Biomedical subjects

Houston G Wood

Publications and source records attributed to Houston G Wood.

16 recordsLinked to original sources

PubMed2005

Methods of failure and reliability assessment for mechanical heart pumps.

Artificial blood pumps are today's most promising bridge-to-recovery (BTR), bridge-to-transplant (BTT), and destination therapy solutions for patients suffering from intractable congestive heart failure (CHF). Due to an increased need for effective, reliable, and safe long-term artificial blood pumps, each new design must undergo failure and reliability testing, an important step prior to approval from the United States Food and Drug Administration (FDA), for clinical testing and commercial use. The FDA has established no specific standards or protocols for these testing procedures and there are only limited recommendations provided by the scientific community when testing an overall blood pump system and individual system components. Product development of any medical device must follow a systematic and logical approach. As the most critical aspects of the design phase, failure and reliability assessments aid in the successful evaluation and preparation of medical devices prior to clinical application. The extent of testing, associated costs, and lengthy time durations to execute these experiments justify the need for an early evaluation of failure and reliability. During the design stages of blood pump development, a failure modes and effects analysis (FMEA) should be completed to provide a concise evaluation of the occurrence and frequency of failures and their effects on the overall support system. Following this analysis, testing of any pump typically involves four sequential processes: performance and reliability testing in simple hydraulic or mock circulatory loops, acute and chronic animal experiments, human error analysis, and ultimately, clinical testing. This article presents recommendations for failure and reliability testing based on the National Institutes of Health (NIH), Society for Thoracic Surgeons (STS) and American Society for Artificial Internal Organs (ASAIO), American National Standards Institute (ANSI), the Association for Advancement of Medical Instrumentation (AAMI), and the Bethesda Conference. It further discusses studies that evaluate the failure, reliability, and safety of artificial blood pumps including in vitro and in vivo testing. A descriptive summary of mechanical and human error studies and methods of artificial blood pumps is detailed.

Animals↗

PubMed2005

Numerical and experimental analysis of an axial flow left ventricular assist device: the influence of the diffuser on overall pump performance.

Thousands of adult cardiac failure patients may benefit from the availability of an effective, long-term ventricular assist device (VAD). We have developed a fully implantable, axial flow VAD (LEV-VAD) with a magnetically levitated impeller as a viable option for these patients. This pump's streamlined and unobstructed blood flow path provides its unique design and facilitates continuous washing of all surfaces contacting blood. One internal fluid contacting region, the diffuser, is extremely important to the pump's ability to produce adequate pressure but is challenging to manufacture, depending on the complex blade geometries. This study examines the influence of the diffuser on the overall LEV-VAD performance. A combination of theoretical analyses, computational fluid (CFD) simulations, and experimental testing was performed for three different diffuser models: six-bladed, three-bladed, and no-blade configuration. The diffuser configurations were computationally and experimentally investigated for flow rates of 2-10 L/min at rotational speeds of 5000-8000 rpm. For these operating conditions, CFD simulations predicted the LEV-VAD to deliver physiologic pressures with hydraulic efficiencies of 15-32%. These numerical performance results generally agreed within 10% of the experimental measurements over the entire range of rotational speeds tested. Maximum scalar stress levels were estimated to be 450 Pa for 6 L/min at 8000 rpm along the blade tip surface of the impeller. Streakline analysis demonstrated maximum fluid residence times of 200 ms with a majority of particles exiting the pump in 80 ms. Axial fluid forces remained well within counter force generation capabilities of the magnetic suspension design. The no-bladed configuration generated an unacceptable hydraulic performance. The six-diffuser-blade model produced a flow rate of 6 L/min against 100 mm Hg for 6000 rpm rotational speed, while the three-diffuser-blade model produced the same flow rate and pressure rise for a rotational speed of 6500 rpm. The three-bladed diffuser configuration was selected over the six-bladed, requiring only an incremental adjustment in revolution per minute to compensate for and ease manufacturing constraints. The acceptable results of the computational simulations and experimental testing encourage final prototype manufacturing for acute and chronic animal studies.

Blood Circulation↗

PubMed2004

Treatment of full-thickness skin defect with concomitant grafting of 6-fold extended mesh auto-skin and allogeneic cultured dermal substitute.

The aim of this clinical study was to evaluate an allogeneic cultured dermal substitute (CDS) as a biological dressing for highly extended mesh auto-skin grafting. The subjects were five patients with extensive deep burn wounds. Allogeneic CDS was prepared by seeding fibroblasts on a spongy matrix of hyaluronic acid and atelo-collagen. Six-fold extended auto-skin graft was applied to the debrided wound, on which allogeneic CDS was placed. A conventional ointment-gauze dressing was used to protect the CDS. The CDS was applied repeatedly at intervals of 5-7 days. In all cases, the wounds were closed by successful take of mesh auto-skin graft and prompt epithelization from the grafted skin. The skin on the grafted area had a cicatrix appearance, but was soft and thin, maintaining good quality. The application of 6-fold extended auto-skin graft in conjunction with allogeneic CDS is an effective method for treatment of extensive severe burn wounds.

Adult↗

PubMed2004

Inlet and outlet devices for rotary blood pumps.

The purposes of inlet and outlet devices for rotary blood pumps, including inducers and diffusers for axial pumps, inlet and exit volutes for centrifugal pumps, and inlet and outlet cannulas, are to guide the blood into the impeller, where the blood is accelerated, and to convert the high kinetic energy into pressure after the impeller discharge, respectively. The designs of the inlet and outlet devices have an important bearing on the pump performance. Their designs are highly dependent on computational fluid dynamics (CFD) analysis, guided by intuition and experience. For inlet devices, the design objectives are to eliminate separated flow, to minimize recirculation, and to equalize the radial components of velocity. For outlet devices, the design goals are to reduce speed, to minimize energy loss, and to avoid flow separation and whirl. CFD analyses indicate the velocity field and pressure distribution. Geometrical optimization of these components has been implemented in order to improve the flow pattern.

Assisted Circulation↗

PubMed2004

Computational Fluid Dynamics (CFD) study of the 4th generation prototype of a continuous flow Ventricular Assist Device (VAD).

The continuous flow ventricular assist device (VAD) is a miniature centrifugal pump, fully suspended by magnetic bearings, which is being developed for implantation in humans. The CF4 model is the first actual prototype of the final design product. The overall performances of blood flow in CF4 have been simulated using computational fluid dynamics (CFD) software: CFX, which is commercially available from ANSYS Inc. The flow regions modeled in CF4 include the inlet elbow, the five-blade impeller, the clearance gap below the impeller, and the exit volute. According to different needs from patients, a wide range of flow rates and revolutions per minute (RPM) have been studied. The flow rate-pressure curves are given. The streamlines in the flow field are drawn to detect stagnation points and vortices that could lead to thrombosis. The stress is calculated in the fluid field to estimate potential hemolysis. The stress is elevated to the decreased size of the blood flow paths through the smaller pump, but is still within the safe range. The thermal study on the pump, the blood and the surrounding tissue shows the temperature rise due to magnetoelectric heat sources and thermal dissipation is insignificant. CFD simulation proved valuable to demonstrate and to improve the performance of fluid flow in the design of a small size pump.

Blood Flow Velocity↗

PubMed2003

Studies of turbulence models in a computational fluid dynamics model of a blood pump.

Computational fluid dynamics (CFD) is used widely in design of rotary blood pumps. The choice of turbulence model is not obvious and plays an important role on the accuracy of CFD predictions. TASCflow (ANSYS Inc., Canonsburg, PA, U.S.A.) has been used to perform CFD simulations of blood flow in a centrifugal left ventricular assist device; a k-epsilon model with near-wall functions was used in the initial numerical calculation. To improve the simulation, local grids with special distribution to ensure the k-omega model were used. Iterations have been performed to optimize the grid distribution and turbulence modeling and to predict flow performance more accurately comparing to experimental data. A comparison of k-omega model and experimental measurements of the flow field obtained by particle image velocimetry shows better agreement than k-epsilon model does, especially in the near-wall regions.

Equipment Design↗

PubMed2003

Computational fluid dynamics prediction of blood damage in a centrifugal pump.

This study explores a quantitative evaluation of blood damage that occurs in a continuous flow left ventricular assist device due to fluid stress. Computational fluid dynamics (CFD) analysis is used to track the shear stress history of 388 particle streaklines. The accumulation of shear and exposure time is integrated along the streaklines to evaluate the levels of blood trauma. This analysis, which includes viscous and turbulent stresses, provides a statistical estimate of possible damage to cells flowing through the pump. In vitro normalized index of hemolysis values for clinically available ventricular assist devices were compared to our damage indices. This allowed for an order of magnitude comparison between our estimations and experimentally measured hemolysis levels, which resulted in a reasonable correlation. This work ultimately demonstrates that CFD is a convenient and effective approach to analyze the Lagranian behavior of blood in a heart assist device.

Centrifugation↗

PubMed2002

A prototype HeartQuest ventricular assist device for particle image velocimetry measurements.

The objective of this study is to fully characterize the flow within the HeartQuest ventricular assist device (VAD), a magnetically levitated centrifugal VAD, using particle image velocimetry (PIV) to identify regions of potential high shear or stagnation and validate and refine computational models of the flow. An acrylic model of the pump was designed and constructed to allow optical access into all interior regions of the pump. The geometry of the exterior housing and the use of a novel working fluid make quantitative measurements of velocity within the exit volute, blade passage, cut-water, blade tip clearance, and pump inlet possible. Highly accurate velocity measurements using particle PIV have been made in one region (the inlet elbow), and measurements in the other critical regions of the pump will be made. These measurements are used for investigation of regions with potential for hemolysis resulting from high shear stress or with potential for thrombosis caused by recirculation or stagnation. Quantitative velocity data are also needed for comparison with computational fluid dynamics (CFD) models of the VAD. In this study, experiments have again proven to be an essential complement to CFD for thorough investigations of the flow inside the pump.

Blood Flow Velocity↗

PubMedDate not supplied

Pediatric circulatory support systems.

Ventricular assist devices (VADs) are a valid option for long term circulatory support in pediatric patients with postoperative myocardial failure or debilitating heart defects. Most clinical experience to date has involved the short-term support of patients weighing 6 kg and larger. For cases of VAD implementation in pediatric patients, the assist device showed tremendous promise in reversing cardiac failure and providing adequate support as a bridge to cardiac transplantation. The Medos-HIA system, Berlin Heart, Medtronic Bio-Medicus Pump, Abiomed BVS 5000, Toyobo-Zeon pumps, and Hemopumps have proven successful for short-term circulatory support for the pediatric population. The Jarvik 2000 and Pierce-Donachy pediatric system further demonstrate the potential to be used for pediatric circulatory support. The clinical and experimental success of these support systems provide encouragement to believe that long-term support is possible.

Child↗

PubMedDate not supplied

Computational fluid dynamics modeling of impeller designs for the HeartQuest left ventricular assist device.

To finalize the design of the next generation of the HeartQuest left ventricular assist device, a suitable impeller had to be designed and tested. The new prototype was based on calculations and test results of previous designs, but required several changes to decrease the size. For most pump designs, this is a simple matter of altering impeller geometry and rotational speed to achieve the desired pressure rise and flow rate. However, this particular pump was limited by housing geometry and the magnetic bearings that support the impeller. Without much freedom in the overall impeller size, the only parameters open to the designers were the blade profiles and the rotating speed. Rather than build several candidates and test them in a rig at enormous cost, computational models of several designs were tested and analyzed. This not only saved money, but also sped up the development time for the project. The computer models were developed in TASCflow, a computational fluid dynamics software package from AEA Technologies. This paper analyzes the data from several of the selected models, paying close attention to pumping performance and general trends from specific design changes.

Heart-Assist Devices↗

PubMedDate not supplied

Design and transient computational fluid dynamics study of a continuous axial flow ventricular assist device.

A ventricular assist device (VAD), which is a miniaturized axial flow pump from the point of view of mechanism, has been designed and studied in this report. It consists of an inducer, an impeller, and a diffuser. The main design objective of this VAD is to produce an axial pump with a streamlined, idealized, and nonobstructing blood flow path. The magnetic bearings are adapted so that the impeller is completely magnetically levitated. The VAD operates under transient conditions because of the spinning movement of the impeller and the pulsatile inlet flow rate. The design method, procedure, and iterations are presented. The VAD's performance under transient conditions is investigated by means of computational fluid dynamics (CFD). Two reference frames, rotational and stationary, are implemented in the CFD simulations. The inlet and outlet surfaces of the impeller, which are connected to the inducer and diffuser respectively, are allowed to rotate and slide during the calculation to simulate the realistic spinning motion of the impeller. The flow head curves are determined, and the variation of pressure distribution during a cardiac cycle (including systole and diastole) is given. The axial oscillation of impeller is also estimated for the magnetic bearing design. The transient CFD simulation, which requires more computer resources and calculation efforts than the steady simulation, provides a range rather than only a point for the VAD's performance. Because of pulsatile flow phenomena and virtual spinning movement of the impeller, the transient simulation, which is realistically correlated with the in vivo implant scenarios of a VAD, is essential to ensure an effective and reliable VAD design.

Blood Flow Velocity↗

PubMedDate not supplied

Transient and quasi-steady computational fluid dynamics study of a left ventricular assist device.

The HeartQuest continuous flow left ventricle assist device (LVAD) with a magnetically levitated impeller operates under highly transient flow conditions. Due to insertion of the in-flow cannula into the apex of the left ventricle, the inlet flow rate is transient because of ventricular contraction, and the pump's asymmetric circumferential configuration with five rotating blades forces blood intermittently through the pump to the great arteries. These two transient conditions correspond to time varying boundary conditions and transient rotational sliding interfaces in computational fluid dynamics (CFD). CFD was used to investigate the pump's performance under these dynamic flow conditions. A quasi-steady analysis was also conducted to evaluate the difference between the steady and transient analyses and demonstrate the significance of transient analysis, especially for transient rotational sliding interfaces transient simulations. This transient flow analysis can be applied generally in the design process of LVADs; it provides more reliable fluid forces and moments on the impeller for successful design of the magnetic suspension system and motor.

Blood Circulation↗

PubMedDate not supplied

The status of failure and reliability testing of artificial blood pumps.

Artificial blood pumps are today's most promising bridge-to-transplant, bridge-to-recovery, and destination therapy solutions for patients with congestive heart failure. There is a critical need for increased reliability and safety as the next generation of artificial blood pumps approach final development for long-term destination therapy. To date, extensive failure and reliability studies of these devices are considered intellectual property and thus remain unpublished. Presently, the Novacor N100PC, Thoratec VAD, and HeartMate LVAS (IP and XVE) comprise the only four artificial blood pumps commercially available for the treatment of congestive heart failure in the United States. The CardioWest TAH recently received premarket approval from the US Food and Drug Administration. With investigational device exemptions, the AB-180, AbioCor, LionHeart, DeBakey, and Flowmaker are approved for clinical testing. Other blood pumps, such as the American BioMed-Baylor TAH, CorAide, Cleveland Clinic-Nimbus TAH, HeartMate III, Hemadyne, and MagScrew TAH are currently in various stages of mock loop and animal testing, as indicated in published literature. This article extensively reviews in vitro testing, in vivo testing, and the early clinical testing of artificial blood pumps in the United States, as it relates to failure and reliability. This detailed literature review has not been published before and provides a thorough documentation of available data and testing procedures regarding failure and reliability of these various pumps.

Clinical Trials as Topic↗

PubMedDate not supplied

Computational design and experimental performance testing of an axial-flow pediatric ventricular assist device.

The Virginia Artificial Heart Institute continues to design and develop an axial-flow pediatric ventricular assist device (PVAD) for infants and children in the United States. Our research team has created a database to track potential PVAD candidates at the University of Virginia Children's Hospital. The findings of this database aided with need assessment and design optimization of the PVAD. A numerical analysis of the optimized PVAD1 design (PVAD2 model) was also completed using computational fluid dynamics (CFD) to predict pressure-flow performance, fluid force estimations, and blood damage levels in the flow domain. Based on the PVAD2 model and after alterations to accommodate manufacturing, a plastic prototype for experimental flow testing was constructed via rapid prototyping techniques or stereolithography. CFD predictions demonstrated a pressure rise range of 36-118 mm Hg and axial fluid forces of 0.8-1.7 N for flows of 0.5-3 l/min over 7000-9000 rpm. Blood damage indices per CFD ranged from 0.24% to 0.35% for 200 massless and inert particles analyzed. Approximately 187 (93.5%) of the particles took less than 0.14 seconds to travel completely through the PVAD. The mean residence time was 0.105 seconds with a maximum time of 0.224 seconds. Additionally, in a water/glycerin blood analog solution, the plastic prototype produced pressure rises of 20-160 mm Hg for rotational speeds of 5960 +/- 18 rpm to 9975 +/- 31 rpm over flows from 0.5 to 4.5 l/min. The numerical results for the PVAD2 and the prototype hydraulic testing indicate an acceptable design for the pump, represent a significant step in the development phase of this device, and encourage manufacturing of a magnetically levitated prototype for animal experiments.

Adolescent↗

PubMedDate not supplied

Computational design and experimental testing of a novel axial flow LVAD.

Thousands of cardiac failure patients per year in the United States could benefit from long-term mechanical circulatory support as destination therapy. To provide an improvement over currently available devices, we have designed a fully implantable axial-flow ventricular assist device with a magnetically levitated impeller (LEV-VAD). In contrast to currently available devices, the LEV-VAD has an unobstructed blood flow path and no secondary flow regions, generating substantially less retrograde and stagnant flow. The pump design included the extensive use of conventional pump design equations and computational fluid dynamics (CFD) modeling for predicting pressure-flow curves, hydraulic efficiencies, scalar fluid stress levels, exposure times to such stress, and axial fluid forces exerted on the impeller for the suspension design. Flow performance testing was completed on a plastic prototype of the LEV-VAD for comparison with the CFD predictions. Animal fit trials were completed to determine optimum pump location and cannulae configuration for future acute and long-term animal implantations, providing additional insight into the LEV-VAD configuration and implantability. Per the CFD results, the LEV-VAD produces 6 l/min and 100 mm Hg at a rotational speed of approximately 6300 rpm for steady flow conditions. The pressure-flow performance predictions demonstrated the VAD's ability to deliver adequate flow over physiologic pressures for reasonable rotational speeds with best efficiency points ranging from 25% to 30%. The CFD numerical estimations generally agree within 10% of the experimental measurements over the entire range of rotational speeds tested. Animal fit trials revealed that the LEV-VAD's size and configuration were adequate, requiring no alterations to cannulae configurations for future animal testing. These acceptable performance results for LEV-VAD design support proceeding with manufacturing of a prototype for extensive mock loop and initial acute animal testing.

Animals↗

PubMedDate not supplied

Axial flow blood pumps.

Each year, thousands of cardiac patients await healthy donor hearts for transplantation. Due to the current shortage of donor hearts (approximately 2300 per year), these patients often require supplemental circulatory support until a transplant becomes available. This supplemental support is often provided by a mechanical heart pump or left ventricular assist device (LVAD). This article explores one type of LVAD, specifically the design and development of axial flow ventricular assist devices (VAD). We discuss the design details, and experimental or clinical experience with the following axial flow support systems: Hemopump, MicroMed DeBakey VAD, Jarvik 2000, HeartMate II, Streamliner, Impella, Berlin INCOR I, Valvo pump, and IVAP. All of these devices demonstrate promise in providing bridge-to-transplant and ultimately destination therapy for adult cardiac failure patients.

Animals↗