Biomedical subjects

Heinrich Schima

Publications and source records attributed to Heinrich Schima.

At least 19 recordsLinked to original sources

PubMed2007

Endocrine function is not impaired in patients with a continuous MicroMed-DeBakey axial flow pump.

OBJECTIVES: Pulsatile blood flow has been regarded to be of importance for the regulation of endocrine organs. A new generation of continuous flow mechanical blood pumps is now available for clinical application. Patients with implanted MicroMed-DeBakey axial pumps show nonphysiologic low-pulsatile blood flow profiles, and therefore it appeared to be of interest to evaluate their possible effect on the endocrine system. METHODS: Eight male patients and 1 female patients (mean age, 51 +/- 10 years) with end-stage left-sided heart failure were implanted with a MicroMed-DeBakey axial pump. After a mean period of 67 +/- 19 days, basal pituitary hormone concentrations and their responses to a bolus injection of hypothalamic releasing hormones were tested. In addition, thyroid hormones, testosterone, and plasma and urinary catecholamine levels were measured at baseline. RESULTS: Administration of the hypothalamic releasing hormones revealed normal responses of all pituitary hormones (adrenocorticotropic hormone, thyroid-stimulating hormone, luteinizing hormone, and prolactin), except for growth hormone, the response of which was slightly impaired (10.2 +/- 6.8 vs 19.9 +/- 6.5 ng/L, P < .05). Also, the cortisol response to the corticotropin-releasing hormone-stimulated adrenocorticotropic hormone release was normal, as were basal concentrations of thyroid hormones (triiodothyronine, thyroxine, free triiodothyronine, and free thyroxine), testosterone, and urinary catecholamines. CONCLUSIONS: Implantation of a continuous flow axial pump with low-pulsatile blood flow profile appears to have no major effect on the hypothalamic-pituitary-endorgan system and sympathoadrenal functions. This finding is reassuring for the growing number of patients treated with this convenient new pump and could contribute considerably to their prognosis and quality of life.

Catecholamines↗

PubMed2006

Effect of continuous arterial blood flow in patients with rotary cardiac assist device on the washout of a stenosis wake in the carotid bifurcation: a computer simulation study.

In recipients of rotary blood pumps for cardiac assist, the pulsatility of arterial flow is considerably diminished. This influences the shear stress patterns and streamlines in the arterial bed, with potential influence on washout and plaque growth. These effects may be aggravated in the recirculation area of stenoses, and therefore, exclude patients with atherosclerosis from the therapy with these devices. A numerical study was performed for the human carotid artery bifurcation with the assumption of a massive stenosis (75% reduction of cross-section area) in the carotid bulb. Four different flow time patterns (no support to full pump support) were applied. Flow patterns and particle residence time within the recirculation region were calculated, once within the relevant volume behind the stenosis and and once within a small region directly at the posterior heel of the stenosis. The flow patterns showed a considerable radial vorticity behind the stenosis. Mean particle residence time in the whole recirculation region was 15% less for high pump support (nearly continuous flow) compared to the natural flow pattern (0.19s compared to 0.22s), and nearly identical for the small heel region (0.28 to 0.27s). The flow simulation demonstrates, that even in the case of a pre-existing stenosis, the local effects of continuous flow on particle residence times are rather minimal (as was shown previously for intact arterial geometries). Therefore, from the point of macroscopic flow field analysis, continuous flow should not enhance the thromboembolic risk in ventricular assist device recipients.

Blood Flow Velocity↗

PubMed2006

First clinical experience with an automatic control system for rotary blood pumps during ergometry and right-heart catheterization.

BACKGROUND: At present, most clinically implanted rotary blood pumps are operated at constant speed and adjusted by the physician. It is generally assumed that an adaptation of pump speed to the patient's physiologic requirements would be beneficial. The data provided in this paper, based on hemodynamic and spirometric data during exercise in which a pre-load-sensitive control was used, lend quantitative support to this assumption. METHODS: An automatic speed control was developed and implemented with Matlab on a dSpace controller board. The system uses pump speed, pump power, and pump flow as its only input signals. It was connected to the clinical hardware of the DeBakey VAD System. The control is pre-load-sensitive and uses an expert system to detect excessive unloading and eventual suction. This system was used to quantify the cardiovascular reaction of patients to both automatically controlled and constant pump speed. A sub-group of 5 patients underwent bicycle ergometry with Swan-Ganz catheterization and spiroergometry. RESULTS: The automatic, closed-loop speed control showed robust and stable performance. It provided an increase in pump flow (+0.94 +/- 0.5 liters/min, p < 0.05) compared with constant-speed mode in response to physical activity. Pulmonary arterial (PAP) and capillary wedge pressure (PCWP) clearly decreased (-7.4 +/- 4.1 mm Hg for PAP and -8.3 +/- 4.2 mm Hg for PCWP, p < 0.05), and venous oxygen saturation moderately increased (+5.2%). CONCLUSION: An automatic speed-control system for rotary blood pumps was developed and demonstrated by spiroergometry to be appropriately responsive to physiologic demand.

Algorithms↗

PubMed2006

Advanced suction detection for an axial flow pump.

An automatic detection system for ventricular collapse was developed and tested in a first clinical trial as part of a physiological speed control concept for axial flow pumps. From this clinical experience, and based on the acquired data during this trial, an optimization of the developed system was performed. An already-existing database of 784 individual cases was extended. For harmonization of this database an additional 412 snap files were extracted from continuous data recordings and classified manually using a standardized procedure. The already-developed and clinically tested algorithms were supplemented by one additional indicator derived from a preexisting criterion. One threshold value was replaced by application of a numerically optimized nonlinear characteristic curve dependent on heart rate. Finally, in a multidimensional optimization process of the entire suction detection system, 7 individual indicators were adjusted by using 17 independent threshold values. The optimization criteria were applied using a three-level hierarchical system. Within the final database consisting of 1196 snap shots the overall amount of maldetections could be reduced to 23 cases including 5 false positive events (0.42%) and 18 false negative decisions (1.5%). By application of the clinical experience from the first clinical trial of a physiologic control system it became possible to optimize the sensitivity and specificity of the suction detection system to unprecedented accuracy.

Algorithms↗

PubMed2006

The DuraHeart VAD, a magnetically levitated centrifugal pump: the University of Vienna bridge-to-transplant experience.

BACKGROUND: The clinical application of the DuraHeart (Terumo Heart Inc, USA) has begun in Europe as a clinical trial of a third-generation implantable centrifugal blood pump. Four successful clinical implants are presented. METHODS AND RESULTS: Four male patients had end-stage left heart failure and received a DuraHeart VAD as a left ventricular assist device for bridge-to-transplantation. The pump showed good performance with flow rates of 4.9+/-0.5 L/min after gradual weaning of extracorporeal circulation. The pump flow was then maintained at 6.1+/-0.5, 5.5+/-0.3, 5.5+/-0.1, 5.7+/-0.1, 5.5, 6.4 and 6.5 L/min at the 1st, 4th, 8th, 12th, 16th, 20th and 24th postoperative week, respectively. No significant elevation of mean plasma-free hemoglobin was detected. The patients were discharged on the 18th, 42nd, 41st and 31st postoperative day, respectively, and all were successfully transplanted on the 202nd, 84th, 128th and 96th postoperative day, respectively. At the time of transplant surfaces of the removed pumps were free from thrombus formation, although intraventricular pannus growth was observed around the inflow cannulae in all patients. CONCLUSION: The DuraHeart VAD showed stable and sufficient circulatory support for the bridge-to-transplant procedure in this cohort of 4 patients.

Adult↗

PubMed2005

Doppler echocardiographic assessment of valvular regurgitation severity by measurement of the vena contracta: an in vitro validation study.

BACKGROUND: Noninvasive quantitation of valvular regurgitation remains a difficult problem. Measurements of the vena contracta (VC) by color Doppler echocardiography have been proposed but limited data are available on the actual accuracy of this method. METHODS: To evaluate how closely the color Doppler VC reflects the true fluid dynamic VC and the anatomic regurgitant orifice and whether this measurement is affected by flow changes, various models of valvular regurgitation were studied in an in vitro flow circuit. The VC diameter was measured with color Doppler using two different ultrasound systems (Agilent Sonos 5500; Agilent Technologies Inc, Palo Alto, Calif and Vingmed CFM 800; GE Healthcare, Chalfront St Giles, UK). Optical planimetry of the anatomic regurgitant orifice was performed, the true VC diameter was determined by laser particle flow visualization. RESULTS: Because of flow contraction, the true VC diameter was consistently smaller than the anatomic regurgitant orifice diameter. Anatomic orifice and true VC only marginally changed with flow rate. The diameter of the color Doppler VC, however, not only overestimated the anatomic orifice diameter by 45% to 60% and the true VC diameter by 130% to 160%, but was also highly affected by the flow rate and the ultrasound system. Despite these limitations a color Doppler VC diameter of 0.77 cm or more (Agilent) and 0.89 cm or more (Vingmed) detected severe regurgitation with a sensitivity of 93% and 84% and a specificity of 96% and 79%, respectively. CONCLUSIONS: Color Doppler estimates of the VC markedly overestimate regurgitant orifice and true VC. In contrast to the true VC, Doppler measurements are significantly affected by flow rate and by the ultrasound system used. Nevertheless, they allow semiquantitative assessment of valvular regurgitation separating severe from nonsevere regurgitation with acceptable accuracy.

Aortic Valve Insufficiency↗

PubMed2005

Physiology of continuous blood flow in recipients of rotary cardiac assist devices.

The beating heart and the resultant pulse wave have been a symbol of life for centuries. The development history of roller pumps for cardiopulmonary bypass shows that the human body tolerates non-pulsatile blood flow, at least for short-term support. Over the last few years, many types of rotary blood pumps have been developed for clinical use in patients requiring mid- to long-term support. Although early clinical experiences in patients with long-term support have been promising, the matter of whether pulsatile flow is needed or not remains controversial. Therefore, this review summarizes the observed clinical consequences of continuous blood flow in patients supported by rotary blood pumps and relates these consequences to underlying experimental studies.

Animals↗

PubMed2005

Development of a reliable automatic speed control system for rotary blood pumps.

BACKGROUND: Axial blood pumps have been very successfully introduced into the arena of prolonged clinical support. However, they do not offer inherent load-responsive mechanisms for adjusting pumping performance to venous return and changes in physiologic requirements of the patient. To provide for these adjustments we developed an algorithm for demand-responsive pump control based on a reliable suction detection system. METHODS: A PC-based system that analyzes pump performance based on available flow, heart rate and short-term performance history was developed. The physician defines levels of "desired flow" at rest and during exercise, depending on heart rate. In case this desired flow cannot be maintained due to limited venous return, the maximal available flow level is determined from an analysis of the actual pump data (flow, speed and power consumption). An expert system continuously checks the flow signal for any indication of suction. Periodic speed variations then adapt pump performance to the patient's condition. RESULTS: First, stability and functionality were proven under various settings in vitro. The algorithms were then tested in 15 patients in intensive care, in the standard ward, and during bicycle exercise. The system reacted properly to demand changes, at exercise level, in response to coughing and at various Valsalva maneuvers. Suction could also be successfully prevented during severe arrhythmia and in patients with critical cardiac geometry. Exercise tests showed decreases in pulmonary arterial pressure (-22 +/- 9.9%) and pulmonary capillary wedge pressure (-42 +/- 18.54%), and an increase in pump flow (19 +/- 9.5%) and workload (8 +/- 6.1%), all when compared with constant-speed pumping. CONCLUSIONS: A closed-loop control system equipped with an expert system for reliable suction detection was developed that improves response to change in venous return for rotary pump recipients. The system was robust, stable and safe under a wide range of everyday living conditions.

Algorithms↗

PubMed2005

Extracorporeal venovenous cooling for induction of mild hypothermia in human-sized swine.

OBJECTIVE: Several cooling methods have been investigated for inducing mild hypothermia (33-36 degrees C) after cardiac arrest, brain trauma, or stroke. To achieve its best effect, therapeutic hypothermia has to be applied very early after the ischemic insult; otherwise, the beneficial effect would be diminished or even abrogated. The aim of this study was to investigate the effectiveness and safety of extracorporeal venovenous cooling as compared with endovascular cooling. DESIGN: Swine were cooled in a randomized crossover design from 38 degrees C to 33 degrees C brain temperature, either with extracorporeal venovenous cooling or with endovascular cooling. SETTING: Laboratory investigation. SUBJECTS: Six swine of human size (85 to 101 kg). INTERVENTIONS: Swine were randomly cooled with the first device, and after achieving the target brain temperature, re-warmed via the same technique and with heating lamps to baseline temperature. Then the other catheter was inserted and cooling was performed with the second device. MEASUREMENTS: Brain, pulmonary artery and tympanic temperature, blood pressure, and heart rate were recorded continuously. Laboratory samples, including free hemoglobin, were taken at predefined temperature points during cooling. Comparisons between and within (baseline vs. 33 degrees C) the treatment groups were performed with the paired Student's t-test. MAIN RESULTS: The time needed to reduce brain temperature from 38.0 degrees C to 33.0 degrees C was 41 +/- 17 mins with venovenous cooling and 126 +/- 37 mins with endovascular cooling (p = .001). Heart rate and mean arterial pressure decreased moderately during cooling and were significantly lower at 33 degrees C than at baseline in both groups, without differences between groups. None of the swine developed significant hemolysis, arrhythmias, or bleeding. CONCLUSIONS: Extracorporeal venovenous cooling was an effective and safe method to rapidly induce therapeutic mild hypothermia in human-sized swine. It seems to be promising for further application and investigation in patients.

Animals↗

PubMed2005

Comparison of the autoregulatory mechanisms between middle cerebral artery and ophthalmic artery after thigh cuff deflation in healthy subjects.

PURPOSE: To compare dynamic autoregulation in the middle cerebral artery (MCA) and the ophthalmic artery (OA) after a step decrease in systemic blood pressure. METHODS: Eighteen healthy male young subjects were studied. Ultrasound parameters and systemic blood pressures were recorded in each subject before, during, and after a step decrease in blood pressure. Continuous blood pressure recordings were made with a finger plethysmograph system, and flow velocities in the MCA and the OA were continuously measured with Doppler ultrasound. Large bilateral thigh cuffs were inflated and a pressure approximately 20 mm Hg above peak systolic blood pressure was maintained for 3 minutes. A decrease in blood pressure was induced by rapid deflation of bilateral thigh cuffs. Experiments were performed separately for the OA and the MCA. RESULTS: Systemic blood pressure showed a step decrease immediately after thigh cuff release (9%-15%) and returned to baseline 7 to 10 pulse cycles later. Flow velocities in the MCA returned to baseline earlier than systemic blood pressure, indicating peripheral vasodilatation, with a maximum of five to six pulse cycles after the blood pressure decrease. By contrast, flow velocities in the OA returned to baseline later than systemic blood pressure, reflecting peripheral vasoconstriction with a maximum 10 to 15 pulse cycles after cuff release. There was a statistically significant difference in the time course of the resistance changes in the two selected arteries after thigh cuff release (P < 0.001). CONCLUSIONS: The results of the present study suggest substantial differences in the autoregulatory behavior of the vascular beds peripheral to the MCA and the OA. Results in the MCA would be compatible with either metabolic or myogenic vasodilatation, whereas the results in the OA could reflect sympathetic vasoconstriction. Further studies are needed to support this hypothesis. The thigh cuff technique may represent an interesting approach to the study of autoregulation in patients with ocular vascular disease.

Adult↗

PubMed2004

Value and limitations of aortic valve resistance with particular consideration of low flow-low gradient aortic stenosis: an in vitro study.

BACKGROUND: The calculation of valve resistance (R) rather than aortic valve area (AVA) has been proposed for the assessment of aortic stenosis (AS), based on the claim that it is less flow-dependent. Even more importantly, valve resistance has been reported to distinguish between truly severe and "pseudosevere" AS in patients with low cardiac output. However, the diagnostic value of valve resistance remains controversial. METHODS AND RESULTS: Models of stenotic aortic valves (plates and nozzles) and biological stenotic valves were studied in a pulsatile in vitro circuit using Doppler ultrasound and direct pressure and flow measurements. Anatomic AVAs ranged from 0.5 to 1.25 cm2; cardiac output varied from 1.8 to 9.0 l/min. Effective AVA was calculated with the continuity equation. The orifices of the biological valves were recorded with a video camera for planimetry. In low flow-low gradient AS, truly severe stenosis was defined by an AVA remaining <0.85 cm2 after flow normalisation, whereas AVA increased beyond 0.85 cm2 in pseudosevere AS. In rigid stenoses, valve resistance increased significantly with flow, while in bioprostheses this flow dependence was partially masked by an actual increase of the anatomic orifice area. In low flow-low gradient AS, valve resistance was significantly smaller in pseudosevere AS compared to truly severe AS (129 +/- 28 vs. 176 +/- 33 dyne s cm(-5); p<0.001) at a similar baseline effective AVA. After the exclusion of datasets with mean gradients <15 and >35 mmHg, the difference in valve resistance between truly severe and pseudosevere AS was no longer significant (162 +/- 26 vs. 141 +/- 22 dyne s cm(-5); p=0.08). Nevertheless, valve resistance <120 dyne s cm(-5) was found only in pseudosevere stenoses while valve resistance >180 dyne s cm(-5) marked truly severe stenosis. CONCLUSIONS: Valve resistance is flow-dependent and not superior to calculated AVA for the assessment of AS. In low flow-low gradient AS, valve resistance <120 dyne s cm(-5) identifies pseudosevere AS, whereas valve resistance >180 dyne s cm(-5) implies truly severe AS. However, values between 120 and 180 dyne s cm(-5) are nondiagnostic, requiring repeated AVA calculations after flow normalisation.

Aged↗

PubMed2004

Weaning of rotary blood pump recipients after myocardial recovery: a computer study of changes in cardiac energetics.

BACKGROUND: Weaning of patients from mechanical cardiac support after myocardial recovery has always involved multiple, interacting factors, particularly the training of the myocardium during reduction of pump flow. Rotary pumps offer training advantages when support flow is reduced, even to nearly zero. We report a computer analysis that evaluates the work required of the heart during partial unloading and removal of rotary pumps. METHODS AND RESULTS: A computer model of the assisted circulation, previously implemented in MATLAB (The MathWorks Inc, Natick, Mass), has been augmented with a model of the MicroMed DeBakey ventricular assist device (MicroMed Technology, Inc, Houston, Tex). Flow, pressure patterns, and external work (pressure-volume area, calculated as the area of the ventricular pressure-volume loop [external work] plus potential energy) were calculated for nonassisted and various continuously assisted patients. Under low-flow conditions, the heart imposes an oscillating forward-backward flow through the non-occlusive rotary pump, causing an increase in ventricular work. Thus, an assist flow of 1 to 1.5 L/min requires work equivalent to that of the unsupported heart. At 60% contractility, the nonassisted pressure-volume area is 1.10 Ws/beat, and the potential energy is 0.38 Ws/beat. At a Qpump of 1 L/min, the pressure-volume area is 1.21 Ws/beat, and the potential energy is 0.37 Ws/beat. At a Qpump of 3 L/min, the pressure-volume area is 0.93 Ws/beat, and the potential energy is 0.29 Ws/beat. These conditions cannot be achieved with pulsatile systems. CONCLUSION: During weaning and retraining, an implanted rotary pump can provide a workload to the heart like that in the nonassisted situation, thus increasing the predictability of weaning and reducing the risk of reiterating heart failure.

Cardiovascular System↗

PubMed2004

Development of a suction detection system for axial blood pumps.

Axial flow blood pumps for cardiac assistance have proven their clinical viability and benefit in recent years. However, the clinical systems to date have no direct mechanism to decrease pump speed when adequate supply is not available. This may lead to ventricular collapse or increase the probability of hemolysis and thrombotic risks. Based on various experiences with left ventricular assist device (LVAD) patients in various states of recovery, at implant, in the intensive care unit, in the standard ward, and during physical exercise, 11 different algorithms were developed for the automatic detection of ventricular suction. These detection algorithms analyze the flow pattern for the presence of distinct suction indicators. For selection and optimization of the algorithms, 1000 records from approximately 100 patients were collected. Each record contains 5 s of pump flow, current, and arterial pressure. Three experts classified these records in terms of suction probability and other abnormalities. The optimization was developed in Matlab, capable of solving a fifth-dimensional optimization problem with 256 different algorithm combinations. The optimization resulted in a set of 6 algorithms, each with specific thresholds. The system detects 100% of the known suction events with 0.28% of false-positive interpretations. If tuned to avoid any false-positive detection, 90.7% of the certain events would be detected. A strategy for the development of a robust suction detection system for axial blood pumps was found. This system will be integrated into an automatic pump speed control system to provide adequate perfusion for the LVAD recipient, without excessive unloading of the ventricle.

Algorithms↗

PubMed2004

Automatic system for noninvasive blood pressure determination in rotary pump recipients.

In patients with implanted rotary pumps, the arterial pressure pulsatility is usually far lower than in normal individuals. Depending on the remaining degree of pulsatility, cuff-based systems such as the classical Riva-Rocci-determination of arterial blood pressure and correlated sounds or pressure measurements based on cuffpressure oscillations become inaccurate or even impossible. Therefore, a system was developed which evaluates the flow in the radial artery using an ultrasound wristwatch sensor, and this additional information is used for pressure determination. A computerized data acquisition and cuff-control system based on a PC using Matlab software, a wristwatch ultrasound device, and a compressor-driven pressure cuff was set up. The cuff was controlled for automatic inflation and deflation cycles. Cuff pressure and arterial flow was recorded. Several algorithm strategies were developed, which gave data for systolic blood pressure and heart rate together with a reliability index for data quality. Finally, the new algorithms were implemented in a microcontroller system. Comparisons with invasive measurements showed excellent correlation with systolic blood pressure (mean deltaP -0.3 mm Hg, n = 28). During exercise of rotary pump patients and therefore enhanced pulsatility the difference from manual evaluation was -2.1 mm Hg (n = 18). In conclusion, adaptation of the classical cuff-pressure method with ultrasound evaluation of peripheral flow allows reliable determination of blood pressure in patients with low pulsatility resulting from implanted rotary cardiac assist pumps. By development of a wristwatch sensor and an automatic control system a robust method for daily use could be developed.

Blood Pressure↗

PubMed2004

Doppler assessment of mechanical aortic valve prostheses: effect of valve design and size of the aorta.

BACKGROUND AND AIM OF THE STUDY: Discrepancies between Doppler and catheter gradients have been reported for bileaflet aortic valve prostheses. Whether modifications in geometric design of newly developed bileaflet valves lead to a different Doppler-catheter gradient relationship has not been evaluated. Variable results have been reported for tilting-disc prostheses. In addition, the effect of aortic size on the Doppler-catheter gradient relationship remains unclear. METHODS: Various sizes of On-X and Edwards Mira (identical with Sorin Bicarbon) bileaflet valves and Sorin Allcarbon tilting-disc aortic valves (19-25 mm) were studied in a pulsatile flow model. Doppler and catheter gradients were measured simultaneously. Aortic diameters between 1.8 and 4 cm were evaluated. RESULTS: Correlation between Doppler and catheter gradients was excellent (r = 0.98-0.99 for peak and mean gradients), but in bileaflet valves Doppler significantly overestimated the corresponding catheter gradients as reflected by slopes of the regression lines (1.57-1.8). In the range of relevant gradients > or = 10 mmHg, Doppler exceeded catheter gradients by 40 +/- 17% (peak) and 39 +/- 16% (mean) in Mira valves, and by 46 +/- 19% (peak) and 43 +/- 14% (mean) in On-X valves. In the Sorin tilting-disc valve, Doppler accurately reflected catheter gradients (slopes of regression lines 1.05-1.14). The aortic diameter significantly influenced results in only tilting-disc valves, but in absolute terms the effect was clinically less relevant. CONCLUSION: Discrepancies between Doppler and catheter gradients are common to all bileaflet valves, regardless of their specific geometric design, whereas tilting-disc valves must be considered individually. The influence of aortic size on the Doppler-catheter gradient relationship appears clinically to be less relevant in prosthetic valves.

Aorta↗

PubMed2003

Special considerations on the implantation technique for the MicroMed-DeBakey ventricular assist device axial pump.

Implantable continuous flow axial pumps were introduced to clinical settings in November 1998 with the MicroMed-DeBakey ventricular assist device. Axial pumps continuously unload the failing left ventricle and accurate positioning of the inflow cannula prevents substantial reductions in pump flow. Considerations on the implantation technique are made to minimize surgical trauma, to prevent ventricular collapse, to optimize inflow conditions (flow increased from 4.3 +/- 0.6 to 6.7 +/- 0.3 L/min), and to facilitate the subsequent transplantation.

Adolescent↗

PubMed2003

Building a computer model of the Haemobear blood pump.

Further development of the Haemobear blood pump requires theoretical predictions of the dynamic behavior of the rotor. These predictions can be used to compare different rotor geometry at desired operating conditions before a prototype is built. The study focuses on a rotor-dynamic model of the rigid rotor with six degrees of freedom (6-DoF), which is implemented using the Matlab-Simulink software package. The forces acting on the rotor are provided to Simulink in terms of constant values (e.g., gravity force), linear coefficients, or nonlinear functions. These coefficients or functions are obtained using numerical simulation results. Fluid forces and magnetic forces can be calculated using commercial software codes. The output of these codes has to be postprocessed to get the desired values for Simulink. This article will give an overview of how to implement arbitrary physical influences on the rotor in a computational model of the complete pump.

Computer Simulation↗

PubMed2002

Numerical study of hemodynamics and wall mechanics in distal end-to-side anastomoses of bypass grafts.

The development and progress of distal anastomotic intimal hyperplasia seems to be promoted by altered flow conditions and intramural stress distributions at the region of the artery-graft junction of vascular bypass configurations. From clinical observations, it is known that intimal hyperplasia preferentially occurs at outflow anastomoses of prosthetic bypass grafts. In order to gain a deeper insight into post-operative disease processes, and subsequently, to contribute to the development of improved vascular reconstructions with respect to long term patency rates, detailed studies are required. In context with in vivo experiments, this study was designed to analyze the flow dynamics and wall mechanics in anatomically correct bypass configurations related to two different surgical techniques and resulting geometries (conventional geometry and Miller-cuff). The influence of geometric conditions and of different compliance of synthetic graft, the host artery and the interposed venous cuff on the hemodynamic behavior and on the wall stresses are investigated. The flow studies apply the time-dependent, three-dimensional Navier-Stokes equations describing the motion of an incompressible Newtonian fluid. The vessel walls are described by a geometrically non-linear shell structure. In an iterative coupling procedure, the two problems are solved by means of the finite element method. The numerical results demonstrate non-physiological flow patterns in the anastomotic region. Strongly skewed axial velocity profiles and high secondary velocities occur downstream the artery-graft junction. On the artery floor opposite the junction, flow separation and zones of recirculation are found. The wall mechanical studies show that increased compliance mismatch leads to increased intramural stresses, and thus, may have a proliferative influence on suture line hyperplasia, as it is observed in the in vivo study.

Anastomosis, Surgical↗