Kiran H. J. Dellimore

Application of finite element analysis to the design of tissue leaflets for a percutaneous aortic valve

Percutaneous Aortic Valve (PAV) replacement is an attractive alternative to open heart surgery, especially for patients considered to be poor surgical candidates. Despite this, PAV replacement still has its limitations and associated risks. Bioprosthetic heart valves still have poor long-term durability due to calcification and mechanical failure. In addition, the implantation procedure often presents novel challenges, including damage to the expandable stents and bioprosthetic leaflets. In this study, a simplified version of Fungs elastic constitutive model for skin, developed by Sun and Sacks, was implemented using finite element analysis (FEA) and applied to the modelling of bovine and kangaroo pericardium. The FEA implementation was validated by simulating biaxial tests and by comparing the results with experimental data. Concepts for different PAV geometries were developed by incorporating valve design and performance parameters, along with stent constraints. The influence of effects such as different leaflet material, material orientation and abnormal valve dilation on the valve function was investigated. The stress distribution across the valve leaflet was also examined to determine the appropriate fibre direction for the leaflet. The simulated attachment forces were compared with suture tearing tests performed on the pericardium to evaluate suture density. It is concluded that kangaroo pericardium is suitable for PAV applications, and superior to bovine pericardium, due to its lower thickness and greater extensibility.

The impact of backboard size and orientation on sternum-to-spine compression depth and compression stiffness in a manikin study of CPR using two mattress types

OBJECTIVES To explore how backboard orientation and size impact chest compressions during cardiopulmonary resuscitation (CPR). METHODS Experiments were conducted on a full-body CPR training manikin using a custom-built simulator. Two backboards of different sizes were tested in longitudinal (head to toe) and latitudinal (side to side) directions to assess the impact of size and orientation on chest compressions during CPR. The net sternum-to-spine displacement, combined mattress and sternal displacement as well as the axial reaction force were measured during each test. RESULTS The difference in net compression depth between the larger and smaller backboards ranged between 0.08±0.30 cm and 1.47±0.13 cm, while the difference in back support stiffness varied between 103.7±211 N/cm and 688.1±180.3 N/cm. The difference in net compression depth between the longitudinal and latitudinal backboard orientations ranged from 0.07±0.32 cm to 0.34±0.18 cm, while for the back support stiffness the difference was between 13.4±50.0 N/cm and 592.2±211.0 N/cm. CONCLUSIONS The effect of backboard size on chest compression (CC) performance during CPR was found to be significant with the larger backboard producing deeper chest compressions and higher back support stiffness than the smaller backboard. The impact of backboard orientation was found to depend on the size of the backboard and type of mattress used. Clinicians should be aware that although a smaller backboard may be easier for rescuers to manipulate, it does not provide as effective back support or produce as deep chest compressions as a larger backboard.

Development of a diagnostic glove for unobtrusive measurement of chest compression force and depth during neonatal CPR

Optimizing chest compression (CC) performance during neonatal cardiopulmonary resuscitation (CPR) is critical to improving survival outcomes since current clinical protocols often achieve only a fraction of the native cardiovascular perfusion. This study presents the development of a diagnostic tool to unobtrusively measure the CC depth and force during neonatal CPR using sensors mounted on a glove platform. The performance of the glove was evaluated by infant manikin tests using the two-thumb (TT) and two-finger (TF) methods of CC during simulated, unventilated neonatal CPR. The TT method yielded maximum CC depths and forces of as much as 25.7 ± 3.2 mm and 35.9 ± 2.2 N while the TF method produced CC depths and forces of as much as 21.6 ± 2.2 mm and 23.7 ± 2.9 N. These results are consistent with clinical findings which suggest that TT compression is more effective than TF compression since it produces greater CC depths and forces.

Quantification of numerical uncertainty in computational fluid dynamics modelling of hydrocyclones

Abstract Large Eddy Simulations of the flow through a hydrocyclone are used to demonstrate that the Grid Convergence Index (GCI) is a practical method of accounting for numerical uncertainty. The small values of GCI (

A modeling approach to the effects of force guided versus depth guided compression during cardiopulmonary resuscitation on different chests and back support surfaces

OBJECTIVES To validate an existing theoretical model for the mechanics of chest compression (CC) during constant peak force cardiopulmonary resuscitation (CPR) using experimental human and manikin CC data from the literature. Also, to gain insights into the clinical application of force guided CPR. METHODS The experimental CC data from the literature were analyzed and compared to theoretical predictions from the constant peak force CPR model. The CPR model was also used to explore how CC rate and peak sternal force may influence CC performance during the clinical application of force guided CPR. RESULTS The model predictions matched the human CC data to within an average difference of less than 1.5% at CC rates of 60 cpm and 90 cpm, and 0.6% for the manikin data at a CC rate of 90 cpm. The model predictions also showed that the net sternum-to-spine compression depth achieved during force guided CPR strongly depends on the patients thoracic stiffness. CONCLUSIONS Good quantitative agreement between the experimental data from the literature and the theoretical model suggests that the constant peak force CPR model developed by Boe and Babbs provides reasonable prediction of CC mechanics during CPR over a range of clinically relevant CC rates. The model predictions also suggest that the effectiveness of CC during force guided CPR is highly sensitive to the patients thoracic stiffness and insensitive to the back support stiffness. Patients having high thoracic stiffness (≥ 100 Ncm(-1)) were found to require higher CC forces, which may exceed the force above which severe chest wall trauma and abdominal injury occurs, in order to achieve the ERC recommended CC depth range. This suggests that the choice of maximum sternal force applied by clinicians during constant peak force CPR ought to be based on a general assessment of the patients thoracic stiffness.

Two-dimensional analytical model of heat transfer for flames in channels

A two-dimensional model for heat transfer in reacting channel flow with a constant wall temperature is developed along with an analytical solution that relates the temperature field in the channel to the flow Peclet number. The solution is derived from first principles by modeling the flame as a volumetric heat source and by applying jump conditions across the flame for plug and Hagen-Poiseuille velocity profiles and is validated via comparison with more detailed computational fluid dynamics solutions. The analytical solution provides a computationally efficient tool for exploring the effects of varying channel height and gas velocity on the temperature distribution in a channel in which a flame is stabilized. The results show that the Peclet number is the principal parameter controlling the temperature distribution in the channel. It is also found that although the Nusselt number is independent of the Peclet number (or velocity) in the postflame region, it can change by nearly 3 ord ers o f magnitude in the preflame region over the range of Peclet numbers (or velocities) expected in microcombustors. This has important implications for quasi-onedimensional numerical modeling of micro/mesoscale combustion, in which it is usual to select a single Nusselt value from the heat transfer literature.Acorrelation to facilitate incorporation of the streamwise Nusselt number variation is provided.

FUEL -AIR MIXING C HALLENGES IN MICRO -POWER SYSTEMS

A simple model has been developed for the scaling of various Reynolds numbers associated with fuel -air mixing in a micro -power system as a function of its overall size and power output. The model is explained and results are presented showing that flows in micro -power systems can vary widely with Reynolds numbers spanning laminar, transitional, and fully turbulent regimes. In general, however, the Reynolds numbers associated with micro -power systems are much smaller than in their conventional -scale counterparts suggesting that ach ieving adequate fuel -air mixing wil l be more difficult than in conventional - scale power systems. A survey of the literature indicates that there is an absence of data in the appropriate Reynolds number range . As a result, predicting the perfo rmance of micro -scale mixers may prove difficul t.

Optimal chest compression in cardiopulmonary resuscitation depends upon thoracic and back support stiffness

A biomechanical analysis of the constant peak displacement and constant peak force methods of cardiopulmonary resuscitation (CPR) has revealed that optimal CC performance strongly depends on back support stiffness, CC rate, and the thoracic stiffness of the patient being resuscitated. Clinically the results presented in this study suggest that the stiffness of the back support surfaces found in many hospitals may be sub-optimal and that a backboard or a concrete floor can be used to enhance CC effectiveness. In addition, the choice of optimal CC rate and maximum sternal force applied by clinicians during peak force CPR is ought to be based on a general assessment of the patient’s thoracic stiffness, taking into account the patient’s age, gender, and physical condition; which is consistent with current clinical practice. In addition, it is important for clinicians to note that very high peak sternal forces, exceeding the limit above which severe chest wall trauma and abdominal injury occurs, may be required for optimal CC during peak force CPR on patients with very stiff chests. In these cases an alternative CPR technique may be more appropriate.

Feasibility of pulse presence and pulse strength assessment during head-up tilt table testing using an accelerometer located at the carotid artery.

Neurally mediated syncope (NMS) is a disorder of the autonomic regulation of postural tone, which is characterized by hypotension and/or bradycardia, resulting in cerebral hypo-perfusion and finally in a sudden loss of consciousness. Prediction of an impending NMS requires detection of pulse presence to derive heart rate (HR) as well as to assess the pulse strength (PS) related to systolic blood pressure (SBP) preferably from a single body location only. This paper analyses the basic feasibility of using a single accelerometer positioned above the common carotid artery to assess pulse strength and pulse rate towards NMS prediction. A physical model has been investigated to gain insights into expected signal morphologies and potential feature candidates vs. hemodynamic parameters such as SBP, pulse pressure (PP) and PR relevant for NMS detection. Model results are compared with first measurements obtained in a head-up tilt table test (HUTT) from a patient during impending syncope. We show that an accelerometer positioned at the carotid artery is a potential approach offering a valuable tool in syncope management.

Development of a real-time feedback algorithm for chest compression during CPR without assuming full chest decompression

OBJECTIVES To evaluate the performance of a real-time feedback algorithm for chest compression (CC) during cardiopulmonary resuscitation (CPR), which provides accurate estimation of the CC depth based on dual accelerometer signal processing, without assuming full CDC. Also, to explore the influence of incomplete chest decompression (CDC) on the CC depth estimation performance. METHODS The performance of a real-time feedback algorithm for CC during CPR was evaluated by comparison with an offline algorithm using adult CPR manikin CC data obtained under various conditions. RESULTS The real-time algorithm, using non-causal baselining, delivered comparable CC depth estimation accuracy to the offline algorithm on both soft and hard back support surfaces. In addition, for both algorithms incomplete CDC led to underestimation of the CC depth. CONCLUSIONS CPR feedback systems which utilize an assumption of full CDC may be unreliable especially in long duration CPR events where rescuer fatigue can strongly influence CC quality. In addition, these systems may increase the risk of thoracic and abdominal injury during CPR since rescuers may apply excessive compression forces due to underestimation of the CC depth when incomplete CDC occurs. Hence, there is a strong need for CPR feedback systems to accurately measure CDC in order to improve their clinical effectiveness.