Question:
(1)   Is there any particular defibrillator waveform or energy level that offers an advantage in the treatment of human cardiac arrest due to ventricular fibrillation (VF)?
 
Answer:
The influence of defibrillator waveform and energy levels on the outcome from attempted defibrillation has been the subject of a relatively small, but growing number of studies. These have reported the results obtained with a variety of defibrillator waveforms and energy levels. The settings in which these studies have been conducted have varied considerably (in hospital, out-of-hospital, animal studies, electrophysiological laboratory). Similarly, many different waveforms, energy levels and impedance compensation systems have been investigated. This has complicated the interpretation of data and limited the application of the results to routine medical practice.

Many studies have shown that attempted defibrillation with biphasic waveforms using equal or lower energy levels was at least as effective as monophasic waveforms. However, no specific waveform has been shown to offer consistent advantages in either ROSC or discharge from hospital in human cardiac arrest. No trial directly comparing different biphasic waveforms has been published up to the time of the Consensus on Science and Treatment Recommendations (CoSTR 2005) which were published in November 2005.

Several human studies have reported effective defibrillation using biphasic waveforms at shock energies between 120J and 200J without showing an optimum level for the first shock. It was the conclusion of CoSTR 2005 that there was insufficient evidence for or against definitive energy levels for the first or subsequent shocks when a biphasic waveform was used. For the initial shock, energy levels of 150J – 200J are appropriate with biphasic truncated exponential (BTE) waveforms or 120J with a rectilinear biphasic waveform. For monophasic defibrillators, review of the evidence suggested that an energy level of 360J was appropriate.

Studies have also investigated the use of second and third biphasic shocks (where VF proves refractory) using energy ranges from 150J to 360J without demonstrating an optimum level. Only one peer-reviewed human study has compared fixed energy with escalating energies for the second or subsequent shocks. This did not report a clear benefit for either strategy. High rates of conversion have been reported when a fixed energy (i.e. non-escalating) 150J BTE shock was used for second and third shocks in presence of VF refractory to earlier shocks. Similarly high rates of conversion have been reported with a rectilinear waveform that used increasing energy levels (120J to 150J-200J).

At present there is insufficient evidence to recommend any one biphasic waveform or level of energy for the initial or subsequent shocks. Good results have been reported with both escalating and non-escalating strategies with currently available biphasic waveforms and energy levels. Results in clinical practice are more likely to be influenced by the early recognition of cardiac arrest, the prompt provision of chest compressions with the minimum of interruptions, and steps to minimise delays in providing a shock.
 

Question:
(2)   There appears to be some contradiction in Guidelines 2005 about if and when CPR should be given before defibrillation on a victim out of hospital. For example, the chapter, ‘The use of Automated External Defibrillators’ states: ‘Guidelines 2005 continues to recommend an immediate shock as soon as the AED is available’ (page 24), yet in ‘Adult Advanced Life Support’ the following appears: ‘…give CPR before attempted defibrillation outside hospital, unless the arrest is witnessed by a healthcare professional or an AED is being used. This advice does NOT apply to lay responders using an AED outside hospital, who should apply the AED as soon as it is available.’
 
Answer:
For out-of-hospital cardiac arrest, lay responders should start CPR as soon as the diagnosis is made, and attempt defibrillation as soon as an AED becomes available. Healthcare professionals should give about 2 min of CPR before attempting defibrillation (manual or AED), unless the arrest has been witnessed by them.
 

Question: (Added February 2008)
(3)   Is it safe to defibrillate a victim if they are wet or lying on a wet surface?
 
Answer:
Yes – in principle it is safe to defibrillate a victim who is wet or lying on a wet surface. If it is likely that you will be using the defibrillator in these conditions (for example at a swimming pool), it is recommended that the manufacturer / supplier be asked to confirm that their specific model is suitable for such use.
 
Appropriate safety precautions are required. The victim’s chest should be dried so that the adhesive AED pads will stick and particular care should be taken to ensure that no one is touching the victim when a shock is delivered. As long as there is no direct contact between the user and the victim when the shock is delivered, there is no direct pathway that the electricity can take that would cause the user to experience a shock.

 
 
 
 
 
 

	© Resuscitation Council (UK)  2008   This page last updated: 5 February 2008