Causes of Supraventricular Tachycardia (SVT)
Supraventricular tachycardia is primarily caused by three main mechanisms: re-entry phenomena (most common), abnormal automaticity, and triggered activity, with re-entry accounting for the majority of SVT cases. 1
Primary Mechanisms
Re-entry is the dominant mechanism underlying most SVT subtypes, involving repetitive electrical impulse conduction around a fixed obstacle in a defined circuit. 2, 1 This requires:
- Unidirectional conduction block in one limb of a circuit 2
- Slow conduction through tissue (typically the AV node) to allow recovery and activation of the alternate pathway 2
- A premature impulse or acceleration of heart rate that impinges on the refractory period of the pathway 2
Abnormal automaticity occurs when atrial, AV junctional, or atrial vessel tissues exhibit enhanced diastolic phase 4 depolarization, leading to increased firing rates compared to normal pacemaker cells. 1
Triggered activity results from disturbances in repolarization, where afterdepolarizations reach threshold and trigger early action potentials during repolarization. 2, 1
Specific SVT Types and Their Etiologies
Atrioventricular Nodal Reentrant Tachycardia (AVNRT) is caused by a re-entry circuit within the AV node involving dual pathways (fast and slow conducting pathways). 1 This is one of the most common forms of paroxysmal SVT. 2
Atrioventricular Reciprocating Tachycardia (AVRT) involves an accessory pathway (bypass tract) between the atria and ventricles. 1 In orthodromic AVRT, anterograde conduction occurs across the AV node with retrograde conduction over the accessory pathway. 2
Atrial Tachycardia (AT) can arise from enhanced automaticity, triggered activity, or micro-reentry within atrial tissue, characterized by a long RP interval with P wave morphology different from sinus rhythm. 1 Multifocal atrial tachycardia (MAT) is most commonly encountered in patients with pulmonary disease. 2, 1
Atrial Flutter involves a macro-reentrant circuit, typically around the tricuspid annulus (cavotricuspid isthmus-dependent), and is often associated with structural heart disease. 2, 1
Inappropriate Sinus Tachycardia presents with sinus heart rate >100 bpm at rest, with a mean 24-hour heart rate >90 bpm not due to appropriate physiological responses or primary causes such as hyperthyroidism or anemia. 2
Predisposing Factors and Associated Conditions
Structural heart disease is a major predisposing factor, including: 1
Congenital heart disease carries particularly high risk (10-20% incidence in adults with congenital heart disease), especially: 1, 3
- Ebstein anomaly 1, 3
- Tetralogy of Fallot 1, 3
- Transposition of great arteries 1, 3
- Atrial septal defects 1, 3
Acute precipitating events include: 1, 3
Metabolic and endocrine disorders: 1
Medications and substances: 1, 3
- Stimulants 1, 3
- Antiarrhythmics 1, 3
- Caffeine 1, 3
- Digitalis toxicity (particularly for nonparoxysmal junctional tachycardia) 3
Important Clinical Pitfalls
Pre-excitation syndromes (Wolff-Parkinson-White) require prompt evaluation due to risk of sudden cardiac death if atrial fibrillation develops with rapid conduction over the accessory pathway. 1, 3 These patients need expedient referral to a cardiologist because ablation is potentially curative. 4
Prolonged SVT (weeks to months) with fast ventricular response can lead to tachycardia-mediated cardiomyopathy, which is reversible if identified and treated. 2, 1, 3
SVT in congenital heart disease patients carries significantly higher risks of heart failure, stroke, and sudden cardiac death compared to structurally normal hearts. 1, 5 These patients require management by a cardiologist with specialized training. 3
Digitalis toxicity is the most important reversible cause of nonparoxysmal junctional tachycardia and must not be missed. 3