Supraventricular Tachycardia (SVT)

Background

  • The term supraventricular tachycardia (SVT), whilst often used synonymously with AV nodal re-entry tachycardia (AVNRT), can be used to refer to any tachydysrhythmia arising from above the level of the Bundle of His
  • Different types of SVT arise from or are propagated by the atria or AV node, typically producing a narrow-complex tachycardia (unless aberrant conduction is present).
  • Paroxysmal SVT (pSVT) describes an SVT with abrupt onset and offset — characteristically seen with re-entrant tachycardias involving the AV node such as AVNRT or atrioventricular re-entry tachycardia (AVRT).

Supraventricular tachycardia

Classification

  • SVTs can be classified based on site of origin (atria or AV node) or regularity (regular or irregular).
  • Classification based on QRS width is unhelpful as this is also influenced by the presence of pre-existing bundle branch block, rate-related aberrant conduction or presence of accessory pathways.

Classification of SVT by site of origin and regularity

    Regular   Irregular
  Atrial
  Atrioventricular  

AV Nodal Re-entry Tachycardia (AVNRT)

  • This is the commonest cause of palpitations in patients with structurally normal hearts.
  • AVNRT is typically paroxysmal and may occur spontaneously or upon provocation with exertion, caffeine, alcohol, beta-agonists (salbutamol) or sympathomimetics (amphetamines).
  • It is more common in women than men (~ 75% of cases occurring in women) and may occur in young and healthy patients as well as those suffering chronic heart disease.
  • Patients will typically complain of the sudden onset of rapid, regular palpitations.  The patient may experience a brief fall in blood pressure causing presyncope or occasionally syncope.
  • If the patient has underlying coronary artery disease the patient may experience chest pain similar to angina (tight band around the chest radiating to left arm or left jaw).
  • The patient may complain of shortness of breath, anxiety and occasionally polyuria due to elevated atrial pressure releasing atrial natriuretic peptide.
  • The tachycardia typically ranges between 140-280 bpm and is regular in nature.  It may cease spontaneously (and abruptly) or continue indefinitely until medical treatment is sought.
  • The condition is generally well tolerated and is rarely life threatening in patients with pre-existing heart disease.

Pathophysiology

  • In comparison to AVRT, which involves an anatomical re-entry circuit (Bundle of Kent), in AVNRT there is a functional re-entry circuit within the AV node.

Different types of re-entry loops: Functional circuit in AVNRT (left), anatomical circuit in AVRT (right)

Functional pathways within the AV node

In AVNRT, there are two pathways within the AV node:

  • The slow pathway (alpha):  a slowly-conducting pathway with a short refractory period.
  • The fast pathway (beta):  a rapidly-conducting pathway with a long refractory period.

Mechanism of re-entry in "slow-fast" AVNRT (ERP = effective refractory period)

Initiation of re-entry

  • During sinus rhythm, electrical impulses travel down both pathways simultaneously. The impulse transmitted down the fast pathway enters the distal end of the slow pathway and the two impulses cancel each other out.
  • However, if a premature atrial contraction (PAC) arrives while the fast pathway is still refractory, the electrical impulse will be directed solely down the slow pathway (1).
  • By the time the premature impulse reaches the end of the slow pathway, the fast pathway is no longer refractory (2) — hence the impulse is permitted to recycle retrogradely up the fast pathway.
  • This creates a circus movement whereby the impulse continually cycles around the two pathways, activating the Bundle of His anterogradely and the atria retrogradely (3). The short cycle length is responsible for the rapid heart rate.
  • This is the most common type of re-entrant circuit and is termed Slow-Fast AVNRT.
  • Similar mechanisms exist for the other types of AVNRT.

Electrocardiographic Features

General Features of AVNRT

  • Regular tachycardia  ~140-280 bpm.
  • QRS complexes usually  narrow (< 120 ms) unless pre-existing bundle branch block, accessory pathway, or rate related aberrant conduction.
  • ST-segment depression may be seen with or without underlying coronary artery disease.
  • QRS alternans – phasic variation in QRS amplitude associated with AVNRT and AVRT, distinguished from electrical alternans by a normal QRS amplitude.
  • P waves if visible exhibit retrograde conduction with P-wave inversion in leads II, III, aVF.
  • P waves may be buried in the QRS complex, visible after the QRS complex, or very rarely visible before the QRS complex.

Subtypes of AVNRT

Different subtypes vary in terms of the dominant pathway and the R-P interval. The RP interval represents the time between anterograde ventricular activation (R wave) and retrograde atrial activation (P wave).

1. Slow-Fast AVNRT (common type)

  • Accounts for 80-90% of AVNRT
  • Associated with Slow AV nodal pathway for anterograde conduction and Fast AV nodal pathway for retrograde conduction.
  • The retrograde P wave is obscured in the corresponding QRS or occurs at the end of the QRS complex as pseudo r’ or S waves

ECG features:

  • P waves are often hidden – being embedded in the QRS complexes.
  • Pseudo R’ wave may be seen in V1 or V2.
  • Pseudo S waves may be seen in leads II, III or aVF.
  • In most cases this results in a ‘typical’ SVT appearance with absent P waves and tachycardia

Cardiac rhythm strips demonstrating (top) sinus rhythm and (bottom) paroxysmal SVT. The P wave is seen as a pseudo-R wave (circled in bottom strip) in lead V1 during tachycardia. By contrast, the pseudo-R wave is not seen during sinus rhythm (it is absent from circled area in top strip). This very short ventriculo-atrial time is frequently seen in typical Slow-Fast AVNRT.

2. Fast-Slow AVNRT (Uncommon AVNRT)

  • Accounts for 10% of AVNRT
  • Associated with Fast AV nodal pathway for anterograde conduction and Slow AV nodal pathway for retrograde conduction.
  • Due to the relatively long ventriculo-atrial interval, the retrograde P wave is more likely to be visible after the corresponding QRS.

ECG features:

  • QRS-P-T complexes.
  • Retrograde P waves are visible between the QRS and T wave.

 

3. Slow-Slow AVNRT (Atypical AVNRT)

  • 1-5% AVNRT
  • Associated with Slow AV nodal pathway for anterograde conduction and Slow left atrial fibres as the pathway for retrograde conduction.

ECG features:

  • Tachycardia with a P-wave seen in mid-diastole… effectively appearing “before” the QRS complex.
  • Confusing as a P wave appearing before the QRS complex in the face of a tachycardia might be read as a sinus tachycardia.

 

Summary of AVNRT subtypes

  • No visible P waves? –> Slow-Fast
  • P waves visible after the QRS complexes? –> Fast-Slow
  • P waves visible before the QRS complexes? –> Slow-Slow

 

Management of AVNRT

  • May respond to vagal maneuvers with reversion to sinus rhythm.
  • The mainstay of treatment is adenosine.
  • Other agents which may be used include calcium-channel blockers, beta-blockers and amiodarone.
  • DC cardioversion is rarely required.
  • Catheter ablation may be considered in recurrent episodes not amenable to medical treatment.

 

Other types of SVT

Most of the other types of SVT are discussed elsewhere (follow links in table above). Two less-common types are discussed below.

Inappropriate Sinus Tachycardia

  • Typically seen in young healthy female adults.
  • Sinus rate persistently elevated above 100 bpm in absence of physiological stressor.
  • Exaggerated rate response to minimal exercise.
  • ECG indistinguishable from sinus tachycardia.

Sinus Node Reentrant Tachycardia (SNRT)

  • Caused by reentry circuit close to or within the sinus node.
  • Abrupt onset and termination.
  • P wave morphology is normal.
  • Rate usually 100 – 150 bpm.
  • May terminate with vagal manoeuvres.

 

ECG Examples

Example 1a

Slow-Fast (Typical) AVNRT:

  • Narrow complex tachycardia at ~ 150 bpm.
  • No visible P waves.
  • There are pseudo R’ waves in V1-2.

Pseudo R' waves in V1-2

Example 1b

The same patient following resolution of the AVNRT:

  • Sinus rhythm.
  • The pseudo R’ waves have now disappeared.

Pseudo R' waves in V1-2 have resolved

Example 2a

Slow-Fast AVNRT:

  • Narrow complex tachycardia ~ 220 bpm.
  • No visible P waves.
  • Subtle notching of the terminal QRS in V1 (= pseudo R’ wave).
  • Widespread ST depression — this is a common electrocardiographic finding in AVNRT and does not necessarily indicate myocardial ischaemia, provided the changes resolve once the patient is in sinus rhythm.

 

Example 2b

The same patient following resolution of the AVNRT:

  • Sinus rhythm.
  • Pseudo R’ waves have disappeared.
  • There is residual ST depression in the inferior and lateral leads (most evident in V4-6), indicating that the patient did indeed have rate-related myocardial ischaemia (± NSTEMI).

 

Example 3

Patient with Slow-Fast AVNRT undergoing treatment with adenosine:

  • The top section of the rhythm strip shows AVNRT with absent P waves and pseudo R’ waves clearly visible.
  • The middle portion of the strip shows adenosine acting on the AV node to suppress AV conduction — there are several broad complex beats which may be aberrantly-conducted supraventricular impulses or ventricular escape beats (this is extremely common during administration of adenosine for AVNRT).
  • The bottom section shows reversion to sinus rhythm; the pseudo R’ waves have resolved.

 

Example 4a

Fast-Slow (Uncommon) AVNRT:

  • Narrow complex tachycardia ~ 120 bpm.
  • Retrograde P waves are visible after each QRS complex — most evident in V2-3.

Retrograde P waves

Example 4b

The same patient following resolution of the AVNRT:

  • Now in sinus rhythm.
  • The retrograde P waves have disappeared.

Retrograde P waves resolved

Example 5a

Fast-Slow AVNRT:

  • Narrow complex tachycardia ~ 135 bpm.
  • Retrograde P waves following each QRS complex — upright in aVR and V1; inverted in II, III and aVL.

Upright retrograde P waves in aVR

Inverted retrograde P waves lead II

Example 5b

The same patient following resolution of the AVNRT:

  • Sinus rhythm.
  • The retrograde P waves have disappeared.

Retrograde P waves in aVR resolved

Retrograde P waves in lead II resolved

Example 6a

Fast-Slow AVNRT:

  • Narrow complex tachycardia at ~ 125 bpm.
  • Retrograde P waves follow each QRS complex: upright in V1-3; inverted in II, III and aVF.

Upright retrograde P waves in V2

Inverted retrograde P waves in lead II

Example 6b

The same patient following resolution of the AVNRT:

  • Sinus rhythm.
  • Retrograde P waves have disappeared.

Retrograde P waves in V2 have resolved

Retrograde P waves in lead II have resolved

Example 7

SVT with QRS Alternans:

  • Narrow complex tachycardia ~ 215 bpm.
  • Retrograde P waves are visible preceding each QRS complex (upright in V1, inverted in lead II).
  • There is a beat-to-beat variation in the QRS amplitude without evidence of low voltage (= QRS alternans).
  • The PR interval is ~ 120 ms, so this could be either a low atrial tachycardia or possibly an AVNRT with a long RP interval (i.e. either Fast-Slow or Slow-Slow varieties).

Related Topics

Further Reading

Author Credits

References

  • Surawicz B, Knilans TK. Chou’s Electrocardiography in Clinical Practice. 6th Edition. Saunders Elsevier 2008.
  • Chan TC, Brady WJ, Harrigan RA, Ornato JP, Rosen P. ECG in Emergency Medicine and Acute Care. Elsevier Mosby 2005.
  • Brady WJ, Truwit JD. Critical Decisions in Emergency & Acute Care Electrocardiography. Wiley Blackwell 2009.
  • Jazayeri MR, Massumi A, Mihalick MJ, Hall RJ.Sinus node reentry: case report and review of electrocardiographic and electrophysiologic features. Tex Heart Inst J. 1985 Sep;12(3):249-52. PMID: 15227012  Full Text
  • Fox DJ, Tischenko A, Krahn AD, Skanes AC, Gula LJ, Yee RK, Klein GJ. Supraventricular tachycardia: diagnosis and management. Mayo Clin Proc. 2008 Dec;83(12):1400-11. PMID: 19046562 Full Text.
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