How good is your defibrillation technique?

Abstract

In 1956, a patient in ventricular fibrillation (VF) was successfully treated via externally applied electricity.1 The history of human defibrillation, which is still the only effective means of cardioverting VF back into sinus rhythm, began here. By 1962, electric shock had also been found effective in atrial fibrillation and atrial flutter.2 In August 2000 the International Liaison Committee on Resuscitation (ILCOR) published evidence-based guidelines for adult and paediatric life support.3 All the UK training courses on advanced life support are now based on the ILCOR guidelines; yet, in this document of over 400 pages, the defibrillation section is only 4 pages long. This is partly because the topic had been under-researched. New information has emerged since 2000. Our own group has taken a special interest in the defibrillation technique used by hospital practitioners and how it could be improved. Survival to discharge from in-hospital cardiac arrest in the UK is still, at best, only 17% in a monitored area of the hospital such as a coronary care unit.4 It is considerably worse on general hospital wards. The most important factor in determining whether defibrillation will be successful is the time that elapses before delivery of the first shock.5 For every minute post-arrest, the chances of cardioversion decrease by as much as 10%.3 Many defibrillators in hospitals now have an automated facility, so that a nurse suspecting a cardiac arrest can connect the patient to the machine via two pads;6 the device analyses the cardiac rhythm and delivers a shock if appropriate. A study in hospital inpatients has shown that use of the automated facility on new biphasic defibrillators can yield a 2.6-fold increase in survival post-arrest.7 All nurses should therefore be trained to use an automated defibrillator if one is available on their ward.8 Recently, there have been two major developments in defibrillator technology. One is the emergence of pads made of flexible conductive material.9 Our group found that the transthoracic impedance (TTI) to current flow created by use of these pads is the same as that with paddles.10 At present there is uncertainty as to which method is preferable. The second advance concerns the waveform used for defibrillation. Originally, the waveform created by defibrillators was monophasic but over the last ten years the biphasic waveform has taken over and nearly all new models use this technology11 (Figure 1). The relative merits of these two waveforms are beyond the scope of this article. In summary, a biphasic shock of 150 joules is far more likely than a monophasic shock of either 200 or 360 joules to convert VF; yet, so far, no study has shown a survival advantage for the biphasic method.12 Figure 1 Monophasic and biphasic waveforms used by defibrillators Successful defibrillation depends on delivery of the shock to a critical mass of myocardium13 and this in turn depends on the transthoracic current flow (TCF) achieved. If the TCF is too low, defibrillation will fail; if it is too high the result may be myocardial cell damage and even necrosis. On the evidence of post-mortem studies and troponin measurement after defibrillation, necrosis is not a common result of multiple shock delivery.14,15 The most likely reason for failure is a TCF that is too low. TCF is determined by the energy selected and the TTI:16 Clearly a low TTI will result in a high TCF. Numerous studies have shown that a high TTI will decrease the chances of delivering a successful shock to the patient;17-19 therefore, during defibrillation, TTI has to be reduced as far as possible. Some contributory factors, such as thoracic size, are intrinsic and cannot be altered; but extrinsic factors include paddle force, presence of chest hair, use of an electrical coupling agent, 20 the size of the paddles (larger electrodes have a lower impedance but when excessively large can result in less TCF18), the number of shocks delivered21 and paddle position. This article focuses on techniques for minimizing TTI.