What are the differences between biphasic and monophasic defibrillation waveforms and their recommended initial energy settings (joules) for adult cardiac arrest?

Biphasic vs Monophasic Defibrillation: Key Differences and Energy Settings

Biphasic defibrillators are superior to monophasic defibrillators and should be used preferentially, with initial energy settings of 120-200 J for biphasic waveforms compared to 360 J for monophasic waveforms. 1

Fundamental Waveform Differences

Monophasic waveforms deliver electrical current in only one direction (single polarity), with two main subtypes 1:

• Monophasic damped sinusoidal (MDS): Current gradually returns to zero
• Monophasic truncated exponential (MTE): Current abruptly returns to zero

Biphasic waveforms deliver current that reverses direction, flowing in two phases, and are now used in almost all modern AEDs and manual defibrillators 1. Few monophasic defibrillators are currently being manufactured, though many remain in clinical use 1.

Clinical Efficacy: Why Biphasic is Superior

First-Shock Success Rates

Biphasic waveforms achieve dramatically higher first-shock efficacy:

• Biphasic: 85-98% success rate at ≤200 J, with one study showing 90% first-shock success 2, 1
• Monophasic: Lower success rates requiring higher energies 1

At equivalent 200 J energy levels, biphasic shocks reduced the risk of post-shock asystole or persistent VF by 81% compared to monophasic shocks (RR 0.19; 95% CI 0.06-0.60) 3.

Energy Efficiency

Biphasic waveforms achieve equal or superior defibrillation at significantly lower energies 1:

• Biphasic shocks at 115-130 J are as effective as monophasic shocks at 200 J 3
• At very low energies (70-100 J), biphasic waveforms show marked superiority: at 70 J, biphasic achieved 80% success vs. 32% for monophasic 4
• Lower energy delivery results in less myocardial injury as measured by ST segment changes 3

Recommended Initial Energy Settings

For Biphasic Defibrillators (Preferred)

Use manufacturer-recommended dose, typically:

• 150-200 J for biphasic truncated exponential waveforms 1
• 120 J for rectilinear biphasic waveforms 1
• If manufacturer's recommendation unknown, 200 J is reasonable for initial shock 1, 2

For Monophasic Defibrillators (Legacy Devices)

Use 360 J for initial and all subsequent shocks 1. This simplified approach recognizes that most EMS systems may still have monophasic devices in service 1.

Subsequent Shocks

• Biphasic: Use same or higher energy; optimal escalation strategy not definitively established 1
• Monophasic: Continue with 360 J 1
• No evidence of harm from biphasic energies up to 360 J in human studies 1

Critical Clinical Context

Survival Outcomes

Important caveat: While biphasic waveforms show superior shock success (defined as VF termination for ≥5 seconds), no specific waveform has been consistently associated with higher rates of ROSC or survival to hospital discharge 1. This emphasizes that defibrillation is only one component of successful resuscitation—high-quality CPR and minimizing interruptions in chest compressions remain paramount 1.

Modern Resuscitation Protocol

The high first-shock efficacy of biphasic defibrillators (>90%) has fundamentally changed resuscitation protocols 1:

• Single-shock strategy followed by immediate CPR is now recommended, replacing the older 3-stacked-shock sequence 1
• Resume CPR immediately after shock delivery without pulse/rhythm check 1
• The 3-shock sequence created unacceptable 29-37 second delays in chest compressions 1

Common Pitfalls to Avoid

1. Don't assume all defibrillators at the same energy setting deliver identical energy: Actual delivered energy can deviate by +23% to -29% from selected settings 5

2. Don't delay defibrillation searching for "optimal" energy: Use manufacturer's recommended dose or 200 J biphasic/360 J monophasic and proceed immediately 1, 2

3. Don't confuse shock success with patient survival: VF termination doesn't guarantee ROSC or survival—continue high-quality CPR regardless of waveform type 1

4. Don't use synchronized mode for VF: This applies to both waveform types and can result in failure to deliver shock 6

Bottom Line for Clinical Practice

Biphasic defibrillators should be the standard of care due to their superior first-shock efficacy, lower energy requirements, and reduced myocardial injury 1, 2, 3. When using biphasic devices, start with 120-200 J (or manufacturer's recommendation); when using legacy monophasic devices, use 360 J 1. Most importantly, minimize interruptions in chest compressions and deliver shocks as part of an integrated, high-quality resuscitation effort 1.