What is the difference between monophasic and biphasic defibrillators?

Difference Between Monophasic and Biphasic Defibrillators

Biphasic defibrillators are more effective in terminating ventricular fibrillation compared to monophasic defibrillators, achieving higher first-shock success rates with lower energy requirements. 1

Key Differences

Waveform Characteristics

• Monophasic waveforms: Current flows in only one direction between defibrillator pads, delivering a single current pulse in one direction through the heart 1
• Biphasic waveforms: Current flows in one direction for a specified duration and then reverses direction for the remaining milliseconds of the electrical discharge 1

Efficacy

• Biphasic waveforms demonstrate higher first-shock success rates (85-98%) for terminating ventricular fibrillation compared to monophasic waveforms 1
• Multiple randomized trials show biphasic waveforms have superior shock-success rates (defined as termination of VF 5 seconds after shock) 1
• One cohort study demonstrated better hospital discharge and neurological survival outcomes with biphasic waveforms compared to monophasic waveforms 1

Energy Requirements

• Biphasic defibrillators can effectively terminate ventricular fibrillation at lower energy levels (typically 200 J or less) compared to monophasic defibrillators 1
• Lower energy requirements with biphasic waveforms may result in less post-shock myocardial dysfunction 1, 2

Types of Biphasic Waveforms

• Three main types of biphasic waveforms are used in commercial defibrillators 1:
  1. Biphasic Truncated Exponential (BTE)
  2. Rectilinear Biphasic (RLB)
  3. Pulsed Biphasic

Clinical Recommendations

• Defibrillators using biphasic waveforms (BTE or RLB) are preferred over monophasic defibrillators for treatment of both atrial and ventricular arrhythmias (Class IIa, LOE B-R) 1
• In the absence of biphasic defibrillators, monophasic defibrillators remain acceptable for clinical use 1
• No specific biphasic waveform has been proven superior to others for termination of ventricular fibrillation 1

Practical Implications

Energy Selection

• For biphasic defibrillators: It is reasonable to use the manufacturer's recommended energy dose for the first shock 1
• If manufacturer recommendations are unknown, defibrillation at the maximal dose may be considered (Class IIb, LOE C-LD) 1
• For subsequent shocks: Selection of fixed versus escalating energy should be based on specific manufacturer's instructions 1

Impedance Compensation

• Biphasic defibrillators often adjust energy output based on patient impedance, which may improve efficacy 1
• RLB waveform defibrillators typically deliver more energy than selected based on patient impedance (e.g., a selected dose of 120 J typically provides nearly 150 J for most patients) 1

Future Developments

• Multiphasic waveforms (beyond biphasic) have shown promise in animal studies but are not yet commercially available for human use 1
• Animal data suggests multiphasic waveforms may defibrillate at lower energies and induce less post-shock myocardial dysfunction, but human studies are lacking 1

Clinical Bottom Line

Biphasic defibrillators represent the current standard of care due to their superior efficacy in terminating life-threatening arrhythmias at lower energy levels compared to monophasic defibrillators, potentially resulting in less myocardial damage and improved survival outcomes 1.