A Comparison of Two Biphasic Waveforms and Impact of Oxygen on Myocardial Injury Following Cardioversion

Background:

Treatment of cardiac arrest includes effective chest compressions, ventilation using high flow oxygen and the delivery of electric shocks (cardioversion). Prompt termination of cardiac arrhythmias and sufficient airway handling including oxygenation are crucial factors for patient outcome. Studies suggest that cardioversion itself may induce myocardial injury; however it is currently unknown whether hyperoxia exacerbate this injury.

Currently, several different biphasic waveforms are in use for cardioversion.The comparative efficacy and safety of these waveforms has not been compared in clinical studies. Commercially available defibrillators use different electrical waveforms that may affect the efficacy of external defibrillation. Although the superiority of biphasic over monophasic waveforms is well established, the relative efficacy and safety of the available biphasic waveforms is not clear.

The long held belief that oxygen (O2) is always beneficial for ill and injured patients is being challenged by recent studies suggesting that excessive oxygenation may result in free-radical mediated tissue damage, resulting in increased morbidity and mortality. Recent guidelines suggest that O2 should be 'titrated' to achieve an appropriate target saturation of 94-98% rather than being administered in an all-or-nothing fashion, particularly in patients with myocardial ischemia or those following resuscitation from cardiac arrest. The Scottish Intercollegiate Guidelines Network (SIGN), British Thoracic Society, National Institute of Health Care Excellence (NICE), European Resuscitation Council and other respected bodies all restrict O2 use to those with hypoxemia, pulmonary oedema or continuing myocardial ischemia, based on expert opinion and pathophysiological reasoning rather than hard evidence.

Studies using patients presenting with shockable life-threatening arrhythmias (e.g. ventricular fibrillation) are difficult to perform for ethical and practical reasons. This study will therefore use patients requiring elective cardioversion for atrial arrhythmias, who are able to give consent and also present a more controlled population. Elective cardioversion is a standard treatment for patients with atrial arrhythmias and is performed as a day case procedure under general anaesthesia.