Defibrillation Shockable and Non-shockable Rhythms

| In Articles | 18th April 2020

Defibrillator used for defibrillation in cardiac problems to apply electric shocks to the heart

Defibrillator - a device used to control heart activity by applying an electric current to the chest wall and heart. The action is called defibrillation and it means a controlled electric shock.

Types of defibrillators

  • Manual
  • Automated
  • Monophasic or Biphasic waveform

Biphasic defibrillators

  • Require less energy
  • Smaller capacitors and batteries
  • Lighter and more transportable
  • Biphasic shocks have higher success rate for ventricular fibrillation and pulseless ventricular tachycardia

Electrical current flow depends upon:

  • Electrode position
  • Transthoracic electrical resistance
  • Energy delivered
  • Body size
  • The defibrillator recharge speed

Defibrillator safety

  • Must be assumed by the person who applies electrical shocks
  • Never hold both paddles in one hand
  • Charge only with paddles on casualty's chest
  • Avoid direct or indirect contact
  • Wipe any fluid from the patient's chest
  • Remove high flow oxygen from the defibrillation zone

Manual defibrillation

  • Diagnose VF/pVT from ECG
  • Select correct energy level
  • Charge paddles
  • Shout "stand clear"
  • Visual check of area
  • Check monitor
  • Deliver shock

Automatic external defibrillators

  • Analyse cardiac rhythm
  • Prepare for shock delivery
  • Specificity for recognition of shockable rhythm close to 100%

Semi-automatic external defibrillation

  • Analyse cardiac rhythm
  • Prepare for shock delivery
  • Does not automatically apply the shock
  • Specificity for recognition of shockable rhythm close to 100%

Shock energy

  • Shocks energy 150-200 J, if biphasic
  • Shocks energy 360 J, if monophasic

The shock will immediately be followed by CPR 30:2 for 2 minutes without evaluating the rhythm or central pulse!

During CPR:

  • Correct reversible causes
  • Check electrodes, paddle position and contact
  • Attempt/verify: airway and oxygen, intravenous and intraosseous access
  • Start giving drugs after the third shock!


Ventricular Fibrillation (VF)

It is commonly called VF (or V- Fib), an immediately life threatening condition. It produces no effective myocardial contraction. The uncoordinated ventricular muscle quivers, therefore there is no effective cardiac output. Left untreated VF invariably leads to ventricular standstill and death.

Rhythm: indeterminate
Heart rate: indeterminate

Ventricular Fibrillation - life threatening cardiac condition when external electrical defibrillation is the most successful treatment

Ventricular Tachycardia (VT)

It is defined as three or more premature ventricular contractions (PVCs) in a row, with a rate above 100 bpm. PVTs are extra heartbeats that disrupt the regular heart rhythm.

VT is an unstable rhythm. It can occur in short bursts causing few or no symptoms. Alternatively it may be sustained, becoming symptomatic and causing unconsciousness, loss of cardiac output and death.

Rhythm: Regular or slightly irregular ventricular rhythm

Heart rate: Ventricular rate of 100 bpm or above.

Ventricular tachycardia - good response after defibrillation and cardiac electrical shocks

For VT/VF cardiac arrest:


Check rhythm is VT/VF & shock asap ↓

CPR 2min ↓

Check rhythm is VT/VF & shock ↓

Adrenaline + CPR 2min ↓

Check rhythm is VT/VF & shock ↓

Amiodarone + CPR 2min ↓

Check rhythm is VT/VF & shock ↓

Adrenaline + CPR 2min


The cardiac rhythms that do not require treatment with a DC shock are termed “NonShockable” rhythms.
These non-shockable rhythms are asystole and pulseless electrical activity (PEA).


It is the absence of myocardial electrical activity and therefore produces no cardiac output.
The patient is pulseless and non-responsive.
Rapid initiation of CPR and ALS measures is vital.

What you see on the monitor:
Asystole appears as a nearly flat line (except for those changes caused by chest compressions). No electrical activity is evident.
In a patient with a pacemaker, pacing spikes may be visible on the ECG strip.

Graphic representation of no electric activity (shocks) as no defibrillation is done.

Pulseless Electrical Activity (PEA)

It may also be termed Electromechanical Dissociation (EMD).
It exists when there is a coordinated electrical rhythm (this includes a range of rhythms) without a detectable cardiac output. Potential causes must be sought and treated, including hypoxia, hyper/hypothermia, hypovolemia, hyper/hypokalaemia, pulmonary/cardiac embolus, tension pneumothorax, cardiac tamponade, toxins, thrombus, acidosis and ventricular rupture.

What you see on the monitor:

Organised electrical activity, often bradycardia (slower than normal heart rate). Each case of PEA is unique and the rhythm seen on the monitor will therefore differ from case to case. However all patients with PEA will have no detectable cardiac output.

For non-shockable cardiac arrest:

CPR 2min + Adrenaline ↓

Check rhythm, if non-shockable ↓

CPR 2min ↓

Check rhythm, if non-shockable ↓

Adrenaline + CPR 2min ↓

Check rhythm, if non-shockable ↓

CPR 2min ....etc.

Adrenaline is a medication and hormone naturally produced by the human body in "fight or flight'' situations (a body mechanism that enables it to mobilise a lot of energy rapidly in order to cope with threats to survival). As a medication it belongs to the sympathomimetic drugs class and is used to treat a number of conditions, including anaphylaxis, cardiac arrest, and superficial bleeding.

Amiodarone is an antiarrhythmic medication used to treat and prevent a number of types of irregular heartbeats.

!!! To remember: rescuers must minimise interruptions to compressions, consider reversible causes, intubate if necessary (using a laryngeal mask airway LMA or endotracheal tube ETT), give oxygen, measure waveform capnography (monitoring the partial pressure of carbon dioxide CO2 in the exhaled air using a non invasive monitoring device) and plan actions before interrupting compressions.