Biomechanical Tests And Finite Element Analysis Of External Chest Compression Upon The Human Thorax

Background and objectivesIncidence of sudden death approximates up to 60 million each year in China. However,the patients who can get to hospital for rescue are only 5%-15%,and only 1%-20%of them can survive.In case of sudden heart arrest,if cardiopulmonary resuscitation(CPR) is postponed one minute,the survival rate is reduced by 10%-7%. So world medical domain pays close attention to CPR year by year.After almost 50 years,modern CPR has developed into three levels of nine steps.The three levels include:①Basic Life Support(BLS).②Advanced Life Support(ALS).③Prolonged Life Support(PLS).BLS is basis and key to entire resuscitation and the external chest compression(ECC) is an important procedure in BLS.The theory of CPR has been developing constantly from the last century to this day.In 2005,the newest international CPR guideline especially emphasized the effectiveness and continuity of ECC.The concerns that CPR academic circles show include always its frequency,definite mechanism and effect.(1) The frequency of ECC has increased from previous 60 times/min to current 100 times/min.In recent years,some scholars suggested high frequency compression(above 120 times/min).(2) The validity of ECC:The newest CPR guideline clearly regulated the compression depth,location and strength.In order to be effective,the compression must be quick and forceful. The location of compression is between two mammilla,the palm should maintain correct position after compression,and compression should extend for 4～5cm for adults and 1/3～1/2 of thorax for babies.The thorax must be completely resilient,and relaxing time should be basically equal to pressing time.However,some clinical studies display:50%of ECC is unqualified,for the extent of ECC is insufficient, moreover,24%～49%of the time is not devoted to ECC process in CPR.Chest wall elastic recover is able to increase the returned blood volume.If chest wall does not recover,the reduction of the returned blood volume will induce cardiac output loss. Accordingly,too much discontinuance compression will lead to aeteria coronaria and cerebral blood flow discontinuance,and the success ratio of CPR is obviously reduced.The mechanism of ECC is also a focal point of resuscitation development. The mechanism of cardiac pump was first suggested that regular compression extrudes the heart between breastbone and backbone so that the pressure in heart ventricle increases and blood marches.When the pressure relieved,heart ventricle revives for suction effect and blood come back to heart.Eventually,the artificial circulation is established through recompression.In 1980's,scholars begin to study the mechanism of CPR in blood flow,and proposed the theory of thorax pump.Some researchers proposed the theory of resonance frequency between thorax,great vessels and heart.Traditional research methods of resuscitation include animal model, clinical research,and human model.Empirical study of ECC is limited by time, environment and different anatomic structure of different species,so the use of the research method is also limited.Through the vitodynamic experiment of body samples,however,the present research tested stress and straining of thorax during ECC.The finite element model of human thorax was re-established to reflect the practical problems reasonably,clearly and accurately.After testing the availability of the model,the present research utilized finite element analysis to investigate the stresses and deformations during the ECC process,providing biomechanic evidence and new thinking for addressing the mechanisms and clinic effects of CPR.Materials and Methods1.Anatomical foundation of ECC:The anatomical structures of human thorax,chest wall, as well as the provisional location of heart and thorax were studied systematically.2.Biomechanical tests of ECC upon the human thorax:One case of fresh male cadaver specimen was selected and its chest radiography was taken before the experiment to exclude any disease(including fractures,deformity,etc) that may exist. The chest was scanned by double spiral CT in the 157th Clinical Department of the 458^th People's Liberation Army Hospital.The conditions of withstanding different loads during human thorax compression were simulated respectively,by use of MTS testing machine and extension device.The thorax displacement and strain were tested under the vertical compression of a known gravity 0-200 N(Newton).This study enrolled two groups:groups A and B.The tests were performed on the entire and intact chest in group A,but after the soft tissues in the chest wall and the whole organs inside the cavitas thoracis were removed in group B.The points of experimental displacement measurement were the same.The seven pressing points were at the anterior midline of the sternum and between the intercostal 4^th to 5^th(7^th point),from the left rib section 5 before the midline 38.0mm(6^th point),left rib section 4 from the center line before being 42.0mm(5th point),from the left rib section 3 before the midline 37.6mm(4^th point),from the left rib section 2 before midline 28.7mm(3^rd point),before the midline of the horizontal section 3 rib junction point(2^nd point),and the center line and the junction point of the horizontal ribbed(1^st point).3.Constructed the 3D model of human thorax:The spiral CT images of the same cadaver specimen in the last experiment were selected(biomechanical tests completed).Spiral CT scan was conducted continuously from the first to the upper edge of the 12^th rib margin,bone tissue fault observation windows were chosen to observe the thorax and reconstruction of the skeleton.The window level was 396 and window width 1536. The slice thickness of scan was 1.0mm,and scan time was 36.53 s.A total of 440 two-dimensional images of human thorax were obtained,and the images were stored in DICOM format.The two-dimensional images in DICOM format were imported into Mimics10.1 software.A 3D model of spine,stemum and rib bone of thoracic was established by Mimics10.1 software after spatial orientation of images and the image segmentation were performed.4.Constructed the 3D finite element model of human thorax:The 3D images of thorax were modified and surface-meshed with the software of Mimics 10.1.The model was discriminated into six kinds of material.Then the model of surface-meshed could be kept as the document ANSYS named.lis as the suffix and be introduced into ANSYS software directly to establish a 3D finite element model.5.Validity test of 3D finite element model of human thorax:The results of biomechanic tests and 3D finite element model of thorax were compared,and the two sets of bias and straining under the same force were compared.At last,the validity of 3D finite element model was tested by the tendency of the two sets of data.6.The finite element analysis of ECC upon the human thorax:The 3D finite element models of human thorax which had been confirmed effective were subjected to perpendicular loading in simulation.The bias,stresses,and straining of the models were analyzed during perpendicular loadings of 0-400N,then the results were analyzed in comparison with clinical data.Results1.All the bones of the thorax are connected with costovertebral joints, costotransverse joints and stemocostal joints.Thorax is flexible and active.When ECC is performed,the vertical compression force impacts on sternum,then the articulation of the sternum and ribs,ribs and costovertebral joints,so that the sagittal and transverse diameters of thorax change.Because the rib cartilage is rich in elasticity,the external cardiac compression or artificial breathing is preferred for the patients who suffer from sudden breath arrest.The locations of ECC points are nearly identical to the closest positions of heart to the thorax.2.Biomechanical tests on the thorax samples of fresh adult male showed that the displacements and straining of human body samples thorax were as follows:(1)Under the static compression,the results of strength-offset relation were:①The offset displayed pressure-dependent relationship when loading changed from 0 from 200 N. The offset increased at every measurement point while loading increased.②When the external force was identical,the offset varied with measurement points.The offset at the 7^th pressing point was maximal.The proper order was:offset at pressing point 7＞at point 6＞at point 5＞at point 2＞at point 4＞at point 1＞at point 3.③The pressing point moved down 40mm in depth only when it accepted the loading force of 200 N.④The comparison between groups A and B:The offset in Group B was bigger under the same force.When the external force was 200N,the offset at the pressing point reached 60 mms,but the sternum and rib of the sample did not fracture.(2) Under dynamic compression,the relations between loading and displacement of thorax were shown as follows.When the clinical chest compression at a frequency of 100se/min was simulated and the relaxing equaled the compression in time,the tendency line between external force and straining showed that the displacement offset was enlarged as soon as the external force increased.(3)The column diagram of displacements between the dynamic and static states demonstrated that the displacement in the dynamic state at the same measurement point is much smaller than that in the static state.According to relation curve of force-strain,when the external force acted on the thorax,straining was shown to be pressure-dependent.As the external force(loading) increased,the strain at every measurement point increased.3.The three-dimensional finite element model was built with 1158 085 nodes and 736 022 elements,imitating complicated structures of human thorax,including clavicle,rib,and vertebral column.The model could display distinctly and directly the entire images of structure of human thorax which could be used in calculation.4.Since the entire thorax elements and net lattice data were too numerous,and the 11^th and 12^th floating ribs could be ignored,a model of 356 562 nodes and 215 808 elements was created by subtracting the part below the 11^th rib before calculation. Comparing the offset and strain between the biomechanic tests group and the model group under identical force effects,we found that the offsets between the original model and the biomechanic tests group were greatly different.After the model material was adjusted,the model was compared and verified with the former experiment group once again before the reasonable thorax model was calculated.5.The analyses of the thorax finite element model showed that the stresses distributed homogeneously and symmetrically after the loading was exerted on normal thorax. The concentrated location of straining on thorax was different from that of stress when compression acted on the thorax.The former was located in the border of sternum and rib,and the latter in the rib where the degree of freedom was restrained. The maximal deformation during compressing was located in the border of sternum and the rib,while the straining was located in the back at the same time.The present research compared the stress with the strain of every rib.Conclusions1.The heart will be deformed obviously,and the deformation increases as the external chest compression becomes more forceful.The anatomic features of heart and chest bone support the theory of cardiac pump and mechanism.The thorax is a complicated structure consisting of many bones and synostoses.During ECC,a vertical downward force affects chest bone,and changes the sagittal and the transverse diameters through the sternum and rib connections,ribs,thoracic vertebral joints.The intrathoracic pressure becomes greater consequently.This is the anatomical basis of thoracic pump mechanism.2.Under the same force,the nearer the palpating point,the larger the bias.In the same rib,the bias at the measuring point of sternum is larger than that at the measurement point of rib.Thoracic deformation is in direct ratio to the force during ECC.3.The heart deformation occurs definitely during ECC.4.The biomechanical tests show that in the dynamic circumstances,the bias is to increase with force.The trend line of relation between the force and bias shows the following force and displacement formula:y=8.0173x+38.698.It will offer momentous reference data for quantifying the effect of ECC.5.The three-dimensional finite element model of human thorax is established successfully on the computer.6.Under the same external force,the tropism of bias in the human thoracic 3D finite element model and that in the biomechanic tests group are consistent.The tropism of straining curve is consistent in the two groups too.Thus,we think that this model is effective and provides meaningful finite element analysis data for our future clinical studies.7.The biomechanic experimental results show that the fifth costicartilage is likely to get fractured during clinical compression.It is reasonable that ECC should be conducted on the quartus spatium intercostale. 8.One weakness of this study is that all the biomaterials are supposed to be homogenous,continuous and isotropic.But that is different from the fact.Another weakness is that the thorax of the body has the following links:articulationes stemocostales,articulatio costovertebralis,and articulatio costotransversaria,but the model does not.The absence of articulationes sternocostales matters especially, because it can influence the change of bias.The finite-element method and method of biomaterial tests of the specimen have its own advantages and disadvantages.The two methods should supplement each other so that results obtained can be more scientific and reasonable.