Re-evaluation of priorities: Good CPR vs Defibrillation
Introduction
Over the past 5 years it has become clear that the standard approach to ventricular fibrillation (ie. defibrillate ASAP!) is not necessarily always the best. Recent lessons learnt from the published literature include the importance of timing of compressions with regard to defibrillation, the harm associated with interruptions to compressions, the potential for a period of CPR to improve the likelihood of defibrillation, the importance of “good CPR” (including the avoidance of hyperventilation). In addition a number of changes have been made to the process of defibrillation, including the single shock technique, the use of an adequate energy level for the waveform used, and commencement of compressions immediately after a shock rather than checking the rhythm and pulse.
Interruptions to compressions
Interruptions in chest compressions (“hands-off time”) are common, often prolonged, and are associated with a decrease in coronary perfusion pressure and a deceased likelihood of defibrillation success. These adverse effects commence within 10 seconds, but appear to be at least partially reversible with the re-commencement of chest compressions. Obviously some pauses in compressions for rhythm recognition or specific interventions (such as ventilations, defibrillation, or intubation) are at times essential, but they should be minimised.
Technique for compressions
The optimal rate of cardiac compression is approximately 100 compressions/minute. The ideal depth of chest compression is at least 4-5 cm (or approximately one third of its depth).
Ventilation
The minute ventilation requirements during cardiac arrest are less than that in the non-arrested state, so the baseline respiratory rate can be decreased. Hyperventilation during cardiac arrest is associated with increased intrathoracic pressure, decreased coronary and cerebral perfusion, and, at least in animals, a decreased rate of return of spontaneous circulation. To increase the number of compressions given per minute, minimise interruptions to chest compressions, and simplify instruction for teaching and skills retention, a single compression:ventilation ratio of 30:2 has been recommended for adult BLS before the airway is secured. After the airway is secured the recommended ventilation rate is 8-10/minute (eg 15:1). To avoid the “Lazarus phenomenon” a period of disconnection from the ventilation circuit may be beneficial.
Monitoring CPR
A number of different techniques are available to monitor the quality of CPR. These vary from simple observation (rate, depth and positioning of chest compressions, the rate and depth of ventilation, and palpation of central pulses) to end-tidal carbon dioxide and new monitor/defibrillators that can monitor the depth and rate of compressions and ventilation.
Compressions and defibrillation
Chest compressions should be continued up until the delivery of the shock, and should be commenced again immediately following defibrillation (without checking the rhythm). Even if defibrillation has successfully reverted the rhythm into one that could generate a pulse, in the vast majority of cases this is not initially so. Immediate compressions in these situations avoid the detrimental effects of interruptions to compressions, and maintain the coronary perfusion pressure. The underlying rhythm should be checked after each 2 minutes of CPR (or if signs of life return), If a rhythm compatible with a return of spontaneous circulation is observed at this stage, then the pulse should also be checked.
Single shocks
The use of a single shock strategy for defibrillation is now recommended. This approach decreases the interruptions to chest compressions that were inherent in the stacked shock approach, so the benefits will be dependent on the quality of CPR, as the next shock will be delayed for at least 2 minutes while CPR is performed.
The Australian Resuscitation Council recommends a stacked shock strategy (up to 3 shocks as necessary) for the first defibrillation attempt in a witnessed arrest, where a manual defibrillator is immediately available, and the time required for rhythm recognition and charging of the defibrillator is short.
Energy levels
Recommendations for energy levels depend on the type of defibrillator and the waveform being used. Current recommendations are based on maximising the likelihood of the success of each shock. The recommended energy level for defibrillation in adults where monophasic defibrillators are used is 360 Joules. When using biphasic waveforms, the energy level should be set at 150-200J. There is no evidence supporting an escalation of energy levels for subsequent shocks.
Summary
Defibrillation retains its importance as the definitive treatment for VF, but it is now recognised that the metabolic milieu should be optimised before attempts at defibrillation are made. Good CPR (compressions and ventilation) is crucial, and interruptions to this life sustaining procedure should be minimised.
(The Australian Resuscitation Council guidelines are freely available on the internet: www.resus.org.au).
Further reading
1. Aufderheide TP. The problem with and benefit of ventilations: should our approach be the same in cardiac and respiratory arrest? Curr Opin Crit Care. 2006 Jun;12(3):207-12.
2. Edelson DP, Abella BS, Kramer-Johansen J, et al. Effects of compression depth and pre-shock pauses predict defibrillation failure during cardiac arrest. Resuscitation. 2006 Nov;71(2):137-45.
3. Morley PT. Monitoring the quality of CPR. Curr Opin Crit Care. 2007 Jun;13(3):In press.
4. Proceedings of the 2005 International Consensus on Cardiopulmonary Resuscitation and Emergency Cardiovascular Care Science with Treatment Recommendations. Resuscitation. 2005 Nov-Dec;67(2-3):157-341.
5. Wik L, Hansen TB, Fylling F, et al. Delaying Defibrillation to Give Basic Cardiopulmonary Resuscitation to Patients with Out-of-Hospital Ventricular Fibrillation: A Randomized Trial. JAMA. 2003;289(11):1389-95.
Peter Morley, May 2007
