Defibrillation is the passage of electrical current across the myocardium to depolarise a critical myocardial mass. It is the only treatment proven to be effective for terminating VF. The depolarisation allows the restoration of coordinated myocardial electrical activity following recapture by a single pacemaker. Success depends on the ability of the depolarized myocardium to begin synchronous excitation and contraction.
Defibrillation must be initiated as soon as possible, and never delayed by other interventions, as the probability of successful defibrillation decreases rapidly with time.
- Asystole when fine VF cannot be excluded.
- Manual defibrillator (all modern defibrillators are biphasic. Older models may be monophasic)
- Semi-automatic external defibrillator (SAED): often found in hospitals, where there is some degree of medical expertise in its use
- Automated external defibrillator (AED) or shock-advisory defibrillator: usually biphasic, and found in public places where expertise is limited.
Pads or paddles
- Disposable self-adhesive gel pads with electrode wires that connect directly to defibrillator are now preferred to paddles.
- Defibrillator paddles are attached to the defibrillator by insulated leads. Contact is made with the patients skin over gel pads placed on chest wall.
Note: Although self-adhesive defibrillation pads and paddles are both safe and effective with a similar transthoracic impedance and efficacy, self-adhesive pads allow for longer term monitoring in the peri-arrest situation; allow the operator to defibrillate a safe distance from the patient and enable quicker delivery of the first shock.
Ensure own safety and place pads/paddles on chest.
- Check chest is dry and that operator is not standing in a pool of blood, fluid or water.
- Ensure no metal objects, ECG electrodes or GTN patches are underneath fast patches or paddles and that internal pacemakers are at least 5 cm away from contact points.
- Move oxygen source (oxygen mask, nasal prongs or disconnected ventilation tubing) at least 1 m away from patient to reduce the risk of fires, burns or explosions.
- All personnel should be clear of any contact with the patient or bed prior to administration of the shocks.
- The paddles should either remain held on the patient’s chest, or be placed back in the machine in between shocks, and never moved while charged.
Reducing transthoracic impedance
Reducing the transthoracic impedance to a minimum will increase the amount of electrical energy reaching the heart.
- Choose the right paddle size
- Optimum size in adults is 10–13 cm in diameter (smaller in children)
- If too small, focal myocardial damage may occur
- If too large, contact with the chest wall becomes difficult and impedance is increased, and the charge delivered decreased.
- Choose the right paddle/pad site
- Do not place over female breast (increased impedance).
- Standard placement
- Right chest: 2nd intercostal space, mid-clavicular line
- Left chest: 5–6th intercostal space, mid-axillary line.
- Anteroposterior placement
- Anterior chest: 5-6th intercostal space, anterior or mid-axillary line
- Posterior chest: over left or right infrascapular region.
- Use conductive pads: when applying paddles, always use gel pads for the skin as this will greatly reduce the transthoracic impedance.
- Shave the chest (if time): this prevents air trapping beneath the electrode and increases electrical conduction.
- Apply firm pressure with paddles (5–8 kg on each paddle).
- Watch the respiratory phase delivery: defibrillation should occur in end expiration (ventilation) when transthoracic impedance is least.
Note: Asthmatic subjects often require higher than normal energy levels for defibrillation, as they generate auto PEEP (gas trapping).
- Biphasic (modern) defibrillators use 150–200 J for each shock.
- Biphasic truncated rectilinear waveform use 120 J for all shocks.
- Monophasic dampened sinusoidal waveform (older machines) use 360 J (4 J/kg in children) for the initial shock and all subsequent shocks.
Manual defibrillation with paddles
- Review ECG trace on monitor and identify rhythm.
- Attach gel pads to chest wall.
- Turn on defibrillator and select required energy level.
- Put paddles on patient’s chest.
- Charge using ‘Charge’ button on right hand (apex) paddle.
- Ensure all personnel (and yourself) are clear of the patient, the bed rails and metallic contact points. State ‘All clear’.
- Discharge pads with ‘Shock’ button on paddles.
- Do not lift the paddles from the chest wall. Review ECG tracing.
- Repeat as required.
Manual defibrillation with self-adhesive pads (preferred to paddles)
- Place pads safely on chest (as above).
- Turn defibrillator on, observe ECG trace and identify rhythm.
- Select energy level.
- Check that all personnel are not in physical contact with the patient and state ‘All clear’.
- Visually confirm the shockable rhythm is still present and press ‘Shock’ on the defibrillator.
- Repeat as required
- Burns to skin
- Myocardial injury
- Electrical injury to bystanders
- Failure (check correct charge is set, correct pressure applied if using paddles, charge was actually delivered i.e. tonic patient muscle contraction; then consider anteroposterior pad or paddle placement).