness, especially under critical care with inotropic infusion. MAT is often associated with hypoxia and sympathetic stimulation. Digitalis toxicity also may be present in persons with MAT, with triggered activity as the mechanism.

Other underlying conditions that are commonly associated with MAT are the following:

• Valvular heart disease

• Diabetes mellitus

• Hypokalemia

• Hypomagnesemia

• Azotemia

• Postoperative state

• Sepsis

• Methylxanthine toxicity

• Myocardial infarction

• Pneumonia

Unusual forms of atrial tachycardias can be seen in patients with an infiltrative process involving the pericardium and, by extension, the atrial wall.

A very unusual form of recipient-to-donor atrial tachycardia may be seen in patients who have undergone bi-atrial orthotropic cardiac transplantation. ^[13] In this situation, atrial arrhythmia originates in the recipient remnant of the heart or from reentry around the incision suture lines and break through to the donor heart. Because of cardiac denervation, the recipient patients usually do not develop symptoms of palpitations but rather develop tachycardia-induced cardiomyopathy in the donor heart. These atrial tachycardias are extremely difficult to treat with drugs, but they can be cured by catheter ablation. Sometimes, atrial fibrillation and atrial flutter in the recipient heart indicate rejection; hence, close vigilance is necessary.

Atrial tachycardia is relatively rare, constituting 5-15% of all SVTs. Atrial tachycardia has no known racial or ethnic predilection and no known predilection for either sex. There may be some association with pregnancy.

Atrial tachycardia may occur at any age, although it is more common in children and adults with congenital heart disease. MAT is a relatively infrequent arrhythmia, with a prevalence rate of 0.05-0.32% in patients who are hospitalized. It is predominantly observed in males and in older patients—in particular, elderly patients with multiple medical problems. The average age of patients from 9 studies was 72 years.

In patients with structurally normal hearts, atrial tachycardia is associated with a low mortality rate. However, tachycardia-induced cardiomyopathies have developed in patients with persistent or frequent atrial tachycardia. Patients with underlying structural heart disease, congenital heart disease, or lung disease are less likely to be able to tolerate atrial tachycardia. Other morbidity is associated with lifestyle changes and associated symptoms.

A study by Chung et al indicated that in patients with acute ischemic stroke and nonsustained atrial tachycardia, an enlarged left atrium is a risk factor for stroke recurrence. The study involved 252 patients, who were followed up for a mean period of 35 months. ^[14]

Multifocal atrial tachycardia

MAT itself is seldom life threatening. The condition is transient and resolves when the underlying condition improves. The prognosis depends on the prognosis of any comorbid disease.

Many patients with MAT have significant comorbidities, especially COPD and respiratory failure, that often require treatment in an intensive care unit. Consequently, a high mortality rate (up to 45%) is associated with this arrhythmia, although the mortality is not a direct consequence of the rhythm abnormality.

Potential complications of MAT include development of tachycardia-induced cardiomyopathy if the arrhythmia is persistent. Other complications include the following:

• Atrial thrombi with embolization and subsequent stroke

• Myocardial infarction from incongruous myocardial supply and demand

• Pulmonary emboli

For patient education information, see the Heart Health Center, as well as Supraventricular Tachycardia and Palpitations.

In the case of multifocal atrial tachycardia (MAT) related to medication, education regarding correct medication usage and the monitoring of such medications should be considered. In the case of a pulmonary source, education about prevention and recognition of developing pulmonary conditions may be helpful.