How to manage Supraventricular Tachycardia (SVT) with second-degree type 1 Atrioventricular (AV) block?

Management of SVT with Second-Degree Type 1 AV Block

For SVT with second-degree type 1 AV block, synchronized cardioversion is recommended as first-line treatment due to the risk of worsening AV block with pharmacological agents. 1, 2

Initial Assessment and Stabilization

• Assess hemodynamic stability immediately:
  • If unstable (hypotension, altered mental status, chest pain, heart failure): Proceed directly to synchronized cardioversion
  • If stable: Consider treatment options based on specific SVT mechanism and presence of AV block

Treatment Algorithm

For Hemodynamically Unstable Patients:

1. Synchronized cardioversion (Class I, Level B-NR recommendation) 1, 2
   • Begin with 50-100 J (biphasic)
   • Ensure proper sedation if patient is conscious
   • Have emergency equipment readily available

For Hemodynamically Stable Patients:

1. Vagal maneuvers (Class I, Level B-R recommendation) 1, 2

   • Modified Valsalva maneuver (strain for 10-30 seconds equivalent to 30-40 mmHg)
   • Carotid sinus massage (after confirming absence of carotid bruits)
   • Cold, wet towel to face
   • Caution: Monitor for worsening AV block during vagal maneuvers

2. Synchronized cardioversion if vagal maneuvers fail (Class I, Level B-NR) 1

   • Preferred over pharmacological options due to pre-existing AV block

3. Pharmacological options (with extreme caution):

   • Avoid adenosine in second-degree AV block as it can worsen the block and potentially cause asystole
   • Avoid calcium channel blockers (verapamil, diltiazem) as they can worsen AV conduction 3, 4
   • Avoid beta-blockers as they can further depress AV nodal conduction

Special Considerations

Pre-existing AV Block:

• The presence of second-degree type 1 AV block significantly impacts management decisions
• Medications that slow AV conduction (adenosine, calcium channel blockers, beta-blockers) can worsen the block and potentially cause complete heart block 1, 3
• Continuous ECG monitoring is essential during any intervention

Mechanism-Specific Concerns:

• If SVT involves an accessory pathway (WPW syndrome):
  • Avoid AV nodal blocking agents (can enhance conduction through accessory pathway)
  • Procainamide may be considered if cardioversion is not immediately available 1

Post-Conversion Management:

• After successful conversion:
  • Monitor for recurrence and worsening AV block
  • Obtain 12-lead ECG to document rhythm
  • Consider referral to electrophysiology for definitive management

Long-term Management

• Catheter ablation (Class I, Level B-R recommendation) is recommended for recurrent, symptomatic SVT 2
• For patients not suitable for ablation:
  • Consider permanent pacemaker placement if significant AV block persists
  • Avoid long-term use of medications that worsen AV conduction

Pitfalls and Caveats

• Do not apply pressure to the eyeball as a vagal maneuver - this practice is dangerous and has been abandoned 1, 2
• Do not administer verapamil or diltiazem in patients with pre-excited AF as this can lead to ventricular fibrillation 3, 4
• Do not delay cardioversion in hemodynamically unstable patients 2
• Always have defibrillation equipment immediately available when treating SVT, especially with pre-existing conduction abnormalities 4
• Monitor for tachycardia-mediated cardiomyopathy if SVT persists for weeks to months with fast ventricular response 2

The management of SVT with second-degree type 1 AV block requires careful consideration of the risks associated with worsening AV block. Synchronized cardioversion represents the safest and most effective approach in this specific scenario.