Supraventricular Tachycardia (SVT)

Definition

The term supraventricular tachycardia (SVT) refers to any tachydysrhythmia arising from above the level of the Bundle of His, and encompasses regular atrial, irregular atrial, and regular atrioventricular tachycardias

  • It is often used synonymously with AV nodal re-entry tachycardia (AVNRT), a form of SVT
  • In the absence of aberrant conduction (e.g. bundle branch block), the ECG will demonstrate a narrow-complex tachycardia
  • 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 SVT Rhythm Strip
Supraventricular tachycardia (SVT): Rhythm strip demonstrating a regular, narrow-complex 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 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. Other associated symptoms may include:
    • Presyncope or syncope due to a transient fall in blood pressure
    • Chest pain, especially in the context of underlying coronary artery disease
    • Dyspnoea
    • Anxiety
    • Rarely, polyuria due to elevated atrial pressures causing release of atrial natriuretic peptide
  • The tachycardia typically ranges between 140-280 bpm and is regular in nature. It may self-resolve 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.

re-entry-tachycardias

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


Functional pathways within the AV node

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.
AV_reentry_circuit
Mechanism of re-entry in “slow-fast” AVNRT:
1) A premature atrial contraction (PAC) arrives while the fast pathway is still refractory, and is directed down the slow pathway
2) The ERP in the fast pathway ends, and the PAC impulse travels retrogradely up the fast pathway
3) The impulse continually cycles around the two pathways
Initiation of re-entry
  • During normal 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, and the impulse is permitted to recycle retrogradely up the fast pathway (2)
  • 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 most common type of re-entrant circuit is termed Slow-Fast AVNRT.
  • Similar mechanisms exist for the other types of AVNRT
Electrocardiographic Features

ECG features of AVNRT

  • Regular tachycardia ~140-280 bpm
  • Narrow QRS complexes (< 120ms) unless there is co-existing bundle branch block, accessory pathway, or rate-related aberrant conduction
  • P waves if visible exhibit retrograde conduction with P-wave inversion in leads II, III, aVF. They may be buried within, visible after, or very rarely visible before the QRS complex

Associated features include:

  • ST-segment depression, which 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.

Subtypes of AVNRT

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

  1. Slow-Fast AVNRT (80-90%)
  2. Fast-Slow AVNRT (10%)
  3. Slow-Slow AVNRT (1-5%)
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
AVNRT-Slow-Fast
Top strip: Normal sinus rhythm. Absence of pseudo-R waves
Bottom strip: Paroxysmal SVT. The P wave is seen as a pseudo-R Wave (circled) in lead V1 during tachycardia. 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.
  • May be misinterpreted as 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 other types of SVT are discussed elsewhere (follow links in classification table). Two less common types include:

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

Slow-Fast (Typical) AVNRT:

  • Narrow complex tachycardia at ~ 150 bpm
  • No visible P waves
  • There are pseudo R’ waves in V1-2
Slow-Fast (Typical) AVNRT Pseudo R’ waves in V1-2
Pseudo R’ waves in V1-2

Example 1b
Typical-AVNRT-resolved 2

The same patient following resolution of the AVNRT:

  • Sinus rhythm
  • The pseudo R’ waves have now disappeared
Typical-AVNRT-Pseudo R' waves in V1-2 have resolved
Pseudo R’ waves in V1-2 have resolved

Example 2a
Slow-Fast AVNRT

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
Slow-Fast AVNRT resolved 2

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
AVNRT-resolves-with-adenosine 3

Patient with Slow-Fast AVNRT undergoing treatment with adenosine:

  • The top rhythm strip shows AVNRT with absent P waves and pseudo R’ waves clearly visible
  • The middle 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-AVNRT ECG 4 2

Fast-Slow (Uncommon) AVNRT:

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

Example 4b
Fast-Slow-AVNRT resolved 4b 2

The same patient following resolution of the AVNRT:

  • Now in sinus rhythm.
  • The retrograde P waves have disappeared.
Fast-Slow-AVNRT Retrograde P waves resolved 4b 2
Retrograde P waves

Example 5a
Fast-Slow AVNRT ECG 5

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
Upright retrograde P waves in aVR
Inverted retrograde P waves lead II
Inverted retrograde P waves lead II

Example 5b
Fast-Slow-AVNRT resolved 3

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 aVR resolved
Retrograde P waves in lead II resolved
Retrograde P waves in lead II resolved

Example 6a
Fast-Slow-AVNRT

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
Inverted retrograde P waves in lead II
Inverted retrograde P waves in lead II
Upright retrograde P waves in V2
Upright retrograde P waves in V2

Example 6b
ECG Fast-Slow-3-resolved 2

The same patient following resolution of the AVNRT:

  • Sinus rhythm.
  • Retrograde P waves have disappeared.
Retrograde P waves in lead II have resolved 2
Retrograde P waves in lead II have resolved
Retrograde P waves in V2 have resolved 2
Retrograde P waves in V2 have resolved

Example 7
Electrical-alternans 2

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)


References

Advanced Reading

Online

Textbooks


LITFL Further Reading

ECG LIBRARY 700

ECG LIBRARY

Electrocardiogram

Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

MBBS (UWA) CCPU Emergency Medicine Advanced Trainee based in Melbourne, Australia. Co-author of the LITFL ECG Library. Likes Ultrasound, Echo, ECGs, and anything and everything with caffeine. Part of the 2021 ANZCEN Clinician Educator Incubator Programme | @rob_buttner | ECG Library |

2 Comments

  1. Most useful information I have been looking for, cheers and thumbs up for dr Ed Burns.
    Dr Bukhari
    FACRRM, MBBS

  2. Dear Ed,

    Few conflicting information out there in the lit about treatment of SVT w WPW. I feel safest is to just shock because at that speed, u cant afford to make the error of diagnosising regular vs irregular rhythm. I.e AF w WPW.

    Your thoughts please?

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.