What is the difference between monophasic and biphasic waveforms in cardioversion?

Differences Between Monophasic and Biphasic Waveforms in Cardioversion

Biphasic waveforms are superior to monophasic waveforms for cardioversion, with higher first-shock success rates (99.1% vs 92.4%), requiring lower energy levels, and resulting in less dermal injury. 1, 2

Waveform Characteristics

Monophasic Waveforms

• Current flows in only one direction (single polarity)
• Types include:
  • Monophasic damped sinusoidal (MDS): current returns to zero gradually
  • Monophasic truncated exponential (MTE): current returns abruptly to zero 1
• Require higher energy levels (typically 200-360J) for effective cardioversion 1
• Associated with greater skin burns and dermal injury (41% vs 17% with biphasic) 2
• Less commonly manufactured today but many still in use 1

Biphasic Waveforms

• Current flows in two directions (bidirectional current flow)
• Types include:
  • Biphasic truncated exponential (BTE)
  • Rectilinear biphasic
  • Pulsed biphasic 1
• Require lower energy levels (typically 100-200J) for effective cardioversion 1
• Higher first-shock success rates compared to monophasic waveforms 1
• Require fewer shocks and lower total energy delivered 2
• Standard in most modern defibrillators and AEDs 1

Clinical Efficacy Comparison

Success Rates

• Biphasic waveforms have higher first-shock success rates:
  • For atrial fibrillation: 99.1% biphasic vs 92.4% monophasic 1
  • At 100J: 60% biphasic vs 22% monophasic 2
  • At 200J: 90% biphasic vs 53% monophasic 2
• In shock-resistant atrial fibrillation, biphasic waveforms achieve 61% success vs 18% with monophasic waveforms 3
• For atrial flutter, first-shock efficacy with 50J biphasic (48%) is significantly better than 100J monophasic (28%) 4

Energy Requirements

• Biphasic waveforms require approximately half the energy of monophasic waveforms 2, 5
• Mean energy delivered: 217J biphasic vs 548J monophasic 2
• For atrial flutter: 76J mean energy with biphasic vs 177J with monophasic 4

Practical Implications

Initial Energy Selection

• For monophasic waveforms:
  • Start with 200J (not 100J, which has only 14% success rate)
  • Escalate in 100J increments up to 400J 1, 6
• For biphasic waveforms:
  • Start with 100-150J depending on AF duration
  • Can escalate to 200J if needed 6

Safety Considerations

• Biphasic waveforms cause less myocardial damage and dermal injury 2
• Both waveforms can cause transient ST-segment elevation and creatine kinase elevation, but this is more pronounced with monophasic waveforms 1, 5
• Maintain at least 1-minute intervals between shocks to prevent myocardial damage 6

Special Considerations

Patients with Implanted Devices

• Position paddles as far as possible from implanted devices
• Anterior-posterior configuration is preferred with implanted devices
• Device should be interrogated before and after cardioversion 1, 6

Electrode Placement

• Anterior-posterior paddle configuration is generally more effective than anterior-lateral 1
• Proper synchronization with QRS complex is critical for both waveforms 6

Common Pitfalls to Avoid

1. Starting with too low energy for monophasic waveforms (100J has only 14% success)
2. Not allowing sufficient time between shocks (minimum 1 minute)
3. Improper paddle placement or poor contact (increases impedance)
4. Not synchronizing properly with QRS complex
5. Not considering thoracic impedance (affects energy delivery)

In conclusion, while both waveform types can be effective for cardioversion, biphasic waveforms represent the current standard of care due to their superior efficacy, lower energy requirements, and improved safety profile.