What is the difference between biphasic and monophasic shocks in cardiac arrest management?

Difference Between Biphasic and Monophasic Defibrillation Shocks

Biphasic defibrillators are preferred over monophasic defibrillators for treating ventricular fibrillation and ventricular tachycardia because they achieve significantly higher first-shock success rates at lower energy levels. 1

Technical Differences

Waveform Characteristics

• Monophasic shocks deliver electrical current in one direction only, with the current gradually (damped sinusoidal) or abruptly (truncated exponential) returning to zero 1
• Biphasic shocks deliver current that reverses polarity during the shock, flowing in one direction then reversing to flow in the opposite direction 1

Energy Requirements

• Biphasic waveforms effectively terminate ventricular fibrillation at lower energy levels (typically 115-200 J) compared to monophasic waveforms which typically require 200-360 J 1, 2
• Lower energy delivery with biphasic shocks results in less myocardial injury as measured by ST segment changes and cardiac biomarkers 2

Clinical Efficacy Differences

First-Shock Success Rates

• Biphasic waveforms demonstrate 85-98% first-shock success for terminating ventricular fibrillation 3
• Multiple randomized trials show biphasic shocks have significantly higher shock-success rates compared to monophasic defibrillation 1
• One high-quality randomized trial found 200 J biphasic shocks achieved 100% first-shock success versus 90% for 200 J monophasic shocks 4
• Another blinded randomized trial demonstrated 69% success for biphasic versus 45% for monophasic shocks at 200 J (odds ratio 4.01) 5

Survival Outcomes

• One cohort study demonstrated better hospital discharge and neurological survival with biphasic versus monophasic waveforms, though confounding factors were present 1
• However, no conclusive randomized trial evidence proves biphasic defibrillators save more lives than monophasic defibrillators in out-of-hospital cardiac arrest 1
• A 2013 meta-analysis of 4 randomized trials (572 patients) found no significant difference in survival to hospital discharge between waveforms (RR 1.14,95% CI 0.84-1.54) 6
• The largest randomized trial (TIMBER study, 168 patients) found no statistically significant differences in hospital admission (73% monophasic vs 76% biphasic) or survival (34% vs 41%, p=0.35) 7

Current Guideline Recommendations

American Heart Association 2015 Guidelines

• Defibrillators using biphasic waveforms (BTE or RLB) are preferred to monophasic defibrillators for treatment of both atrial and ventricular arrhythmias (Class IIa, LOE B-R) 1
• Both waveform types are recommended to treat arrhythmias, but biphasic is preferred based on greater success in arrhythmia termination 1
• In the absence of biphasic defibrillators, monophasic defibrillators are acceptable 1

Energy Dosing Recommendations

• For biphasic defibrillators, use the manufacturer's recommended energy dose for the first shock 1, 3
• If manufacturer recommendations are unknown, defibrillation at maximal dose may be considered 1
• No specific biphasic waveform (truncated exponential vs rectilinear) has been proven superior to another 1, 3

Practical Clinical Implications

Advantages of Biphasic Waveforms

• Higher first-shock efficacy allows earlier termination of ventricular fibrillation 4
• Lower energy requirements reduce potential myocardial damage 2
• Biphasic defibrillators often adjust energy output based on patient impedance, which may improve efficacy 3
• All new defibrillators currently manufactured use biphasic waveforms 1

Important Caveats

• While biphasic shocks have superior shock success rates (defined as termination of VF 5 seconds after shock), this has not translated into conclusive survival benefit in randomized trials 1, 6, 7
• The evidence for survival benefit comes primarily from observational studies with confounding factors rather than high-quality randomized trials 1
• Most comparative studies were conducted during controlled electrophysiology procedures or ICD testing, not actual cardiac arrests 2

Current Practice Reality

• Monophasic defibrillators are no longer being manufactured but many remain in clinical use 1
• When monophasic defibrillators are used, higher initial energy (360 J) is preferable 1
• The transition to biphasic technology is based primarily on improved shock success rates, not proven mortality reduction 1