FSI Analysis of a healthy and a stenotic human trachea under impedance-based boundary conditions.

In this work, a fluid-solid interaction (FSI) analysis of a healthy and a stenotic human trachea was studied to evaluate flow patterns, wall stresses, and deformations under physiological and pathological conditions. The two analyzed tracheal geometries, which include the first bifurcation after the carina, were obtained from computed tomography images of healthy and diseased patients, respectively. A finite element-based commercial software code was used to perform the simulations. The tracheal wall was modeled as a fiber reinforced hyperelastic solid material in which the anisotropy due to the orientation of the fibers was introduced. Impedance-based pressure waveforms were computed using a method developed for the cardiovascular system, where the resistance of the respiratory system was calculated taking into account the entire bronchial tree, modeled as binary fractal network. Intratracheal flow patterns and tracheal wall deformation were analyzed under different scenarios. The simulations show the possibility of predicting, with FSI computations, flow and wall behavior for healthy and pathological tracheas. The computational modeling procedure presented herein can be a useful tool capable of evaluating quantities that cannot be assessed in vivo, such as wall stresses, pressure drop, and flow patterns, and to derive parameters that could help clinical decisions and improve surgical outcomes.

FSI analysis of a human trachea before and after prosthesis implantation.

In this work we analyzed the response of a stenotic trachea after a stent implantation. An endotracheal stent is the common treatment for tracheal diseases such as stenosis, chronic cough, or dispnoea episodes. Medical treatment and surgical techniques are still challenging due to the difficulties in overcoming potential complications after prosthesis implantation. A finite element model of a diseased and stented trachea was developed starting from a patient specific computerized tomography (CT) scan. The tracheal wall was modeled as a fiber reinforced hyperelastic material in which we modeled the anisotropy due to the orientation of the collagen fibers. Deformations of the tracheal cartilage rings and of the muscular membrane, as well as the maximum principal stresses, are analyzed using a fluid solid interaction (FSI) approach. For this reason, as boundary conditions, impedance-based pressure waveforms were computed modeling the nonreconstructed vessels as a binary fractal network. The results showed that the presence of the stent prevents tracheal muscle deflections and indicated a local recirculatory flow on the stent top surface which may play a role in the process of mucous accumulation. The present work gives new insight into clinical procedures, predicting their mechanical consequences. This tool could be used in the future as preoperative planning software to help the thoracic surgeons in deciding the optimal prosthesis type as well as its size and positioning.

Cryopreserved homografts in aortic and mitral prosthetic endocarditis: expanding the use of biological tissues in complex cardiac infections.

The case of a 27-year-old male heroin addict suffering from mitral and aortic prosthetic valve endocarditis is presented. Double valve re-replacement was performed using cryopreserved aortic homografts. Aortic root replacement with coronary re-implantation and intra-atrial valve implantation for mitral valve replacement were the techniques used. Despite the fatal outcome of this case, it clearly illustrates the possibilities of expanding the indications for combined complex replacement of heart valves by using fully biological tissue of human origin.

A pre-operative planning for endoprosthetic human tracheal implantation: a decision support system based on robust design of experiments.

Swallowing depends on physiological variables that have a decisive influence on the swallowing capacity and on the tracheal stress distribution. Prosthetic implantation modifies these values and the overall performance of the trachea. The objective of this work was to develop a decision support system based on experimental, numerical and statistical approaches, with clinical verification, to help the thoracic surgeon in deciding the position and appropriate dimensions of a Dumon prosthesis for a specific patient in an optimal time and with sufficient robustness. A code for mesh adaptation to any tracheal geometry was implemented and used to develop a robust experimental design, based on the Taguchi's method and the analysis of variance. This design was able to establish the main swallowing influencing factors. The equations to fit the stress and the vertical displacement distributions were obtained. The resulting fitted values were compared to those calculated directly by the finite element method (FEM). Finally, a checking and clinical validation of the statistical study were made, by studying two cases of real patients. The vertical displacements and principal stress distribution obtained for the specific tracheal model were in agreement with those calculated by FE simulations with a maximum absolute error of 1.2 mm and 0.17 MPa, respectively. It was concluded that the resulting decision support tool provides a fast, accurate and simple tool for the thoracic surgeon to predict the stress state of the trachea and the reduction in the ability to swallow after implantation. Thus, it will help them in taking decisions during pre-operative planning of tracheal interventions.

CFD analysis of the human airways under impedance-based boundary conditions: application to healthy, diseased and stented trachea.

A computational fluid dynamics model of a healthy, a stenotic and a post-operatory stented human trachea was developed to study the respiration under physiological boundary conditions. For this, outflow pressure waveforms were computed from patient-specific spirometries by means of a method that allows to compute the peripheral impedance of the truncated bronchial generation, modelling the lungs as fractal networks. Intratracheal flow pattern was analysed under different scenarios. First, results obtained using different outflow conditions were compared for the healthy trachea in order to assess the importance of using impedance-based conditions. The resulted intratracheal pressures were affected by the different boundary conditions, while the resulted velocity field was unaffected. Impedance conditions were finally applied to the diseased and the stented trachea. The proposed impedance method represents an attractive tool to compute physiological pressure conditions that are not possible to extract in vivo. This method can be applied to healthy, pre- and post-operatory tracheas showing the possibility of predicting, through numerical simulation, the flow and the pressure field before and after surgery.

Atrial Fibrillation and Stroke Risk After Coronary Artery Bypass Grafting Surgery.

Background: The present multicentre study was aimed at determining the effect of preoperative atrial fibrillation (preop-AF) as stroke risk factor in coronary artery bypass graft surgery (CABG) during the perioperative period. Methods: Patients undergoing isolated CABG surgery were enrolled from 21 Spanish centers. Baseline variables related with perioperative stroke risk were recorded and analysed. The Northern New England Cardiovascular Disease Study Group (NNECVDSG) stroke risk schema was used to stratify stroke risk and compare predicted vs observed neurologic outcomes in this study. Results: 26347 patients were enrolled in the study. Prevalence of preop-AF was 4.2%, and was associated significantly with major cardiovascular comorbidities. The stroke rate was 1.38% (365 strokes), and it was slightly higher for patients with preop-AF vs non preop-AF, 1.82% vs 1.36%, p = 0.2. NNECVDSG schema showed good predictive ability calculating the area under the receiver operating characteristic curve (c-statistic 0.696; 95% CI 0.668 to 0.723). To investigate the associations of baseline preoperative variables with perioperative CABG-stroke a logistic regression model was performed. Preop-AF impact on perioperative stroke was lower that other variables. Preop-AF did not show an adverse impact in the quartiles groups according to NNECVDSG Stroke Risk Index. Conclusion: Risk of perioperative stroke in isolated CABG surgery patients is not significantly increased by preop-AF.

FSI analysis of the coughing mechanism in a human trachea.

The main physiological function of coughing is to remove from the airways the mucus and foreign particles that enter the lungs with respirable air. However, in patients with endotracheal tubes, further surgery has to be performed to improve cough effectiveness. Thus, it is necessary to analyze how this process is carried out in healthy tracheas to suggest ways to improve its efficacy in operated patients. A finite element model of a human trachea is developed and used to analyze the deformability of the tracheal walls under coughing. The geometry of the trachea is obtained from CT of a 70-year-old male patient. A fluid structure interaction approach is used to analyze the deformation of the wall when the fluid (in this case, air) flows inside the trachea. A structured hexahedral-based grid for the tracheal walls and an unstructured tetrahedral-based mesh with coincident nodes for the fluid are used to perform the simulations with the finite element-based commercial software code (ADINA R&D Inc.). Tracheal wall is modeled as an anisotropic fiber reinforced hyperelastic solid material in which the different orientation of the fibers is introduced. The implantation of an endotracheal prosthesis is simulated. Boundary conditions for breathing and coughing are applied at the inlet and at the outlet surfaces of the fluid mesh. The collapsibility of a human trachea under breathing and coughing is shown in terms of flow patterns and wall stresses. The ability of the model to reproduce the normal breathing and coughing is proved by comparing the deformed shape of the trachea with experimental results. Moreover the implantation of an endotracheal prosthesis would be related with a decrease of coughing efficiency, as clinically seen.

Experimental characterization and constitutive modeling of the mechanical behavior of the human trachea.

BACKGROUND AND AIMS: Cartilage and smooth muscle constitute the main structural components of the human central airways, their mechanical properties affect the flow in the trachea and contribute to the biological function of the respiratory system. The aim of this work is to find out the mechanical passive response of the principal constituents of the human trachea under static tensile conditions and to propose constitutive models to describe their behavior. METHODS: Histological analyses to characterize the tissues and mechanical tests have been made on three human trachea specimens obtained from autopsies. Uniaxial tensile tests on cartilaginous rings and smooth muscle were performed. Tracheal cartilage was considered an elastic material and its Young's modulus and Poisson's coefficient were determined fitting the experimental curves using a Neo-Hookean model. The smooth muscle was proved to behave as a reinforced hyperelastic material with two families of collagen fibers, and its non-linearity was investigated using the Holzapfel strain-energy density function for two families of fibers to fit the experimental data obtained from longitudinal and transversal cuts. RESULTS: For cartilage, fitting the experimental curves to an elastic model, a Young's modulus of 3.33 MPa and nu=0.49 were obtained. For smooth muscle, several parameters of the Holzapfel function were found out (C(10)=0.877 kPa, k(1)=0.154 kPa, k(2)=34.157, k(3)=0.347 kPa and k(4)=13.889) and demonstrated that the tracheal muscle was stiffer in the longitudinal direction. CONCLUSION: The better understanding of how these tissues mechanically behave is essential for a correct modeling of the human trachea, a better simulation of its response under different loading conditions, and the development of strategies for the design of new endotracheal prostheses.

Patient-specific models of human trachea to predict mechanical consequences of endoprosthesis implantation.

Nowadays, interventions associated with the implantation of tracheal prostheses in patients with airway pathologies are very common. This surgery may promote problems such as migration of the prosthesis, development of granulation tissue at the edges of the stent with overgrowth of the tracheal lumen or accumulation of secretions inside the prosthesis. Among the movements that the trachea carries out, swallowing seems to have harmful consequences for the tracheal tissues surrounding the prosthesis. In this work, a finite-element-based tool is presented to construct patient-specific tracheal models, introducing the endotracheal prosthesis and analysing the mechanical consequences of this surgery during swallowing. A complete description of a patient-specific tracheal model is given, and a fully experimental characterization of the tracheal tissues is presented. To construct patient-specific grids, a mesh adaptation algorithm has been developed and the implantation of a tracheal prosthesis is simulated. The ascending deglutition movement of the trachea is recorded using real data from each specific patient from fluoroscopic images before and after implantation. The overall behaviour of the trachea is modified when a prosthesis is introduced. The presented tool has been particularized for two different patients (patient A and patient B), allowing prediction of the consequences of this kind of surgery. In particular, patient A had a decrease of almost 30 per cent in his ability to swallow, and an increase in stresses that were three times higher after prosthesis implantation. In contrast, patient B, who had a shorter trachea and who seemed to undergo more damaging effects, did not have a significant reduction in his ability to swallow and did not present an increase in stress in the tissues. In both cases, there are clinical studies that validate our results: namely, patient A underwent a further intervention whereas the outcome of patient B's surgery was completely successful. Notwithstanding the fact that there are a lot of uncertainties relating to the implantation of endotracheal prostheses, the present work gives a new insight into these procedures, predicting their mechanical consequences. This tool could be used in the future as pre-operative planning software to help thoracic surgeons in deciding the optimal prosthesis as well as its size and positioning.