Key Facts

Supraventricular tachycardia (SVT) is a broad term for tachyarrhythmias involving tissue at or above the bundle of His. In clinical practice, it most commonly refers to paroxysmal SVT (PSVT), particularly AVNRT and AVRT.

Epidemiology

• Prevalence: ~2.3 per 1,000 population
• More common in women (2:1 for AVNRT)
• AVNRT peak onset: 20-40 years
• AVRT (WPW): often presents in younger patients
• Rarely life-threatening unless associated with WPW and pre-excited AF

Aetiology

• AVNRT: dual AV nodal pathways (slow and fast) forming a re-entry circuit within/around the AV node
• AVRT: accessory pathway between atria and ventricles (overt = WPW with delta wave; concealed = no delta wave at rest)
• Atrial tachycardia: ectopic focus or micro-re-entry in atrial tissue
• Triggers: caffeine, alcohol, stress, exercise, sympathomimetics

Pathophysiology

• AVNRT: re-entry circuit using slow (anterograde) and fast (retrograde) AV nodal pathways — typical form; atypical = reverse direction
• AVRT: re-entry circuit using AV node (anterograde) and accessory pathway (retrograde) = orthodromic; antidromic = reverse (wide complex)
• Sudden onset and termination is characteristic
• Regular narrow-complex tachycardia (unless aberrant conduction or antidromic AVRT)

Typical Presentation

• Sudden-onset palpitations ('heart racing')
• Regular, rapid heartbeat (140-250 bpm)
• Typically abrupt onset and offset
• Associated: dizziness, dyspnoea, chest discomfort, anxiety
• Polyuria after episode (due to ANP release)
• Episodes may last seconds to hours

Signs

• Regular tachycardia with narrow QRS
• Hypotension if prolonged or haemodynamically compromising
• Visible jugular venous 'cannon A waves' (AVNRT)

Red Flags

• Haemodynamic instability (syncope, hypotension, chest pain)
• Wide-complex tachycardia (consider antidromic AVRT or VT until proven otherwise)
• Pre-excited AF in WPW (irregular, wide-complex, very rapid)

Bedside

• 12-lead ECG (during tachycardia): regular narrow-complex tachycardia; look for retrograde P waves
• ECG (post-reversion): look for delta wave (WPW), short PR interval
• Vagal manoeuvres: diagnostic and therapeutic

Bloods

• TFTs: thyrotoxicosis
• U&Es: electrolyte abnormalities
• FBC: anaemia

Imaging

• Echocardiography: structural heart assessment if recurrent episodes

Special Tests

• 24h/7-day Holter/event recorder: capture paroxysmal episodes
• Electrophysiology study: definitive diagnosis and ablation

Non-pharmacological

Acute termination:

• Modified Valsalva manoeuvre (REVERT trial): blow into 10mL syringe for 15 seconds in semi-recumbent position, then lie flat with legs elevated 45° for 15 seconds
• Carotid sinus massage: with continuous ECG monitoring (avoid if carotid bruit/stenosis)
• Facial ice immersion: (diving reflex)

Pharmacological

Acute management (haemodynamically stable):

• Adenosine IV: 6mg rapid push → 12mg → 12mg (flush with 20mL saline, via large-bore cannula, antecubital fossa)
  • Warn patient: transient chest tightness, flushing, sense of impending doom
  • Contraindicated in: asthma, 2nd/3rd degree heart block, severe hypotension
  • Caution with dipyridamole (potentiates effect) and carbamazepine
• If adenosine contraindicated: verapamil 2.5-5mg IV over 2 minutes (avoid in HFrEF or concurrent beta-blocker)

Haemodynamically unstable:

• Synchronised DC cardioversion: 70-120J biphasic

Prophylaxis (recurrent SVT):

• Pill-in-pocket: flecainide 100-300mg PO (if no structural heart disease) + beta-blocker
• Regular prophylaxis: beta-blocker (bisoprolol 2.5-10mg), verapamil (80-120mg TDS), or flecainide (50-150mg BD)
• Definitive: catheter ablation

Surgical/Interventional

• Catheter ablation: first-line definitive treatment
  • AVNRT ablation (slow pathway modification): success >95%, <1% risk of complete heart block
  • AVRT ablation (accessory pathway): success >95%
  • Recommended for recurrent symptomatic SVT or patient preference
  • Particularly recommended in WPW with pre-excited AF or high-risk pathway

Referral Criteria

• Recurrent symptomatic SVT: cardiology/electrophysiology referral for ablation
• WPW pattern on ECG: risk stratification and ablation consideration
• Pre-excited AF or syncope in WPW: urgent referral

• SVT is generally benign with excellent prognosis
• Catheter ablation: cure rate >95% with low complication rate
• Without ablation: recurrent episodes likely but rarely life-threatening
• WPW with pre-excited AF: rare but potentially fatal (sudden cardiac death risk ~0.15% per year)
• Quality of life significantly improved with ablation
• Post-ablation recurrence: ~2-5%

SVT Differentiating Features

REVERT Modified Valsalva Technique