Electrical Therapy for Cardiac Arrhythmias: Energy Selection

Electrical cardioversion and defibrillation are among the most effective interventions in acute medicine – provided you choose the right energy. Joule values set too low lead to failed shock deliveries, unnecessary myocardial stress, and time loss. Energies set too high can cause myocardial damage. There are significant differences in recommended energy selection between monophasic and biphasic technology, between synchronized cardioversion and unsynchronized defibrillation, and between the various arrhythmias. This article gives you a systematic, practical overview – from atrial fibrillation to pulseless ventricular tachycardia.

Fundamentals: Monophasic vs. Biphasic

Before you deal with specific joule values, you need to know the underlying technology of your defibrillator. The waveform significantly determines which energy level you should choose.

Monophasic Defibrillators

With monophasic devices, current flows in one direction through the myocardium. This older technology requires higher energies to achieve sufficient myocardial depolarization. Monophasic devices are still in use in many facilities but are increasingly being replaced by biphasic models.

• Maximum energy: typically 360 J
• Higher impedance dependence
• Greater myocardial damage with repeated shocks

Biphasic Defibrillators

Biphasic devices deliver current in two directions – first in one direction, then the polarity reverses. This achieves equivalent or even superior efficacy at lower energy levels. Two relevant biphasic waveforms exist:

• Biphasic Truncated Exponential (BTE): Used by manufacturers such as Philips and Physio-Control/Stryker. The waveform is truncated after a defined time.
• Rectilinear Biphasic (RLB): Used by ZOLL. The waveform dynamically adapts to transthoracic impedance.

The clinical consequence: With biphasic devices, the effective energies are manufacturer-dependent. There is no uniform "biphasic standard." You need to know the manufacturer's recommendation for your specific device.

Practical Consequence

If you don't know which biphasic waveform your device uses, refer to the information on the device itself or in the quick reference guide. The AHA guidelines recommend: If the optimal biphasic dose is unknown, use the maximum energy of the device. This applies particularly to defibrillation for ventricular fibrillation and pulseless ventricular tachycardia.

Synchronized Cardioversion vs. Defibrillation

A fundamental distinction that is occasionally forgotten in stressful situations:

• Synchronized cardioversion: The shock is synchronized with the R-wave to avoid the vulnerable phase (T-wave). Indication: Any unstable rhythm with a palpable pulse where organized electrical activity is present (atrial fibrillation, atrial flutter, SVT, monomorphic VT with pulse).
• Unsynchronized defibrillation: The shock is delivered immediately, without waiting for the R-wave. Indication: Ventricular fibrillation (VF), pulseless ventricular tachycardia (pVT), and polymorphic VT (since reliable R-wave detection is not possible here).

Clinical key point: Polymorphic ventricular tachycardia is treated like ventricular fibrillation – unsynchronized with defibrillation energy. Attempting synchronized cardioversion in polymorphic VT frequently results in the device failing to detect an R-wave and not delivering a shock. This costs life-saving time.

Energy Selection by Arrhythmia

Atrial Fibrillation (AF)

Atrial fibrillation shows disorganized atrial activity with irregular conduction to the ventricles. For electrical cardioversion you need comparatively high energies, since the disorganized atrial activity is harder to terminate than organized reentry circuits.

Recommended energy levels:

Technology	Starting Energy	Escalation
Biphasic	120–200 J (manufacturer-dependent)	Stepwise increase to maximum
Monophasic	200 J	300 J → 360 J

Specific manufacturer recommendations for initial biphasic cardioversion of atrial fibrillation:

• ZOLL (RLB): 120 J
• Philips (BTE): 150 J
• Physio-Control/Stryker (BTE): 150–200 J

The AHA guidelines recommend a starting energy of 120–200 J for biphasic devices, with the lower end being typical for RLB devices and the upper end for BTE devices.

Practical tip: Many experienced clinicians start directly with 200 J biphasic for atrial fibrillation to avoid failed shock attempts – especially in obese patients or long-standing atrial fibrillation. This approach is supported by the guidelines and is pragmatically sensible.

Atrial Flutter and Supraventricular Tachycardia (SVT)

Atrial flutter is based on an organized macro-reentry in the right atrium; SVT typically on an AV nodal reentry or an accessory pathway. These organized reentry circuits can be terminated with significantly lower energies than atrial fibrillation.

Recommended energy levels:

Technology	Starting Energy	Escalation
Biphasic	50–100 J	Stepwise increase
Monophasic	100 J	200 J → 300 J → 360 J

The AHA guidelines indicate that a low starting energy of 50–100 J is reasonable for atrial flutter and SVT. Many practitioners start with 50 J biphasic for atrial flutter – the conversion rate is already high with low energies for this organized rhythm.

Important: Before electrical cardioversion of SVT – if the patient's condition permits – vagal maneuvers and adenosine should be attempted. Synchronized cardioversion for SVT is generally reserved for hemodynamically unstable patients.

Monomorphic Ventricular Tachycardia (VT) with Pulse

Monomorphic VT with a pulse represents a special case: the patient has a palpable pulse, but the rhythm is potentially life-threatening and can degenerate into ventricular fibrillation at any time.

Recommended energy levels:

Technology	Starting Energy	Escalation
Biphasic	100 J	Stepwise increase
Monophasic	200 J	300 J → 360 J

The AHA guidelines recommend a starting energy of 100 J biphasic for synchronized cardioversion of monomorphic VT. If unsuccessful, escalate stepwise.

Clinical decision point: If the patient is stable (no shock, no pulmonary edema, no loss of consciousness, no severe chest pain), you can initially treat pharmacologically – for example with amiodarone 150 mg IV over 10 minutes. If hemodynamic instability develops, synchronized cardioversion takes priority. Never wait for medication to take effect when the patient is hemodynamically decompensating.

Polymorphic Ventricular Tachycardia

Polymorphic VT – including the special form Torsades de Pointes – shows varying QRS morphologies and is treated like ventricular fibrillation: unsynchronized defibrillation.

Recommended energy levels:

Technology	Starting Energy	Escalation
Biphasic	120–200 J (manufacturer-dependent)	Maximum of the device
Monophasic	360 J	–

Caution with Torsades de Pointes: In addition to defibrillation, magnesium sulfate 2 g IV over 10 minutes is the specific therapy. Furthermore, the cause (QT-prolonging medications, electrolyte disturbances) must be identified and corrected. Overdrive pacing may be considered.

Ventricular Fibrillation (VF) and Pulseless Ventricular Tachycardia (pVT)

This is cardiac arrest – maximum urgency, no synchronization.

Recommended energy levels:

Technology	First Shock	Subsequent Shocks
Biphasic	120–200 J (manufacturer-dependent)	Same or higher, up to maximum
Monophasic	360 J	360 J

Manufacturer recommendations for the first biphasic defibrillation shock:

• ZOLL (RLB): 120 J
• Philips (BTE): 150 J
• Physio-Control/Stryker (BTE): 200 J

The AHA guidelines recommend: If the optimal dose is unknown, use the maximum energy of the device. When in doubt, go higher rather than too low. A failed shock in VF means two more minutes of CPR without rhythm conversion.

Summary Table: Energy Selection at a Glance

Rhythm	Synchronization	Biphasic	Monophasic
Atrial Fibrillation	Synchronized	120–200 J	200 J initial
Atrial Flutter	Synchronized	50–100 J	100 J initial
SVT	Synchronized	50–100 J	100 J initial
Monomorphic VT (with pulse)	Synchronized	100 J	200 J initial
Polymorphic VT	Unsynchronized	120–200 J	360 J
VF / pVT	Unsynchronized	120–200 J	360 J

Common Errors and Pitfalls

Sync Mode Forgotten or Falsely Activated

One of the most common and potentially fatal errors: you want to defibrillate for VF, but the sync mode is still activated from the last cardioversion. The device searches for an R-wave, finds none – and delivers no shock. Many defibrillators automatically reset the sync mode after each shock, but not all. Know your device.

Conversely: you want to perform synchronized cardioversion for monomorphic VT and forget to activate the sync mode. The unsynchronized shock may fall into the vulnerable phase and trigger VF.

Misinterpretation of Polymorphic VT

Polymorphic VT is not infrequently misinterpreted as an "irregular wide complex tachycardia," and an attempt is made to perform synchronized cardioversion. The device cannot reliably detect the varying QRS complexes. Result: no shock. Always defibrillate unsynchronized for polymorphic VT.

Failure to Escalate Energy

After a failed first cardioversion shock, the same energy is frequently selected again. Escalate stepwise – there is no reason to repeat the same unsuccessful energy level.

Pad Position and Impedance Underestimated

The best energy selection is of little use if electrode position is suboptimal or transthoracic impedance is unnecessarily high:

• Anterolateral (right parasternal infraclavicular – left mid-axillary line) or anteroposterior (sternal – interscapular)
• Apply pads firmly, no air pockets
• In patients with chest hair: shave
• In obese patients: anteroposterior is often superior
• Sufficient pressure when using paddles (at least 8 kg per paddle)

Not Knowing Device-Specific Differences

It bears repeating: There is no uniform biphasic standard. The optimal starting energy varies by waveform and manufacturer. Read your device's quick reference guide. Ideally, the recommended starting energy is displayed as a sticker on the device.

Special Situations

Pacemakers and ICDs

In patients with an implanted pacemaker or ICD, pads should be placed at least 8 cm away from the generator. The anteroposterior position is often advantageous here. After cardioversion/defibrillation, the device must be checked, as threshold changes may occur.

Pediatric Patients

Pediatric energy selection differs fundamentally:

• Defibrillation: 2 J/kg → 4 J/kg → up to a maximum of 10 J/kg or adult dose
• Synchronized cardioversion: 0.5–1 J/kg → 2 J/kg if unsuccessful

Elective Cardioversion

For planned cardioversions (e.g., for persistent atrial fibrillation), additional considerations apply:

• Adequate sedation/anesthesia (e.g., propofol)
• Observe fasting requirements
• Anticoagulation according to guidelines (at least 3 weeks of therapeutic anticoagulation or TEE to exclude atrial thrombus)
• Readiness for airway management

Algorithm: Decision Tree for Energy Selection

1. Pulse present?

   • No → Unsynchronized defibrillation → VF/pVT energy (120–200 J biphasic / 360 J monophasic)
   • Yes → Proceed to 2

2. Narrow or wide QRS complex?

   • Narrow → SVT / AF / Atrial Flutter → Synchronized cardioversion
     • Atrial fibrillation: 120–200 J biphasic
     • Atrial flutter/SVT: 50–100 J biphasic
   • Wide → Proceed to 3

3. Regular or irregular?

   • Regular → Monomorphic VT → Synchronized, 100 J biphasic
   • Irregular → Polymorphic VT → Unsynchronized, 120–200 J biphasic (treat like VF)

Practical Training

Correct energy selection can be learned in theory – but confident application under stress requires regular hands-on practice. In the ACLS Refresher Course from Simulation Tirol, you train synchronized cardioversion and defibrillation in realistic simulation scenarios, learn the differences between device types, and strengthen team-based decision-making. All courses follow the current AHA guidelines and are led by experienced instructors. Find more information at simulation.tirol.