Current Issue : July - September Volume : 2013 Issue Number : 3 Articles : 4 Articles
Background: The end-systolic pressure-volume relationship is often considered as\r\na load-independent property of the heart and, for this reason, is widely used as\r\nan index of ventricular contractility. However, many criticisms have been\r\nexpressed against this index and the underlying time-varying elastance theory:\r\nfirst, it does not consider the phenomena underlying contraction and second, the\r\nend-systolic pressure volume relationship has been experimentally shown to be\r\nload-dependent.\r\nMethods: In place of the time-varying elastance theory, a microscopic model of\r\nsarcomere contraction is used to infer the pressure generated by the contraction\r\nof the left ventricle, considered as a spherical assembling of sarcomere units. The\r\nleft ventricle model is inserted into a closed-loop model of the cardiovascular\r\nsystem. Finally, parameters of the modified cardiovascular system model are\r\nidentified to reproduce the hemodynamics of a normal dog.\r\nResults: Experiments that have proven the limitations of the time-varying\r\nelastance theory are reproduced with our model: (1) preload reductions, (2)\r\nafterload increases, (3) the same experiments with increased ventricular\r\ncontractility, (4) isovolumic contractions and (5) flow-clamps. All experiments\r\nsimulated with the model generate different end-systolic pressure-volume\r\nrelationships, showing that this relationship is actually load-dependent.\r\nFurthermore, we show that the results of our simulations are in good agreement\r\nwith experiments.\r\nConclusions: We implemented a multi-scale model of the cardiovascular system,\r\nin which ventricular contraction is described by a detailed sarcomere model.\r\nUsing this model, we successfully reproduced a number of experiments that have\r\nshown the failing points of the time-varying elastance theory. In particular, the\r\ndeveloped multi-scale model of the cardiovascular system can capture the loaddependence\r\nof the end-systolic pressure-volume relationship....
Background: The lumbar range of motion has traditionally been used to assess\r\ndisability in patients with low back disorders. Controversy exists about how\r\nmovement ranges in static positions or in a single straight plane is related to the\r\nfunctional status of the patients. The trunk circumduction, as the result of\r\nneuromuscular coordination, is the integrated movements from three dimensions.\r\nThe functional workspace stands for the volume of movement configuration from\r\nthe trunk circumduction and represents all possible positions in three dimensions. By\r\nusing single quantitative value, the functional workspace substitutes the complicated\r\njoint linear or angular motions. The aim of this study is to develop the functional\r\nworkspace of the trunk circumduction (FWTC) considering possible functional\r\npositions in three dimensional planes. The reliability of the trunk circumduction is\r\nexamined.\r\nMethods: Test-retest reliability was performed with 18 healthy young subjects. A\r\nthree-dimensional (3-D) Motion Analysis System was used to record the trunk\r\ncircumduction. The FWTC was defined and calculated based on the volume of the\r\ncone that was formed as the resultant scanned area of markers, multiplied by the\r\nlength of the body segment. The statistical analysis of correlation was performed to\r\ndescribe the relation of maximal displacements of trunk circumduction and straight\r\nplanes: sagittal and coronal.\r\nResults: The results of this study indicate that the movement of trunk circumduction\r\nmeasured by motion analysis instruments is a reliable tool. The ICC value is 0.90-0.96,\r\nand the means and standard deviations of the normalized workspace are: C7 0.425\r\n(0.1162); L1 0.843 (0.2965); and knee 0.014 (0.0106). Little correlations between the\r\nmaximal displacement of trunk circumduction and that of straight planes are shown\r\nand therefore suggest different movement patterns exist.\r\nConclusions: This study demonstrates high statistical reliability for the FWTC, which\r\nis important for the potential development as the functional assessment technique.\r\nThe FWTC provides a single integrated value to represent angular and linear\r\nmeasurements of different joints and planes. Future study is expected to carry out\r\nthe FWTC to evaluate the amount of workspace for the functional status of patients\r\nwith low back injuries or patients with spinal surgery....
Background: Extracorporeal membrane oxygenation (ECMO) can replace the lungs�\r\ngas exchange capacity in refractory lung failure. However, its limited\r\nhemocompatibility, the activation of the coagulation and complement system as\r\nwell as plasma leakage and protein deposition hamper mid- to long-term use and\r\nhave constrained the development of an implantable lung assist device. In a tissue\r\nengineering approach, lining the blood contact surfaces of the ECMO device with\r\nendothelial cells might overcome these limitations. As a first step towards this aim,\r\nwe hypothesized that coating the oxygenator�s gas exchange membrane with\r\nproteins might positively influence the attachment and proliferation of arterial\r\nendothelial cells.\r\nMethods: Sheets of polypropylene (PP), polyoxymethylpentene (TPX) and\r\npolydimethylsiloxane (PDMS), typical material used for oxygenator gas exchange\r\nmembranes, were coated with collagen, fibrinogen, gelatin or fibronectin. Tissue\r\nculture treated well plates served as controls. Endothelial cell attachment and\r\nproliferation were analyzed for a period of 4 days by microscopic examination and\r\ncomputer assisted cell counting.\r\nResults: Endothelial cell seeding efficiency is within range of tissue culture treated\r\ncontrols for fibronectin treated surfaces only. Uncoated membranes as well as all\r\nother coatings lead to lower cell attachment. A confluent endothelial cell layer\r\ndevelops on fibronectin coated PDMS and the control surface only.\r\nConclusions: Fibronectin increases endothelial cells� seeding efficiency on different\r\noxygenator membrane material. PDMS coated with fibronectin shows sustained cell\r\nattachment for a period of four days in static culture conditions....
Background: Predictions of the forces transmitted by the redundant force-bearing\r\nstructures in the knee are often performed using optimization methods considering\r\nonly moment equipollence as a result of simplified knee modeling without ligament\r\ncontributions. The current study aimed to investigate the influence of model\r\ncomplexity (with or without ligaments), problem formulation (moment equipollence\r\nwith or without force equipollence) and optimization criteria on the prediction of\r\nthe forces transmitted by the force-bearing structures in the knee.\r\nMethods: Ten healthy young male adults walked in a gait laboratory while their\r\nkinematic and ground reaction forces were measured simultaneously. A validated 3D\r\nmusculoskeletal model of the locomotor system with a knee model that included\r\nmuscles, ligaments and articular surfaces was used to calculate the joint resultant\r\nforces and moments, and subsequently the forces transmitted in the considered\r\nforce-bearing structures via optimization methods. Three problem formulations with\r\neight optimization criteria were evaluated.\r\nResults: Among the three problem formulations, simultaneous consideration of\r\nmoment and force equipollence for the knee model with ligaments and articular\r\ncontacts predicted contact forces (first peak: 3.3-3.5 BW; second peak: 3.2-4.2 BW;\r\nswing: 0.3 BW) that were closest to previously reported theoretical values (2.0-4.0 BW)\r\nand in vivo data telemetered from older adults with total knee replacements (about\r\n2.8 BW during stance; 0.5 BW during swing). Simultaneous consideration of moment\r\nand force equipollence also predicted more physiological ligament forces (< 1.0 BW),\r\nwhich appeared to be independent of the objective functions used. Without\r\nconsidering force equipollence, the calculated contact forces varied from 1.0 to 4.5 BW\r\nand were as large as 2.5 BW during swing phase; the calculated ACL forces ranged from\r\n1 BW to 3.7 BW, and those of the PCL from 3 BW to 7 BW.\r\nConclusions: Model complexity and problem formulation affect the prediction of the\r\nforces transmitted by the force-bearing structures at the knee during normal level\r\nwalking. Inclusion of the ligaments in a knee model enables the simultaneous\r\nconsideration of equations of force and moment equipollence, which is required for\r\naccurately estimating the contact and ligament forces, and is more critical than the\r\nadopted optimization criteria....
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