Definition of defibrillation
Defibrillation is the only effective therapy for cardiac arrest caused by ventricular fibrillation and can be defined as an attempt to depolarise a critical mass of the myocardium in order to restore the synchronicity of the heart’s electrical conduction system.4 The overarching aim is to achieve the highest efficacy with the lowest possible energy and current, allowing for depolarisation of the myocardial cells with minimal or no myocardial damage.5
There are numerous factors that may impact on the effectiveness of defibrillation:
The literature behind defibrillation
Strategies before defibrillation
Precordial thump
There is little high quality evidence to support the use of a precordial thump although 3 old case series suggest that VF or pulseless VT was converted to a perfusing rhythm by a precordial thump.6,7,8 It has been suggested that an effective precordial thump may be delivered by the closed fist from between 5 and 40 cm.9 There are suggestions that a precordial thump may lead to rhythm deterioration but it is not possible to judge the likelihood of this occurring. Current recommendations suggest that a single precordial thump may be considered after a monitored cardiac arrest if a defibrillator is not immediately available.9
CPR before defibrillation
In two studies, 1½–3 minutes of CPR by paramedics or EMS physicians before attempted defibrillation improved return of spontaneous circulation (ROSC) and survival rates for adults with out-of-hospital VF or VT when the response interval and time to defibrillation was ≥4 to 5 minutes.10,11 A more recent trial contradicted these results and found no improvement in ROSC in adults with out-of-hospital VF or VT, in which 1½ minutes of paramedic CPR was delivered before defibrillation.12 In animal studies of VF lasting 5 minutes, CPR (often with administration of epinephrine) before defibrillation improved haemodynamics and survival rates.9
Recommendation
In the case of out-of-hospital cardiac arrest attended, but unwitnessed, by healthcare professionals equipped with manual defibrillators, 2 minutes of CPR should be given prior to defibrillation (approximately 5 cycles of 30:2).13
Transthoracic impedance
Energy selection and transthoracic impedance are the two main determinants of intracardiac current flow during defibrillation. Transthoracic impedance (TTI) is the resistance to current flow created by body size and structure. Factors that determine TTI include energy selected, electrode size, paddle–skin coupling material, number and time interval of previous shocks, phase of ventilation, distance between electrodes (size of the chest), and paddle electrode pressure.9
Pad/paddle positioning and size
See Figure 4.1.
No human studies have evaluated the effect of pad/paddle position on defibrillation success or survival rates; most studies evaluated cardioversion or used secondary endpoints such as TTI.9 A review of the science in 2005 suggests that placement of paddles or electrode pads on the superior-anterior right chest and the inferior-lateral left chest were effective, whilst alternative positions such as apex-posterior and anteroposterior positions were also reported to be effective.9 Where paddles are being used it is suggested that the apical paddle should be placed longitudinally to maximise contact with the chest;14 it is not clear if this is applicable to adhesive pads. Care should be taken to avoid placing pads or paddles directly on breast tissue as this has been shown to increase TTI.15
One human study16 and one animal study17 documented higher success rates with larger 12.8 cm paddles compared with 8 cm paddles. A number of studies have reported reduced TTI with larger paddles15,18-23 and one animal study has shown significant myocardial damage when using small (4.3 cm electrodes) when compared with 8 cm or 12.8 cm pads.24
Figure 4.1 Defibrillation electrode position.
Recommendations9
- Pads should be placed on the superior-anterior right chest and inferior lateral left chest where possible.
- In large-breasted women it is reasonable to place the left electrode pad/paddle lateral to or beneath the breast, where paddles are used; the apical paddle should be placed longitudinally.
- Defibrillation success may be improved with 12.3 cm pads rather than 8 cm pads, small pads should be avoided to reduce the risk of myocardial injury.
- Where paddles are being used, the apical paddle should be placed longitudinally.
Adhesive pads or paddles?
Several studies reported in the 2005 International Consensus on Cardiopulmonary Resuscitation suggest that levels of TTI are similar with both paddles and pads. It has been suggested that TTI is reduced where an optimum 8 kg of pressure is applied to paddles but there are several safety and practical advantages to the use of pads, particularly in the prehospital environment.9
Ventilation status
During the inspiratory cycle the patient’s lungs are filling with air, which increases TTI. Delivering shocks at the end of expiration when the lungs are deflated will reduce TTI and increase the chances of successful defibrillation.4
Recommendations
- Adhesive pads are safe and effective and suitable as an alternative to paddles.
- Defibrillation should coincide with the peak of expiration to minimise TTI.
Defibrillation waveform
Biphasic v monophasic defibrillation
Defibrillation waveforms are complex interventions; it is not important to understand the waveforms used by these defibrillators but it is important that the most effective devices are used. Monophasic waveforms vary in the speed at which the waveform returns to the zero voltage point – gradually (damped sinusoidal) or instantaneously (truncated exponential) – and deliver a current that proceeds in a single direction.25 All new defibrillators produce a biphasic waveform, which means that the current flows initially in a positive direction and then, after a predetermined time reverses to a negative direction. The modern generation of biphasic defibrillators are calibrated to alter the waveform delivered to the patient based on TTI (that is, impedance compensated biphasic waveforms (ICB)). These devices aim to deliver a shock ‘dose’ that is proportional to each patient.26
This waveform has been shown to be more effective than the monophasic waveform defibrillators,27 and is successful with fewer shocks.28 Biphasic defibrillators are smaller and lighter than monophasic defibrillators and use lower energy levels so require less battery power.
Recommendation
- Biphasic waveform shocks are safe and effective for termination of VF when compared with monophasic waveform shocks.9
Energy levels for defibrillation
A metanalysis identified insufficient evidence for or against a specific energy level for either first or subsequent shocks when using a defibrillator.9 It is reasonable to use energy levels of 150–200 joules with BTE waveform biphasic defibrillators, and 120 joules with the rectilinear biphasic waveform. An initial shock of 360 joules is considered reasonable when using a monophasic waveform defibrillator.9
Automated external defibrillation
Automated external defibrillators (AED) are sophisticated, computerised devices that deliver defibrillatory shocks to those in cardiac arrest caused by VF or ventricular tachycardia (VT).29 AEDs use voice and visual prompts to guide the practitioner in the delivery of defibrillatory shocks and have become more sophisticated and safer over recent years. For AEDs to perform reliable ECG signal analysis and make a shock/no-shock decision, CPR must be discontinued due to the artefacts introduced by chest compressions and ventilations;30 this introduces periods where there is no blood flow from compressions. One study identified that patients were not perfused for approximately 50% of the time when an AED was used in out-of-hospital cardiac arrest.31 Animal studies have shown these delays to be linked to a worse outcome in cases of prolonged VF.32
A number of studies indicate that the use of AEDs by trained lay and professional responders has significantly improved the outcome for those who suffer an out-of-hospital VF cardiac arrest where an effective response plan is in place.33–37 The evidence for the use of AEDs by trained responders (e.g. police and fire) is less clear with some studies indicating improved survival whilst others show no improvement.9
Recommendations
Use of AEDs by trained lay and professional responders is recommended to increase survival rates in patients with cardiac arrest. Use of AEDs in public settings (airports, casinos, sports facilities, etc) where witnessed cardiac arrest is likely to occur can be useful if an effective response plan is in place.9 In order to minimise the time where there is no blood flow due to interruptions in compressions, professional ambulance staff should be trained in and have the use of manual defibrillators.
Procedure for defibrillation
| Procedure | Additional Information rationale |
| 1. Perform CPR until defibrillator/monitor is attached. | Maximises perfusion whilst awaiting equipment. |
| 2. Prepare patient’s chest. Check for: • Patches (e.g. GTN) remove if found. • Jewellery or piercings in the pathway of defibrillation; remove if found. • Moisture: dry wet chests. • Pacemaker sites: avoid defibrillating over pacemaker sites. • Underwired bras: remove if found. | Maximises contact between pads and chest and reduces TTI. |
| 2. It may be necessary to shave chest hair for pad positioning but this should not delay defibrillation if no razor is available. | |
| 3. Place the right (sternal) electrode to the right of the sternum, below the clavicle, and the apical paddle vertically in the mid-axillary line, approximately level with the V6 ECG electrode position or the female breast. | Ensures that the current passes through a critical mass of the myocardium. |
| 4. Assess rhythm. | To identify shockable or non-shockable rhythm. |
| 5. Select appropriate energy for defibrillator. | |
| 6. Use strong verbal commands: ‘VF/VT seen’ ‘Charging at 150 joules’ (energy stated here is an example only). | Ensure safety during defibrillation |
| 7. Confirm that nobody is in direct or indirect contact with the patient. Consider commands such as: ‘Top, middle, bottom clear’ whilst checking those specific areas of the patient. | |
| 8. Ensure that oxygen is off and intravenous lines are down. | |
| 9. State: ‘stand clear’. | |
| 10. Perform a final quick visual check to ensure that everyone | |
| 11. Deliver shock. | |
| 12. Immediately recommence CPR irrespective of the presenting rhythm. |
Definition of transcutaneous cardiac pacing (TCP)
External (transcutaneous) pacing is a temporary method of pacing a patient’s heart during an emergency. Pulses of electrical current are delivered through the patient’s chest to stimulate the cardiac muscle to contract. It restores electrical stimulation to the myocardium in an emergency setting and can be initiated quickly by any healthcare professional who has undertaken the necessary training.38
Terminology used in transcutaneous cardiac pacing
- Capture: The presence of a QRS complex after a pacing spike (electrical capture). If the patient has a palpable pulse consistent with the heart rate on the monitor then mechanical capture has also been achieved.
- Demand pacing: The provision of a pacing stimulus only when the patient’s heart rate falls below a predetermined limit.
- Fist (percussion) pacing: The use of serial rhythmic blows with the closed fist over the left lower edge of the sternum to pace the heart. This will only be used in the emergency setting in P-wave asystole and profound bradycardia resulting in clinical cardiac arrest.39
- Fixed-rate pacing: The provision of a pacing stimulus irrespective of the patient’s intrinsic heart rate.
- Threshold: The minimum energy required to maintain consistent capture.
Indications for use of transcutaneous cardiac pacing
The European Resuscitation Council Guidelines for resuscitation 2005 advocate TCP in bradycardia where there is no response to atropine, if atropine is unlikely to be effective or if the patient is severely symptomatic, particularly if there is high-degree block (Möbitz Type II or third-degree block).40 The Joint Royal Colleges Ambulance Liaison Committee (JRCALC) also advocates the use of TCP where available.41
