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Biomedical subjects

H Reul

Publications and source records attributed to H Reul.

At least 73 records · Page 4Linked to original sources
PubMed1994

Design improvements of the HIA-VAD based on animal experiments.

Since 1990, development of the Helmholtz-Institute at Aachen ventricular assist device (HIA-VAD) was mainly based on animal tests performed at the University of Groningen. Although various in vitro tests had been performed previously, animal testing resulted in significant improvements of the HIA-VAD with regard to hemodynamics and pump handling. The most important design improvement was a new trileaflet polyurethane valve, which was designed as a blood-pump valve with an emphasis on opening behavior and flow resistance. Excellent hydrodynamic performance and sufficient durability of this new valve were confirmed by various in vitro tests. Further design and manufacturing improvements resulted in a completely transparent pump, which can be easily deaired and optically controlled. Final pump design was evaluated during subsequent animal tests, and the results are very promising in view of an efficient cardiac support system.

Animals↗
PubMed1994

Concept, realization, and first in vitro testing of an intraarterial microaxial blood pump with an integrated drive unit.

The intraaortically located micoraxial pump represents a promising device for temporary cardiac assistance in terms of efficiency and practicability. Due to well-documented problems arising from the concept of a pump unit being driven by an extracorporeally placed motor via a flexible drive shaft cable, a new pump concept is presented. The cable is replaced by a proximally attached microelectric motor that needs only an extracorporeal power supply.

Aorta↗
PubMed1994

The Helmholtz Total Artificial Heart Labtype.

To perform the first experimental tests for validation of a new gear unit concept, the pump chamber, diaphragm, and pusher plate design of an orthotopic electromechanical total artificial heart (TAH) (Helmholtz Labtype) was manufactured. In its early stage of development, it provides some of the most important features of the conceptual final artificial heart. The new gear unit transforms a uniform unidirectional rotational motor movement into translatory pusher plate movements, with resting phase in the end-diastolic position, and the angled pump chamber orientation determines the available space for the motor and gear unit. Furthermore, this labtype provides flexibility with regard to use of different types of structural parts for experimental investigations. The first in vitro test results, obtained with specially designed circulatory mockloops that simulate physiological preload and afterload conditions, are presented. They comprise pressure and flow generation, motor performance, efficiency, and energy consumption. The results prove the feasibility of the new gear unit concept for an electromechanical artificial heart and allow a reliable determination of the necessary performance of the future brushless DC motor for the first in vivo TAH model.

Blood Pressure↗
PubMed1994

Technical requirements and limitations of miniaturized axial flow pumps for circulatory support.

The engineering principles of rotary blood pumps are elucidated by means of a basic introduction into turbomachinery. Additionally, some important dimensionless quantities which relate to pumping characteristics and pump type are introduced. These theoretical fundamentals are applied to the design of high-speed microaxial pumps, especially the two Hemopump versions. The theoretical estimates clearly show that each pump version has its physical limits, especially at small impeller sizes. It is also demonstrated that not any clinically desirable working point of an axial pump can be achieved with an arbitrarily small pump size.

Animals↗
PubMed1994

Causes and formation of cavitation in mechanical heart valves.

Cavitation may develop on mechanical valvular prostheses in the mitral position; it causes blood damage and, under particularly adverse conditions, it may result in sudden failure of the prosthesis. Therefore, with regard to future development of mechanical heart valves, the pattern of cavitation and its predisposing factors in different types of prostheses were investigated in in vitro studies, which focused on the analysis of valve closure dynamics and the influence of design parameters on the cavitation-inducing pressure drop at the artificial valve. It was found that cavitation is produced primarily by the deceleration of the closing body of the valve. At 900g, the measured deceleration of the closing bodies falls in the range of the decelerations determined in oscillation experiments for investigating cavitation-induced material erosion. The pressure drop produced thereby is overlapped by the pressure drop in accelerated or turbulent flow regions produced by design characteristics at outlet struts, stop faces or sealing lips during backflow through the closing disc. These phenomena exist particularly in regions of high flow velocity, i.e. at the instant of closure at the maximum distance from the bearing axis of the closing body (12 o'clock position). The onset of cavitation is additionally promoted in this position by a tight joint between the closing body and the ring. Oscillations of the closing body generally have a negligible effect on the cavitation behavior. From these relationships one can infer that cavitation can be avoided in future in mechanical heart valves by locally limited design measures. Especially, unsteadiness in the backflow through the closing valve is to be avoided.

Acceleration↗
PubMed1993

Prosthetic valve function under simulated low cardiac output conditions: preliminary observations.

An experimental protocol was developed and a series of laboratory experiments started to assess the function and behaviour of replacement valves under simulated low cardiac output, that is low flow and low pressure conditions. The Helmholtz Institute's pulse duplicator (1) and instrumentation were carefully tuned in order to achieve accurate and reproducible delivery of the required extremely small stroke volumes. A pilot study was completed with 27 mm St. Jude Medical and CarboMedics bileaflet valves and Wessex porcine bioprostheses, three each, in the simulated aortic position. The results of these experiments suggest that the performances of the two bileaflet prostheses are inadequate if the flow is less than 2.0 l/min; the porcine bioprosthesis needs relatively low flow to start proper function, but its stenotic nature soon becomes apparent; increase in pressure without a corresponding increase in flow has a deleterious effect on valve performances by increasing regurgitation; increasing pulse rate is inversely correlated with valve efficiency; and there is no "all-or-none" cut off point between functional and non-functional situation for the tested devices; there is a semilogarithmic relation between increasing flow and valve performance. The clinical consequences are manifold and may be related both to the stabilization of circulation and to the increased thromboembolic risk during the early postoperative period.

Bioprosthesis↗
PubMed1993

In vitro comparison of bileaflet aortic heart valve prostheses. St. Jude Medical, CarboMedics, modified Edwards-Duromedics, and Sorin-Bicarbon valves.

The hydrodynamic performance of four currently used bileaflet heart valve prostheses (St. Jude Medical, CarboMedics, modified Edwards-Duromedics, and Sorin-Bicarbon) with a nominal tissue anulus diameter of 27 mm were measured in the aortic position. All experiments were performed in nonpulsatile flow and in an electrohydraulic, computer-controlled pulse duplicator simulating the left side of the human circulatory system. Testing conditions were set at cardiac outputs of 3.0, 4.5, 6.5, and 8.0 L/min at a constant heart rate of 70 beats/min. The Sorin-Bicarbon valve had the lowest pressure difference with regard to nonpulsatile (mean 5.4 mm Hg at 30 L/min) and pulsatile (mean 2.2 mm Hg at 8 L/min) flow, followed by the St. Jude Medical, CarboMedics, and modified Edwards-Duromedics valves. The leakage volumes under static and pulsatile flow conditions were lowest for the modified Edwards-Duromedics and Sorin-Bicarbon valves. The energy loss in pulsatile flow was lowest for the Sorin-Bicarbon valve, mainly because its systolic, closure, and leakage energy losses were low. Systolic sequential velocity profiles showed the most even flow distribution pattern for the St. Jude Medical and Sorin-Bicarbon valves. These findings correspond with lower overall Reynolds shear stress levels for the St. Jude Medical and the Sorin-Bicarbon valves than for the modified Edwards-Duromedics and CarboMedics valves.

Aortic Valve↗
PubMed1993

Cavitation threshold with respect to dP/dt: evaluation in 29 mm bileaflet, pyrolitic carbon heart valves.

A total of 15 bileaflet mechanical heart valves were studied in a pulse duplicator at the Helmholtz Institute (Aachen, Germany) under conditions approximating first, a physiological pressure curve and subsequently, a sinusoidal pressure curve. In this study Edwards-Duromedics valves of the modified specification were compared with the earlier version of the Edwards-Duromedics valve as well as with St. Jude Medical valves. Each valve was tested at a series of nine (9) conditions. At each condition, without altering the valve installation or the systemic conditions, each valve was filmed by two separate video systems: the Helmholtz Institute strobe light system and a high speed video recording system. All data, as recorded by each system, was then independently analyzed by both of the two contributing groups and subsequently compared. In this manner, it was possible to objectively verify not only the consistency of the data obtained, but to also determine the relative reliability of the methods for cavitation threshold detection.

Carbon↗
PubMed1992

Two-dimensional color-mapping of turbulent shear stress distribution downstream of two aortic bioprosthetic valves in vitro.

Since artificial heart valve related complications such as thrombus formation, hemolysis and calcification are considered related to flow disturbances caused by the inserted valve, a thorough hemodynamic characterization of heart valve prostheses is essential. In a pulsatile flow model, fluid velocities were measured one diameter downstream of a Hancock Porcine (HAPO) and a Ionescu-Shiley Pericardial Standard (ISPS) aortic valve. Hot-film anemometry (HFA) was used for velocity measurements at 41 points in the cross-sectional area of the ascending aorta. Three-dimensional visualization of the velocity profiles, at 100 different instants during one mean pump cycle, was performed. Turbulence analysis was performed as a function of time by calculating the axial turbulence energy within 50 ms overlapping time windows during the systole. The turbulent shear stresses were estimated by using the correlation equation between Reynolds normal stress and turbulent (Reynolds) shear stress. The turbulent shear stress distribution was visualized by two-dimensional color-mapping at different instants during one mean pump cycle. Based on the velocity profiles and the turbulent shear stress distribution, a relative blood damage index (RBDI) was calculated. It has the feature of combining the magnitude and exposure time of the estimated shear stresses in one index, covering the entire cross-sectional area. The HAPO valve showed a skewed jet-type velocity profile with the highest velocities towards the left posterior aortic wall. The ISPS valve revealed a more parabolic-shaped velocity profile during systole. The turbulent shear stresses were highest in areas of high or rapidly changing velocity gradients. For the HAPO valve the maximum estimated turbulent shear stress was 194 N m-2 and for the ISPS valve 154 Nm-2. The RBDI was the same for the two valves. The turbulent shear stresses had magnitudes and exposure times that might cause endothelial damage and sublethal or lethal damage to blood corpuscules. The RBDI makes comparison between different heart valves easier and may prove important when making correlation with clinical observations.

Aortic Valve↗
PubMed1992

CAD-design, stress analysis and in vitro evaluation of three leaflet blood-pump valves.

The computer-supported development of valves for cardiac-assist devices or artificial hearts is shown in relation to plastic technology. A CAD-system is used for the design development, whereas the dimensioning of the critical and highly stressed membranes is examined by FEM-analyses. Economic manufacture is permitted by the combined thermoforming-dip moulding technique; the blood-side components are made from biocompatible polyurethane to minimize blood damage. The first long-term results in the test set-up are compared to the FEM results.

Equipment Design↗
PubMed1992

Pressure recovery in aortic stenosis: an in vitro study in a pulsatile flow model.

OBJECTIVES: This study was designed to study pressure recovery in various models of aortic valve stenosis by performing hemodynamic measurements under physiologic conditions in a pulsatile aortic flow circuit. The results were used to validate calculations of pressure recovery based on theoretic considerations derived from fluid dynamics. BACKGROUND: Pressure recovery in aortic stenosis has not been systematically analyzed. METHODS: Stenoses varying in size, shape (circular, Y-shaped, slitlike) and inlet configuration (sharp-edged, nozzle-shaped inlet, artificially stenosed bioprostheses) were used. Aortic pressures were measured at multiple sites distal to the stenotic orifice to determine pressure gradients and recovery. RESULTS: With decreasing orifice area (2, 1.5, 1 and 0.5 cm2) pressure recovery increased (5, 7, 10 and 16 mm Hg, respectively) and the index pressure recovery to maximal peak to peak gradient decreased (56%, 37%, 24% and 14%, respectively). For a given orifice size of 0.5 cm2, this index ranged between 12% for a Y-shaped orifice and 15% for a circular orifice with a nozzle (cardiac output 4 liters/min). Increasing the cardiac output increased pressure recovery, whereas the ratio of pressure recovery to maximal pressure gradient remained constant. CONCLUSIONS: The index pressure recovery to transvalvular pressure gradient, which expresses the hemodynamic relevance of pressure recovery, decreases with increasing severity of aortic stenosis but is independent of transvalvular flow. Thus, pressure recovery is of minor importance in severe aortic stenosis but may account for discrepancies between Doppler and manometric gradients observed in patients with mild to moderate aortic stenosis or a prosthetic valve in the aortic position.

Aortic Valve Stenosis↗
PubMed1992

Detachable shape-memory sewing ring for heart valves.

The novel sewing ring concept consists of a detachable ring snap connection between valve housing or stent and suture cuff. The suture cuff itself is a memory metal ring of nickel-titanium (NiTi) that is embedded within a textile fabric torus. After a special training program the ring can assume two geometric shapes when alternately cooled and heated. Due to this physical property, the sewing ring can be safely fixed within a groove of the housing or stent by changing between room and physiological temperature. This allows the surgeon first to implant the sewing ring followed by arbitrary positioning and final fixation of the valve by the shape-memory effect. Additional advantages are an increased stiffening of the valve base for minimizing potential leaflet dysfunction, and reduced time and risk of implantation, particularly when a reoperation is necessary.

Heart Valve Prosthesis↗
PubMed1992

Closing sounds and related complaints after heart valve replacement with St Jude Medical, Duromedics Edwards, Björk-Shiley Monostrut, and Carbomedics prostheses.

OBJECTIVE: To measure the noise produced and related subjective complaints after implantation of four different mechanical heart valve prostheses and to identify further factors related to the patient and prosthesis that influence noise generation and complaints. DESIGN: Sound pressure was measured 5 and 10 cm and 1 m from the point of maximal impulse on the body surface by a calibrated meter in quiet rooms with either a decibel(A) filter or octave filters. The patients were asked about their complaints and examined physically. SETTING: The measurements were conducted in silent rooms of ear, nose, and throat departments. The patients had been operated on either in a university hospital or a community hospital. MAIN OUTCOME MEASURES: Sound pressures of frequency bands and sound pressures measured in dB(A) at various distances. Complaints registerd were: sleep disturbance, disturbance during daytime, "wants a less noisy prosthesis," and "can hear the closing click". PATIENTS: 143 patients after heart valve replacement with St Jude Medical (n = 35), Duromedics Edwards (n = 38), Carbomedics (n = 34) and Björk-Shiley Monostrut (n = 36) prostheses operated on between 1984 and 1988 were matched for valve position, ring size, and body surface area. RESULTS: Duromedics Edwards (33.5 (6) dB(A)) and Björk-Shiley Monostrut valves (31 (4) dB(A)) were significantly louder than St Jude Medical (24 (4) dB(A)) and Carbomedics (25 (6) dB(A)) prostheses (p = 0.0001) (mean (SD)). The louder valves were significantly more often heard by the patients (p = 0.0012) and caused more complaints both during sleep (p = 0.024) and during the daytime (p = 0.07). Patients with these valves were more likely to want a less noisy valve (p = 0.0047). Patients with symptoms were younger, had better hearing, and were more likely to be in sinus rhythm. As well as the type of prostheses, the valve diameter and body height also had an effect on sound emission. CONCLUSIONS: The intensity of the closing click of mechanical valve prostheses was significantly different for various designs. Patient complaints were related to the objectively measured sound pressure. Noise production should be considered when a mechanical valve is selected.

Adult↗
PubMed1992

In vitro evaluation of the calcification behavior of polyurethane biomaterials for cardiovascular applications.

Scope of the study is the calcification behavior of medical grade polyurethanes for cardiovascular devices. Within these applications, implant material calcification is a serious complication. The calcification behavior of these materials is investigated with a dynamic in vitro testing method for evaluating the influence of dynamic mechanical strain and parameters of the solubility of calcium salts.

Calcification, Physiologic↗
PubMed1992

Cavitation of mechanical heart valves under physiologic conditions.

Cavitation has recently entered the discussion of factors leading to blood and material damage after implantation of mechanical heart valves. Since direct evidence for this phenomenon cannot yet be demonstrated in vivo, an in vitro method had to be developed to permit the investigation whether cavitation actually occurs under (simulated) physiologic conditions and to clarify its clinical relevance. Previous studies have shown that different types of commercially available mechanical valves exhibit different tendencies to generate cavitation in vitro. The present study presents a test protocol for the assessment of cavitation generated by different replacement valves under simulated physiologic conditions. Comparative investigations were performed in order to transfer the in vitro results in vivo conditions. Although mechanical valves with an overlapping closing body/valve ring configuration tend to create cavitation earlier, cavitation could not be demonstrated for any investigated mechanical valve type under simulated resting conditions. The danger of continuous cavitation damage is therefore low. Certain valve types exhibit cavitation only at higher heart rates (more than 120-140 beats/min). Some valve types do not show cavitation even during heavy exercise. Based on mathematical models and experimental investigations, it is very likely that if cavitation does occur, blood and material damage may be expected.

Blood↗
PubMed1992

Electromechanical artificial heart with a new gear type and angled pump chambers.

The intrathoracic anatomical situation after explantation of the natural heart defines the maximum available space for the design of the housing as well as of the inlet- and outlet connectors of a fully implantable electromechanical artificial heart. Based on computer-assisted anatomical studies, a total artificial heart housing is designed which facilitates an oblique orientation of the pumping chambers for a better fluidmechanical and anatomical arrangement of the in- and outlet connectors. The pumping chamber geometry is based on modifications of an existing cardiac assist-system. Subsequently a mechanical gear which conforms to this anatomically adapted housing is developed.

Heart, Artificial↗
PubMed1992

Compact mock loops of the systemic and pulmonary circulation for blood pump testing.

Mock loops are an important tool for in vitro investigations of artificial blood pumps. The simple windkessel, throttle, and atrium principle was used for the mock loop design presented. The components of the systemic and the pulmonary mock loop were designed according to calculated numerical simulation parameters. The loops offer a compact design and simple handling. For simulating biventricular assist or total artificial heart (TAH), both loops can be coupled correspondingly. The numerical simulation and the first results with the loops show very good similarity to physiological data of systemic and pulmonary circulation. The measurements of pump characteristics are significant for quantitative comparison of different pump sizes and types, or driving systems.

Blood Circulation↗