Background and purpose:
High-quality chest compression plays an important role for patients with cardia arrest. Chest compression is a complex dynamic system, and the magnitude and exertion method of force are important factors for high-quality chest compression. Dynamic indexes such as Force (F), velocity (V), acceleration (a) and impulse (I) can reflect the magnitude of force and the way of exertion. Previous studies have confirmed that F is closely related to chest compression quality, but the impact of V, a, I, etc. on compression quality is not clear. So, the purpose of this study is to explore the effect of dynamic indicators on high-quality chest compression. 

Materials and Methods: 
This was a prospective self-control study，carried out from 1st Sep 2017 to 30th December 2017. Subjects were recruited if they were more than 18 years old, and had been trained for standard BLS from the Emergency Department of West China Hospital of Sichuan University and West China School of Medicine. Then we collected their general characteristics, such as sex, age, height, weight, waist circumference, thigh length, arm length, maximum grip strength of hands, CPR experience. All subjects were asked to perform 2 minutes of continuous chest compression on the same CPR simulator through a compression-monitoring machine which can automatically record the data of depth, frequency, recoil ratio and force value every 0.01 seconds, and then parameters-maximum depth, frequency, recoil rate, maximum compression force(F), maximum velocity(V), maximum acceleration(a), maximum impulse(I) of each compression were computed. Furthermore, the rate of qualified compression (depth of 5-6cm, frequency at 100-120 beats per minute, and recoil rate ≥95%) was calculated. Then multiple linear regression analysis was done to find the independent risk factors of qualified compression for 2 minutes of continuous chest compression. All statistical analysis was implemented with SPSS software version 19.0(USA, IBM analytics).
 
Results:
A total of 159 subjects were included in the study, including 80 males (50.31%) and 79 females (49.69%), with an average age of 29.36±9.0 years. Single-factor analysis showed that sex, height, weight, waist circumference, thigh length, arm length, single-handed maximum grip strength, F, V, a, I, etc. might affect the rate of qualified compression(P<0.05). Multiple linear regression analysis showed that both V (standardized Beta= 0.937) and a (standardized Beta= 0.404) were independent risk factors of qualified compression for 2 minutes of continuous chest compression. 

Conclusions: 
Maximum velocity and maximum acceleration are important factors related with high-quality chest compression. Further experiments on animals and patients are on demanded to verify this result and to clarify its mechanism. 

Acknowledgement: 
We wish to thank all the people from west china hospital and west china school of medicine that participated in the study. 

Funding: 
This study was supported by the NSFC of China (No.81772037, 81471836 and 81801883) and the Chengdu science and technology fund (2016-HM02-00099-SI).

7 - The impact of dynamic indexes on high-quality chest compression.