How does atrial tachycardia cause supraventricular tachycardia?

Atrial Tachycardia Does Not "Cause" SVT—It IS a Type of SVT

Atrial tachycardia (AT) is a specific subtype of supraventricular tachycardia, not a cause of it. SVT is an umbrella term encompassing multiple distinct arrhythmias that originate from tissue at or above the His bundle, and AT represents one category within this broader classification 1, 2.

Understanding the Taxonomic Relationship

SVT is defined as any tachycardia exceeding 100 bpm at rest where the mechanism involves tissue from the His bundle or above 1, 2. This umbrella term includes:

• Atrial tachycardias (focal AT, multifocal AT, macroreentrant AT including atrial flutter) 1, 3
• Atrioventricular nodal reentrant tachycardia (AVNRT) - the most common form of paroxysmal SVT 4, 3
• Atrioventricular reentrant tachycardia (AVRT) - requiring an accessory pathway 1, 3
• Junctional tachycardias 3

Focal Atrial Tachycardia Characteristics

Focal AT arises from a localized atrial site with regular, organized atrial activity characterized by discrete P waves with typically an isoelectric segment between P waves 1, 2. Key distinguishing features include:

• Atrial rates typically between 100-250 bpm, rarely reaching 300 bpm 1
• Neither the sinus node nor AV node plays a role in initiation or perpetuation 1
• P waves generally occur in the second half of the tachycardia cycle, often obscured by the preceding T wave 1
• The presence of AV block during tachycardia excludes AVRT and makes AVNRT very unlikely, helping confirm AT as the diagnosis 1

Mechanisms of Focal AT

Focal AT can result from three distinct electrophysiologic mechanisms: abnormal automaticity, triggered activity (delayed afterdepolarization), or microreentry 1. Clinical clues to mechanism include:

• "Warm-up" (progressive rate increase at onset) and "cool-down" (progressive rate decrease before termination) suggest automatic mechanism 1
• Automatic ATs tend to be incessant, especially in children 1
• Triggered activity ATs may be either incessant or paroxysmal 1

Anatomic Distribution

Focal ATs cluster in specific anatomical zones rather than occurring randomly 1:

• Right-sided ATs predominantly originate along the crista terminalis from the sinoatrial node to the AV node 1
• Left atrial foci commonly arise from pulmonary veins, atrial septum, or mitral annulus 1
• Many left atrial foci serve as generators for atrial fibrillation 1

Clinical Significance

Focal AT is diagnosed in only 10-15% of patients referred for catheter ablation of SVT, making it relatively uncommon 1. The prevalence is:

• 0.34% in asymptomatic patients 1
• 0.46% in symptomatic patients 1

The outlook is usually benign except for incessant forms, which may lead to tachycardia-induced cardiomyopathy—a critical complication affecting morbidity and quality of life 1, 5.

Common Pitfall: Drug-Induced AT

Digitalis is the drug most commonly associated with focal AT, typically presenting as AT with AV block 1. This presentation is distinctive because:

• The ventricular rate is not excessively rapid due to AV block 1
• Hypokalemia may exacerbate the arrhythmia 1
• Treatment consists of discontinuing digitalis; in cases of persistent advanced AV block, digitalis-binding agents should be considered 1