The management of out-of-hospital cardiac arrest is complex and multifactorial. With an incidence between 5 and 15 per 10,000 (46,000 patients per year in France) and a survival rate of only 5% to 15%, the room for improvement remains significant even today and is based on fast and optimal care. Thus French and international recommendations insist on the central element of external chest compression (ECC) and especially its quality (Monsieurs KG and Al. Resuscitation 2015; 95: 1-80). Improving the chest compression fraction (CCF) by limiting time without cardiac massage (No-Flow) is a second major point of the recommendations (Vaillancourt C and Al. Resuscitation 2011; 82: 1501-7). The survival of cardiac arrest victims is closely related on this No-Flow time. The principle of the chain of survival (early warning - ECC - defibrillation - resuscitation) implies that the deterioration of a single link threaten the whole of the care. To meet these qualitative needs, ECC guidance devices have been developed. They make possible to improve the quality of the ECC achieved (Hostler D and Al. BMJ 2011; 342d512). Their use is one of the areas of improvement mentioned in the recommendations. Our team studied in simulation the prolonged effects of guidance on the quality of the ECC during a prolonged resuscitation, with encouraging results (Buléon C and Al. Am J Emerg Med 2016; 34: 1754-60). The investigators propose a study evaluating the efficiency of the guidance of the ECC and the impact of the time of relay on the CCF. The investigators formulate two hypotheses that they wish to test simultaneously using a 2x2 factorial design, in a multicenter randomized trial. The first assumption is that a 4-minute relay rate improves the CCF (by reducing the No-Flow time) compared to the currently recommended 2-minute relay rate. The second hypothesis is that a guiding device improves the quality of the ECC.

This study should, over a period of 2 years, include 500 patients with cardiac arrest for whom specialized resuscitation is undertaken. The investigators hope by this study to improve the knowledge on the optimal rhythm of the ECC and to validate "in vivo" the interest for the guidance found on manikin. This study should make it possible to clarify the recommendations with a high level of evidence in this field and thus contribute to improving the prognosis of the victims of an out-of-hospital cardiac arrest.

Cardiac arrest (CA) remains a challenge for pre-hospital care. With an incidence of between 5 and 15 per 10,000 (46,000 patients per year in France) and a survival rate of only 5% to 15%, there is yet room for improvement in treatment to reduce morbi-mortality of these patients. The quality of cardiopulmonary resuscitation (CPR) is at the heart of the last three five-year recommendations. (1-3) The latest recommendations emphasize the importance for professionals to work at the highest quality of CPR and External Chest Compression (ECC) possible. (3) The ratio of the time during which the ECC is performed (Low-Flow) to the total time of the resuscitation is referred to as the Chest Compression Fraction (CCF). During CPR, it is essential for the patient's survival to minimize ECC disruption times and therefore to increase the CCF, as this is an independent element in CA survival's improvement. (4,5) ECC interruptions are deleterious to at least two titles. First, they are a source of direct stop in cerebral and coronary perfusions potentially altering the neurological prognosis and the probability of Return of Spontaneous Circulation. (6) Secondly, the quality of the cardiac output generated by the ECC at the time of resuming of the ECC after an interruption is less good for more than 30 seconds: time need for that several chest compressions can restore the best flow possible. (6,7) Reducing these interruptions and improving the ECC is therefore a major goal of improving CPR. The guidelines are that CCF must be greater than 60% and some experts estimate that a CCF of 80% is possible. (8,9) The outcome of patients with pre-hospital CA is significantly, positively and independently correlated with the consistency to different CCF targets, ECC frequency, ECC depth, and brief pre-external electric shock pause (\<10 seconds). (10) There is evidence that ECC's guidance improves adequacy to guidelines and allows to be closer with the ECC frequency, depth and release objectives. (11) The investigators have proved in simulation that the guidance of the ECC delays the deterioration of the overall quality of the ECC and its components (frequency, depth and relaxation) related to fatigue during a prolonged ECC beyond the 2 ECC relay minutes currently recommended. (12) Strategies to get closer with the guidelines regarding the quality of the ECC associated with an improvement in CCF should add or even enhance their beneficial effects for the management of CA victims. Achieving high quality CPR requires the measurement of quality of CPR (ECC and CCF). (13,14) This idea of a support strategy enhanced by "bundles" of concepts is developing in the literature. Thus Cheskes S et al. Describe a "high quality CPR" such as the association of a CCF greater than 70% and achievement of the objectives of the recommendations for the frequency and depth of the ECC. (15) The place of devices for guiding the quality of the ECC needs to be specified. Indeed, studies of their use in real-life situations are criticized for their methodological qualities and their size. (16) The use of a real-time guidance device is proposed as a possibility in the latest guidelines without being an indispensable element due to the lack of current evidence. (3) Its use or non-use does not imply any obvious loss of chance for patients. Evidence as to its usefulness therefore remains to be sought.

For this reason, the investigators wish, through an original, randomized, multi-center study, to provide some answers to the questions about the possibility of an improvement in CCF by the lengthening of the time between two ECC relays and the effect of guidance on the quality of the ECC. The design of the study will also allow to approach a possible combined effect of ECC relays rhythm and guidance. The currently recommended duration of a two-minute ECC cycle between two relays does not have a consistent evidence based and corresponds to a duration for which the ECC effort can be maintained in principle with efficiency. (3) Objective measures have shown that the quality of the ECC can be maintained beyond 2 minutes. Extending the duration of an ECC cycle could reduce the number of ECC interruptions and thus improve the CCF.

The investigators therefore formulate two hypotheses that they wish to test simultaneously using a 2x2 factorial design, in a multicenter randomized trial. The first assumption is that a 4-minutes relay rhythm improves the CCF (by reducing the No-Flow time) compared to the currently recommended 2-minutes relay rhythm. The second hypothesis is that a guiding device improves the quality of the ECC.

The CPRmeter® (guidance device used in this study) will record data on the ECC and its quality (depth, frequency, relaxation, CPRmeter® use time, No-Flow time and Low-Flow time) as well as ECC guidance for the group which will benefit from it (the other group will have the screen masked by a screen cap).

This study should, over a period of 2 years, include 500 major patients presenting a non-traumatic CA for whom a specialized CPR is undertaken. The investigators hope by this study to improve the knowledge on the optimal rhythm of the CEE and to validate "in vivo" the interest for the guidance found on manikin. This study should clarify the guidelines with a high level of evidence in this area and thus contribute to improving the prognosis of victims of out-hospital CA.