Study On Assesseing And Analyzing Viscoelastic Properties Of Relaxed Myocardium

At present,cardiac diastolic dysfunction does harm to the health of human increasingly. More and more emphasis is given to relaxation dysfunction of myocardium in clinic. Now diagnosis and treatment of diastolic heart failure ( DHF ) has become an important topic in cardiovascular field. Incommensurately, the recognition of DHF is insufficient and accurate and accepted in accord with assessing method is absent in clinic. Further physiopathologic recognition of DHF is necessary and the level on diagnosis and treatment of DHF needs improve. Drawing assistance from the ground of cardiac catheter and ultrasound catheter manufacture, this study improved intervention ultrasound indentation system (IUIS) which mainly consisted of ultrasound(US) transducer and pressure transducer, and on basis of stress-strain relation correlating with time in viscoelasticity, investigated using IUIS to assess the creep viscoelastic properties in vivo in diastole, which is deformation changing with time under constant indentation force. These viscoelastic parameters were obtained from the biomechanical model and measurement signal (e.g., force and corresponding deformation ) in myocardium. The difference of viscoelastic parameters between normal and infarcted myocardium will be investigate and further more the pathomechanism of the difference will also be investigated. It will deepen physiopathologic understanding of DHF from biomechanical angle, and lead to new detection techniques and information for diagnosis of DHF, lead to potential target treating cardiac diastolic dysfunction.1. The improvement of IUISDrawing assistance from the ground of cardiac catheter and ultrasound catheter manufacture, this study improved IUIS which consisted of intervention ultrasound catheter (composed of US micro-transducer assessing myocardium deformation and pressure transducer assessing applied force) and integrated electrocardiosignal through increasing san frame frequency. Improved IUIS can get the good time resolution and satisfy the detection of strain-stress-time relation in soft tissue. Improved IUIS can be used to assess viscoelastic properties of relaxed myocardium in diastole in vivo.2. Experimental study on viscoelasticity of relaxed myocardiumOld myocardial infarction model of canine is set up through ligating left anterior descending branch ( LAD)for three months. Creep of myocardium in sham and the old myocardial infarction (OMI) group were assessed respectively using IUIS in middle and advanced diastole stage in vivo. Then standard indentation creep test was carried out in isolated myocardium in above-mentioned canine using load-deformation indentation experimental facility. Test data in vivo and vitro were processed with three-parameter solid viscoelastic model and viscoelastic parameters of normal and infarcted myocardium could be obtained. Results: Creep data curves of sham and OMI group were obtained in vivo and vitro. These data were processed through curve fitting with three-parameter model. The results of curve fitting is good (p<0.01). In vivo, comparing with sham group, parameters of E₁,E_∞,E₂,ηin OMI group all increased obviously(p<0.05), they were as follows: 26.48±7.87 VS 6.97±2.95 kPa,16.73±3.85 VS 4.40±1.61 kPa,49.04±15.03 VS 16.30±12.63 kPa,1.84±0.84×103 VS 0.63±0.51×10³ kPa.s. In vitro, comparing with sham group, parameters of E₁,E_∞,E₂,ηin OMI group all increased obviously(p<0.05), they were as follows: 23.36±3.47 VS 12.77±2.76 kPa,21.48±2.70 VS 11.91±2.52 kPa,300.30±105.66 VS 183.80±46.54 kPa,2.32±0.40×10⁴ VS 1.51±0.66×10⁴ kPa.s. The two approaches were highly correlated ( r>0.8)with linear correlation analysis. Conclusion: In the later period of myocardial infarction, elastic modulus and viscosity resistance of infarcted myocardium both increase, myocardial stiffness increased, while myocardial complaisance decreased. Intervention indentation method is a direct and feasible method used to assess viscoelastic properties of relaxed myocardium in vivo. Three-parameter viscoelastic model can be used to describe creep viscoelastic properties of relaxed myocardium.3.Study on pathomechanism affecting viscoelasticity of relaxed myocardium in old myocardial infarctionThe myocardium samples mentioned above were respectively detected by the following methods: hematoxylin-eosin staining(HE) observing myocardium pathology structural change, triphenytetrazolium chloride staining(TTC) assessing myocardial infarction degree, hydroxyproline concentration determination evaluating collagen contents, sirius red staining and polarizing glass determining collagen volume fraction(CVF) and ratio of I/III collagen of myocardium. Then correlated analysis was carried out between above-mentioned pathology changes and myocardium viscoelastic parameters, so as to investigate pathological factors which affected myocardial elasticity and viscidity. Results: High positive correlation existed between E₁ and myocardial infarction degree,collagen contents,and ratio of I/III collagen. High positive correlation existed betweenηand collagen contents,CVF. High negative correlation existed betweenηand ratio of I/III collagen in sham group. Conclusion:After myocardial infarction, myocardial stiffness increased with infarction degree,collagen deposition,and ratio of I/III collagen increasing; viscosity resistance of myocardium increased when collagen deposition increased. In the latter period in myocardial infarction infarction degree,collagen deposition,ratio of I/III collagen will obviously affect viscoelastic properties of myocardium.4.Change on viscoelasticity of relaxed myocardium after AMI and it's pathomechanismMyocardium creep tests were respectively carried out using IUIS on three healthy mongrels before myocardial infarction, at 1 hour after acute myocardial infarction (AMI) , and 3 hour after AMI in diastole in vivo. Then myocardium samples were detected by these methods as follows : HE staining, transmission electron microscope (TEM) , immunohistochemistry of cytoskeletal protein (microtubulin and desmin). Correlation analysis was carried out between the detection results and viscoelastic parameters. Results: Comparing with NAMI group, parameters of E_∞,E₁ in AMI-1 and AMI-3 group both increased obviously(p<0.05), they were as follows: 5.28±0.15,5.61±0.08 VS 4.31±0.33 kPa;6.84±0.18,11.71±0.89 VS 5.04±0.37 kPa. Comparing with NAMI group, parameters ofÏ„in AMI-3 group increased obviously(p<0.05), they were as follows: 58.98±5.97 VS 23.37±1.99s. Comparing with NAMI group, parameters of E2,ηin AMI-1 and AMI-3 group both decreased obviously(p<0.05), they were as follows: 23.09±1.26,1 0.83±0.75 VS 29.54±2.78 kPa; 0.62±0.25×10³,0.64±0.21×10³ VS 0.75±0.49×10³ kPa.s. Myocardium interstitial edema, myofilament contracture, myofilament dissolving, and myofilament disruption were observed at 3 hour after AMI. High positive correlation existed between the decrease of intracellular microtubule and E₂,Ï„at 3 hour after AMI (|r|>0.8). Conclusion : Myocardial stiffness increased while viscosity resistance decreased in 3 hours after AMI, partly being attributed to myocardium interstitial edema and decrease of intracellular microtubule.