What causes Supraventricular Tachycardia (SVT)?

Causes of Supraventricular Tachycardia (SVT)

Supraventricular tachycardia is primarily caused by reentry phenomena or abnormal automaticity in atrial or atrioventricular nodal tissue above the bundle of His, with the most common mechanisms being atrioventricular nodal reentrant tachycardia (AVNRT), atrioventricular reentrant tachycardia (AVRT), and atrial tachycardia (AT). 1

Primary Mechanisms of SVT

1. Reentry Mechanisms

• Atrioventricular Nodal Reentrant Tachycardia (AVNRT)

  • Most common form of SVT (50-60% of cases)
  • Caused by dual AV nodal pathways (alpha and beta) with different conduction times and refractory periods 2
  • Both pathways are confined to the AV nodal and perinodal atrial tissue

• Atrioventricular Reentrant Tachycardia (AVRT)

  • Second most common form of SVT
  • Depends on an accessory pathway that bypasses the AV node 1
  • Includes Wolff-Parkinson-White syndrome (pre-excitation)
  • Permanent form of junctional reciprocating tachycardia (PJRT) is a specific type 1

• Atrial Flutter

  • Macroreentrant circuit in the atria
  • Typical (cavotricuspid isthmus-dependent) or atypical forms 1

2. Abnormal Automaticity

• Focal Atrial Tachycardia (AT)

  • Arises from a localized atrial site with enhanced automaticity 1
  • Characterized by regular, organized atrial activity with discrete P waves

• Sinus Node Reentry Tachycardia

  • Microreentry arising from the sinus node complex 1
  • P-wave morphology indistinguishable from sinus rhythm

• Multifocal Atrial Tachycardia (MAT)

  • Irregular SVT with ≥3 distinct P-wave morphologies 1
  • Most commonly seen in patients with pulmonary disease 1

Predisposing Factors and Triggers

Structural Heart Disease

• Valvular heart disease (especially mitral valve prolapse)
• Hypertrophic cardiomyopathy
• Congenital heart disease 1
• Untreated SVT can lead to tachycardia-mediated cardiomyopathy 1, 3

Physiological Triggers

• Physical exertion
• Emotional stress
• Caffeine consumption
• Alcohol intake
• Nicotine use 4
• Infection or volume loss (for sinus tachycardia) 1

Medical Conditions

• Hyperthyroidism
• Pulmonary disease (especially for MAT) 1
• Electrolyte abnormalities
• Anemia 4
• Medications (stimulants, sympathomimetics)

Demographic Factors

• Age: Risk increases with age (5-fold greater risk in those ≥65 years) 1
• Gender: Women have twice the risk of developing PSVT compared to men 1, 3

Clinical Presentation and Diagnosis

SVT typically presents with:

• Palpitations (86% of cases) 3
• Chest discomfort/pressure (47%) 3
• Dyspnea (38%) 3
• Lightheadedness
• Fatigue
• Presyncope or syncope (in approximately 15% of patients) 1
• Polyuria (due to release of atrial natriuretic peptide) 1

Diagnosis is confirmed by:

• 12-lead ECG during tachycardia
• Holter monitoring or event recorders for intermittent symptoms 4
• Electrophysiological studies for definitive diagnosis 4

Complications

If left untreated, SVT can lead to:

• Tachycardia-mediated cardiomyopathy (1% of cases) 3
• Heart failure
• Syncope
• Increased risk of stroke in adults with congenital heart disease 1

Important Distinctions

It's crucial to distinguish SVT from:

• Sinus tachycardia (which is nonparoxysmal, accelerates and terminates gradually) 1
• Ventricular tachycardia (which can be life-threatening)
• Atrial fibrillation (which is not typically included in the SVT classification) 1

Understanding the specific mechanism of SVT is essential for appropriate management, as treatment approaches may differ based on the underlying cause.