EM Notes – Supraventricular Tachycardia

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Terminology

  1. SVT includes all tachycardias involving the atria (due to either re-entrancy or automaticity). Atrial flutter, atrial tachycardia, MAT, AF, AV node re-entrant tachycardia & AV re-entrant tachycardia. Atrial flutter, atrial tachycardia & MAT described in Arrhythmias & _AF in Atrial Fibrillation.

AV node re-entrant tachycardia (AVNRT)

  1. 60% of SVTs
  2. Micro re-entrant circuit in AV node.

Management

  1. ABCs, O2,
  2. DC cardiovert if unstable, or stable and failed pharmacological options. Otherwise:
  3. Vagal manoeuvres
  4. Pharmacological options:
    1. Adenosine: 6mg (can repeat at 12mg) rapid bolus IV. SE: bronchospasm (so avoid in asthma). Theophylline blocks it. CI: post cardiac transplant (→cardiac arrest).
    2. Verapamil: 1mg IV q5 min (don’t use β-blocker concurrently) to 15mg
    3. β-blockers, amiodarone and flecainide may also be used
  5. Prophylaxis: β-blockers, sotalol, verapamil, or digoxin

AV re-entrant tachycardia (AVRT)

  1. 30% of SVTs
  2. Pre-excitation syndrome e.g. WPW syndrome, LGL syndrome & Mahaim fibre pathway.
  3. Conducting accessory pathways bypassing Atria-AV node-His bundle-Perkinje fibres.

Wolff-Parkinson-White Syndrome

  1. Bundle of Kent – accessory pathway between an atrium & ventricle, by-passing AV node
  2. ~1:1000 pop (50% symptomatic), M>F. Assoc with Ebstein’s/Tricuspid anomalies, HOCM.
  3. Conduction down accessory pathway may be anterograde, retrograde or both.
  4. ECG:
    1. Normal i.e. concealed WPW, if only retrograde conduction possible via accessory pathway [or if conduction is currently faster through AV node (low vagal tone)]
    2. Else: δ wave (faster antegrade accessory conduction but slower depolarisation of ventricular myocardium compared to AV node-His → early but less steep init part of QRS), PR<120ms. Slightly QRS (>110ms), discordant ST & T changes. May show pseudo-Q or -infarction patterns (neg δ wave in inf/ant leads, prom R wave in V1-3)
  5. Types: Type A or C – pos δ wave & R>S in V1. Type B has neg δ wave or R<S in V1-2.
  6. Tachyarrhythmias (HR 200-300) may be generated by:
    1. Re-entry circuit involving accessory pathway (AVRT) [70-80%]
    2. AF or atrial flutter direct conduction to ventricles via accessory pathway, bypassing AV node (WPW+AF) [~20% AF, ~7% flutter]
  7. When in AVRT, conduction may be:
    1. Orthodromic (~90%, anterograde through AVN) is narrow like AVNRT & no δ waves
    2. Antidromic (10%, resting ECG likely to show δ waves) usually broad. Could → VF
  8. When in WPW+AF or WPW + atrial flutter:
    1. Irregular (AF) or regular (flutter) antidromic broad complex tachycardia. Risk+ of → VT, VF. QRS complexes change in morphology. Rate may be close to 300bpm.

Lown-Ganong-Levine

  1. James Pathway connects atria to AV node, His or fascicles.
  2. Short PR (<120ms), but no δ wave and QRS normal duration.
  3. Some dispute over existence.

Mahaim fibre pathway

  1. Right sided pathway connects AV node to ventricles, fascicles to ventricles or atria to fascicles.
  2. Pathway only allows antegrade conduction.
  3. ECG may appear normal when not in AVRT as AV node conduction preferred unless high vagal tone.
  4. δ waves are not generally seen.
  5. AVRT is always antidromic and usually has LBBB morphology.

Management of AVRT

  1. ABCs, O2
  2. Narrow complex tachycardia (Orthodromic AVRT)
    1. As for AVNRT
  3. Wide complex or irregular tachycardia (Antidromic AVRT, WPW+AF, WPW+flutter)
    1. DC cardioversion, starting @ 100J if unstable
    2. If stable, still consider DC cardioversion as may be safest option OR
    3. Pharmacological options:
      1. Procainamide 50mg/min IV until reversion or ↓BP, ↑QRS>50%, or 17mg/kg max reached. Would be first line but not available in Australia
      2. Flecainide 2mg/kg IV over 30min. But CI if heart not structurally OK.
      3. Amiodarone 150mg IV over 5-10mins (can rpt over 20min) then 900mg/24h – **some concern that amiodarone could also accelerate HR or →VF
      4. **Ibutilide 1mg (0.01mg/kg if <60kg) IV, may rpt.
      5. Avoid adenosine/CCB/BB/digoxin **as may → VT or VF
  4. Prophylaxis: sotalol, flecainide, amiodarone
  5. Electrophysiological studies and radiofrequency ablation