Biomechanical Tests Of Chest Compressions Upon The Human Thorax

Background and Objective1.BackgroundThere are about500,000sudden death patients in china every year, cardiac death is the main reason for sudden death, cardiac arrest account for80%of the cardiac death.The survival rate of out-of-hospital cardiac arrest (CA) subjects is reached6.5-15%[1-4] in developed country, the survive rate of CA subjects performed traditional CPR and performed only-CC can be up to17.7%and33.7%[2] respectively. However, the rate of CA less than1%in China. Cardiopulmonary resuscitation (CPR) is the most important method to rescue cardiac arrest patients, and chest compression (CC) as the basic method of CPR is the key to rescue out-of-hospital cardiac arrest patients especially, that determines whether the salvage is success or not. American heart association(AHA) published "2010American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care(ECC)"(2010AHA Guidelines)in2010,which changed CPR basic life sustain(BLS) significantly, and emphasize"push hard" and "push quick", recommended frequency and depth reaches100cpm and5cm respectively and emphasize high quality CPR. Clinical research[5] confirmed that deeper compression(51mm) more conducive to improving the survival rate of cardiac arrest patients and will help to improve the prognosis. Each additional5mm compression depth raised helps improving survival rate and function recovery of CA subjects. But iatrogenic injuries associated with chest compressions increase significantly[6] at the same time.The highest incidence of complications are ribs and sternum fractures, especially to male patients, which more likely to occur complication than female patients. Though modern CPR was used54years, the survival rate of cardiac arrest patients is not ideal, and the method to raise survival rate is the big puzzle to develop CC.Though modern CPR was performed scince1960s,the compress depth and frequency increased continuous, and CA patients’ rescue rate increased also, but the optimum range of quality indexes are not identified. Reference of CC quality indexes suggested by "guide2010",that demand of medical staff’s physical capacity is increased than before, and they often fail to meet the requirements the "guide2010"recommended[7]. In view of this, some scholars suggested instead of manual CPR by mechanical CPR, but to research high performance and safe CPR instrument requires knowledge of mechanism of CPR, and getting the biomechanics of thorax is basic of CC mechanism. So, mechanism of CPR is the key to resolve CPR problem. The main research method of human CC biomechanism, including clinical research, animal model research, artificial human research, and human cadavers research. Clinical research’s data comes from patient, which is most close to the real man’s data, which limited by instruments and ethics, however. Animal model study is simply and easy, the data is accurate, but the result different from human’s data. So does artificial human model. Human cadaver research was regarded as gold standard.2. ObjectiveThough biomechanical test of human thorax to obtain force-displacement relationship ofthorax, to find how much force needed to meet the compress depth that "guide2010" recommended. Lording under the same force use different CC frequency and different lord position whether can get different chest displacement. Though CPR trainingstudy to know compression quality healthcare professionals can reachand CPR quality whether decreased with the compression circles continue, to provide a reference for future CPR training and clinical work.Materials and Methods 1. To review anatomical structure of chest, the progress on the mechanics of chest compressions mechanism and the biomechanics of chest compressions.2. Choose one male and one female cadaver spicemens,(male:height178cm, weight75kg, age40, thorax transverse diameter28.5cm, thorax antero-posterior diameter19.7cm, chest circumference79cm; female:height163cm, weight52kg, age55, thorax transverse diameter25.6cm, thorax antero-posterior diameter16.8cm, chest circumference73.4cm),the two cadavers were checked by physical examination andscanned by CT, exclusion of thoracic fractures, deformity and masses, etc. According to the thoracic integrity both male and female specimens were divided into four groups:groups A,B,C and D. The tests were performed on the intact chest in group A, after the soft tissues but the internal intercostal muscle, disphragmatic muscle in the chest wall andthe dorsal soft tissues were removed in group B, after the soft tissues but the diaphragmatic muscle in the chest wall andthe dorsal soft tissues were removed in group C, after all the soft tissues and the whole organs inside the cavitas thoracis were removed in group D. Loading position choice:sternum between3rd and4th rib, sternum between4th and5th rib (clinical chest compressions position), sternum between5th and6th rib. They were recorded as position Ⅲ, Ⅰ, Ⅱ. Using BOSE dynamic/static material testing machine (ELF-3510AT, Bose, Inc.,USA) under different loading positions into action. There are static biomechanical testing group and dynamic testing grouprespectively. Static testing group:both male and female specimens of4groups are connected to the known load0～400Newton (Newton, N), sequentially at different loading position to50N as a the level of loading incressed each test.The BOSE test machineconfiguration computer synchronous recording force and displacement data; dynamic testing group:using80cpm,100cpm,120cpm,140cpm loading frequency, with the same force were tested for A, B, C, D groupson three different positions. The BOSE test machine configuration computer synchronous recording force and displacement at the sametime.3.October2011-October2013period, we select185healthcare professionals who participate in CPR training, in which77healthcare professionals are male group and86cases are female group. The depth, frequency, the time to complete5cycles and compression in target of CPR compressions were analyzed between the two groups by using real-time feedback. And we select135of185healthcare professionals to analyze compression depth, rate and efficiency of chest compression in different cycles by using real-time feedback system. The effective compression depth of4～6cm, effective frequency is80-120cpm,efficiency of CPR is defined as the percentage compressions per compression depth and frequency are within the above range of the total number of presses.Result1.Thorax is flexible and active. The heart in partial left side of anterior mediastinum, the locations of CC points are nearly identical to the closest positions of heart to the thorax. Force loads on the chest, transfer energy into the subcutaneous tissue, caused tissues anatomical tensioning, the thorax deflected, anterior-posretior diameter decressed, thoracic volume changed, then increased intrathoracic pressure and (or) pressed the heart, so the vital organ perfusion maintained. This is the the anatomical basis of Cardiac pump theory mechanism and thoracic pump theory mechanism.2.The result of biomechanical tests of the human thorax:(1) There is a good correlation between force and sternal displacement under static loading of the human cadavers thorax. The sternal displacement was increased with increasing load, the force-displacement relationship meet the cubic model. The force-displacement relationship is F=6.715+7.976-0.054-0.005(F:force,N;displacement,mm) to the male’s intact cadaveric thorax and F=7.678+2.135+0.029+0.003to the female’s intact cadaveric thorax under static loading conditions. The force-displacemnet relationship of both male and female cadaver with intact thorax were meet the quadratic model(0-400N), there is a positive correlation between force and displacement under dynamic loading conditions(0-120N). To reach50mm sternal displacement requires646N and432N to male cadaver and female cadaver, that male cadaver need more force than female cadaver to reach the same displacement, and there is need more force under dynamic test condition than static test condition to reach the same sternal displacement.(2)Observed force-displacement relationship curves of thorax that lorded at different positions(positonⅠ, Ⅱ, and Ⅲ), we found that to reach the same displacement there is smaller force when lorded position land II than loaded position III, the loading point II deeper displacement easier, while there were fracture of rib twice.(3)From80cpm to140cpm,the force that transferred to thorax is decreased, and form80cpm to120cpm,the sternal displacement increased when lording with same force, but the sternal displacement decreased at the rate of140cpm.(4)As the thorax demolished, the sternal displacement increased when lording with same force, the female cadaver is significant than the male cadaver. The variance of sternal displacement is smalist between group B and group C.3.After training, the level of basicknowledge of CPR is improved when compared to before training (P>0.05). The mean compression depth is4.84±0.64cm, there is significant difference between male groups and female group (5.20±0.52cm&4.59±0.58cm,P=0.00<0.05);The mean compression frequency is122.63±13.64cpm, there is no significant difference between male groups and female group(122.37±14.5cpm&122.8±12.715cpm, P>0.05); the mean time to complete5cycles is100.79±7.36s, and the total compression in target of all CPR compressions is66.51±34.68%,there is no significant difference between male groups and female group both of two items(P>0.05).As the compressions continue, average compression depth and average compression rate of chest compression decreased gradually, there is significant statistical difference among the5th cycles (P=0.025<0.05&P=0.016<0.05); The average efficiency of compression decreased as the cycles go on, while there is no significant statistical difference among the5cycles (P=0.327>0.05)Conclusion1.The anatomic features of chest and heart support the theory of cardiac pump mechanism and thoracic pump mechanism.2.The force-displacement relationship of intact human cadaveric thorax under static loading conditions meet the cubic model. The displacement increased while loading increased, there is a positive correlation between force and displacement. Chest compressions on different lord positions the sternal displacement variance.The clinical chest compressions press position (sternum,between4th and5th rib)is reasonable. The quality and complication of chest compressions are closely linked with the chest thoracic stiffness, the male thorax are stiffer than female thorax. We should give attention to the thorax stiffness in clinical. Chest compressions should have the best compression rate range, the upper bound of optimal press rate should be in120cpm-140cpm. Thorax is support human chest, the chest to maintain a certain rigidity, the mechanics foundation of chest biomechanics. Thoracic integrity change has an important effect on the biomechanical properties of thorax, the developed degree and integrity are of muscle are main factors. The diaphragm, viscera and the integrity of the thoracic that affect thoracic biomechanical properties can not be ignored.3.After training, healthcare professionals can reach the standards that "guide2010"recommended, and that is easier to male healthcare professionals than female healthcare professionals. The mean depth of female healthcare professionals didn’t reach the standards that "guide2010"recommended. It’s almost reasonable to change rescuer after5th circle, to change rescuer at4th circle can be considered for sometime, especially to female healthcare professionals.