A new in vitro test method for calcification of bioprosthetic heart valves

To investigate the calcification behavior of different bioprosthetic heart valves and verify possible hypotheses of the etiology of valve calcification, an accelerated pulse tester for bioprostheses was developed, whereby up to ten valves can be tested under identical test conditions. Each valve was mounted in a separate compartment on a piston and cyclically moved through a calcifying solution at frequencies of up to 800/min at 37 degrees C: An appropriate calcifying solution was evaluated by incubation tests of bovine and porcine tissue. Calcification was confirmed by measuring Ca and phosphate depletion by atomic absorption spectroscopy, von Kossa staining, EDAX, and microradiography. The first tests were successfully carried out on porcine valves that had been nondestructively assessed for tissue/stress anomalies by holographic interferometry prior to the calcification test. The tests showed that 75% of irregular fringe pattern areas corresponded to the calcification areas.

The implantable fuzzy controlled Helmholtz-left ventricular assist device: first in vitro testing

To perform first experimental tests for validation of a new left ventricular assist device (LVAD) with a high efficiency energy converter, a new pump design and a novel type of perfusion control, a functional labtype, were manufactured. With a stroke volume of 65 ml, a total pump housing volume of 450 ml (including valves and connectors), and a weight of 430 g, it is one of the smallest and lightest implantable pulsatile electromechanical LVADs. Pulsatile operation is generated by a special reduction and displacement gear which transforms a uniform rotational movement of a sensorless, electronically commutated DC motor into a translatory pusher plate movement. A prolonged duration for filling (60% of the cycle time) supports full-empty pumping and consequently a high overall pump efficiency. Active adaptation of output flow to organ perfusion demand is achieved by changing the rotational speed of the motor by means of a sensorless fuzzy controller, which detects preload and afterload induced effects at the motor current input. First in vitro test results obtained within a circulatory mock loop that simulates physiological preloads and afterloads are presented. They comprise preload sensitivity and the function of the novel perfusion controller as well as preload and afterload related flow data. The results prove the feasability of the energy conversion with the novel gear and control concept for an implantable electromechanical pulsatile LVAD.

Gastroesophageal reflux in infants: evaluation of a new intraluminal impedance technique

In this study, pH metry was simultaneously applied with a new technique, the intraluminal multiple electrical impedance (IMP) procedure, for measuring gastrointestinal motility for gastroesophageal reflux (GER) detection. Seventeen infants with clinical symptoms of GER disease such as recurrent apnea, aspiration pneumonia, wheezing, and failure to thrive were investigated during two feeding periods. A single catheter combining a pH electrode with seven electrodes for impedance measurements over a distance of 8.5 cm was used for the investigation. In all patients, 185 acid episodes were detected by pH metry. In 106 of these 185 acid episodes, a unique pattern in the IMP readings was noted, indicated by a retrograde esophageal volume flow. These episodes were regarded as acid GER episodes. Seventy-one of the 185 acid episodes occurred during the clearance process of a preceding acid GER characterized by typical IMP readings of an anterograde bolus transport. Eight of 185 acid episodes were missed in the IMP readings for technical reasons. The IMP pattern described as characteristic for a GER was observed in 490 other episodes not detected by pH metry. More than 75% of all GER detected by IMP reached the pharyngeal space; 73% of all GER occurred during feeding and the first 2 postprandial hours and 27% occurred during the remaining time until the next feeding. Even during the latter period, 34% of GER were detected by IMP only; they were missed by pH metry. Volume clearance indicated by IMP was always completed earlier than acidity clearance. The results show that IMP technique facilitates the detection of all GER, whereas pH metry is confined to the measurement of acid GER. Therefore, this technique might improve the evaluation of GER disease and detection of GER in conditions with gastric hypoacidity.

Concept and development of a measuring system for in vivo recording of orthodontically applied forces and torques in the multiband technique. Part I

The cause of tooth movement with a multiband or multibracket appliance is the force system in the bracket, which is generated by an archwire and transmitted by a bracket to the respective tooth. Continuous arch wires are, from the biomechanical point of view, a statically multiply undetermined system. Quantitative determination of the forces and torques acting at the individual teeth, and thus observance of a non-critical level of applied forces, is therefore extremely problematic in practice, the force is applied by the orthodontist more or less "intuitively" on the basis of recommendations evolved in theory and in vitro. The measuring technique presented in this paper is designed to permit these force systems to be recorded in vivo for the first time, allowing account to be taken of the individual situation "tooth-periodontium-bone". Evolvement of an appropriate measuring principle was followed by development of the individual components of the measuring system. The results of the subsequent initial in vitro test series offer the prospect of successful clinical application.

Comparison of passive and active perfusion catheters: an in vitro study in a pulsatile coronary flow model

Perfusion balloon catheters are designed to provide continuous transcatheter blood flow and thereby reduce myocardial ischemia during coronary angioplasty. To compare the transcatheter flow rates of active and passive (auto-) perfusion catheters, a well-controlled experimental study was performed in a circulation model that duplicates the phasic, predominantly diastolic flow pattern of the left coronary artery. Mean diastolic coronary driving pressure varied between 20 and 100 mm Hg. For the autoperfusion catheters, a strong relationship between transcatheter flow and diastolic coronary driving pressure was found. For example, a coronary driving pressure of 80 mm Hg provided a coronary flow of 30 ml/min (RX-Perfusion [RP], ACS), 28 ml/min (Speedflow [SF], Schneider), 20 ml/min (Lifestream [LS], ACS), and 19 ml/min (Flowtrack [FT], ACS). Reduction of driving pressure to 40 mm Hg decreased the absolute transcatheter flow, which was now 16 ml/min (RP), 13 ml/min (SF), and 10 ml/min (LS and FT). The relative catheter flow (the ratio of absolute flow to baseline coronary flow rate without a catheter in place), was independent of actual coronary driving pressure and ranged between 21% +/- 1% (RP) and 14% +/- 1% (FT and LS). For the active perfusion system (Coreflo, Leocor, a maximal transcatheter flow of 82 ml/min was found. Using this active perfusion system, the relative catheter flow increased with decreasing coronary driving pressure:80 --> 40 mm Hg: 56% --> 107%. For all catheters, the distal perfusion decreased between 30% (3.0 mm RP) and 50% (3.0 mm LS) by a 0.014-inch guidewire placed through the inner channel of the catheter. Because of the strong relationship between coronary driving pressure and transcatheter flow, the residual flow through all autoperfusion catheters becomes critical (<20 ml/min), when the coronary driving pressure drops below 50 mm Hg. By contrast, active perfusion systems are independent of the actual coronary driving pressure and are therefore advantageous for prolonged dilation in patients with low aortic pressure.

Quality control of bioprosthetic heart valves by means of holographic interferometry

BACKGROUND AND AIMS OF THE STUDY

Limited durability of porcine bioprostheses is mainly caused by the progressive development of calcification. We tested the hypothesis that hidden tissue anomalies or unfavorable stress concentrations of commercially available bioprostheses may lead to later calcification and dysfunction. Application of holographic interferometry for non-destructive testing of biological heart valves enables a full-field analysis of heart valves and reveals deformation irregularities of valve tissue.

MATERIAL AND METHODS

We developed an accelerated calcification protocol for bioprosthetic heart valves including an accelerated pulsatile valve tester for simultaneous testing of 10 heart valves under identical conditions and a rapid synthetic calcification fluid containing a final Ca x P of 130 (mg/dl)2 in barbital buffer solution. Ten porcine bioprostheses (St. Jude Medical, Bioimplant) were assessed by holographic interferometry and subjected to the pulsatile accelerated calcification process. Distribution and amount of calcification was evaluated by microradiography after 12 x 10(6) and 19 x 10(6) cycles, respectively. Areas of irregular fringe patterns detected by holography as well as areas of calcification were calculated and compared using a personal computer.

RESULTS

All tested bioprostheses had localized or extended areas with holographic irregularities and the accelerated valve testing protocol resulted in even macroscopically visible calcifications at various sites. Comparative analysis of the obtained microradiographs revealed that 74.2% +/- 6.0% of calcified leaflet areas lay within the previously detected holographic anomalies.

CONCLUSIONS

Our first results show a strong correlation between holographic anomalies and calcification of porcine bioprostheses. We conclude that suitable methods for evaluation and quality control of bioprosthetic heart valves are available and seem to be predictive with regard to valve calcification.

In vitro testing of heart valve wear outside of the manufacturers laboratory--requirements and controversies

BACKGROUND AND AIM OF THE STUDY

Currently, various national and international Standards regulate in vitro and in vivo testing as well as the clinical evaluation of heart valve prostheses. This study concentrates on a single aspect of these protocols: accelerated wear and fatigue testing.

METHODS

The differences in the above Standards were appraised and an experimental study was designed to assess the validity of Standard testing conditions with respect to in vivo relevance. An instrumented BSCC tilting disc valve was tested in two different types of fatigue testers and in a separate study in a sheep model.

RESULTS

By comparing the obtained results it could be clearly demonstrated that under the current Standard conditions (ISO, CEN, FDA) actual in vivo impact loading cannot be reproduced. Also, the two compared test devices, though operating under the same Standard conditions, generated totally different loading conditions on the test valve.

CONCLUSIONS

Based on these findings it is suggested that the actual loading conditions of each valve type should be measured either in vivo in animal models or within a circulatory mock loop which provides physiologic loading conditions, and the test conditions for accelerated wear and fatigue testing should be modified accordingly. Cavitation effects which can be assumed to be associated with high frequency wear testing is another important issue which has to be addressed in future amendments to the Test Standards.

Hemodynamic system analysis of intraarterial microaxial pumps in vitro and in vivo

Because of the lack of a sophisticated pump management system, the performance of the Hemopump in patients cannot assessed successfully. To clarify the interrelationship between an intravascular nonpulsatile pump and a pulsating ventricle, an in vitro study was set up under controlled conditions. Before these in vitro experiments, a series of in vivo experiments were performed in sheep using Hp31 cannulae. As anticipated, the resulting pulsatile pump flow was a function of the momentary pressure difference across the pump. This varying pump flow showed a significant flow loop hysteresis, indicating that the pressure difference across the pump is not the only parameter governing momentary pump flow of a rotary pump operating at constant speed in a pulsatile environment. Furthermore, flow in the Hp31 was significantly influenced by the inflow situation, blood supply, size of the ventricular cavity, and shape and position of the inflow cannula within the ventricle. Pulsatile flow conditions with good as well as impaired inflow into the pump were accordingly simulated in vitro to verify the in vivo measurements, to characterize the various inflow conditions, and to discuss methods of improved pump management. As a result of the in vivo and in vitro experiments, one can rely on the measurement of nonpulsatile in vitro flow and pressure differences across the pump to characterize the momentary pump flow for good inflow conditions into the pump. For these situations, the flow hysteresis produced, caused by fluid inertia within the pump and cannula, can be neglected. In contrast, for an impaired inflow situation, the calculated pump flow based on pressure difference measurements can be misleading. Consequently, an improved pump management system is required to adjust the pump speed, the pump performance, to any kind of impaired inflow.

The heart-Hemopump interaction: a study of Hemopump flow as a function of cardiac activity

The Hemopump is a useful left ventricular assist device. Because it is a rotary blood pump, the pump performance is not constant and is dependent on the cardiac cycle. We measured the static flow delivered by the pump at varying pressure heads (delta P) in a mock circulation. These data are compared to the pump performance in vivo. On the basis of these results, 5 sheep were instrumented for continuous Hemopump flow measurement as well as left ventricular and aortic pressure measurements. The Hemopump flow was relayed instantaneously to the pressure head. Low filling and ventricular failing (through intravenous administration of a beta-blocker) conditions were applied. The in vivo measured flows also are pressure head dependent, but the flow curve shows hysteresis resulting in a loop during each cardiac cycle. The in vivo peak flows (delta P = 0) are similar to the in vitro data. The in vivo means flows (delta = 50 mm Hg) are similar to the in vitro data for the lower pump speeds but are less than that at the higher pump speeds (3.74 +/- 0.55 L/min in vivo at Speed 7 versus 4.6 L/min in vitro). Low filling interrupts the delta P-flow loop and reduces flow. In the failing ventricle, delta P increases and flow is reduced. The cannula leaks and results in aortic insufficiency (0.36 +/- 0.05 L/min) when the pump is turned off. Several conclusions have been drawn from these tests: Cardiac activity is beneficial for the pump performance as well as when the aortic pressure curve is nonpulsatile; the longer the systolic phase, the higher the pump flow; the pump should never be turned off in clinical use, and filling is important for the pump's performance.

In vitro flow dynamics of a new mechanical cardiac valve prosthesis--"tricusp"

The hydrodynamic performance of a newly developed JCL-trileaflet mechanical heart valve prostheses (Tricusp) was measured and compared with some of the currently most used heart valve prostheses types. All experiments were performed in an electrohydraulic, computer-controlled pulse duplicator simulating the left side of the human circulatory system. Testing conditions were set according to a Food and Drugs Administration interlaboratory comparison protocol, with cardiac outputs 3.0, 4.5, 6.5 or 8.0 l/min and a constant heart rate of 70 beats/min. Mean systolic pressure differences, volume and energy losses, dimensionless pressure losses and energy loss coefficients were calculated from the recorded pressure, volume and flow tracings. The results with the Tricusp valve were found to be as good, or even better than those with the currently most used commercially available bileaflet valves.

Three-dimensional versus two-dimensional video endoscopy. A clinical field study in laparoscopic application

Stereoscopic visualisation systems are available for the medical application in clinical routine, especially in the field of endoscopic or minimal invasive surgery. As we have shown in laboratory studies in 1991 the use of stereoscopic visualisation systems leads to a significant improvement of endoscopic manipulation. A broader diffusion of these systems in clinical routine will be based on quantitative evaluation of the influence of stereoscopic visualisation systems on the intervention. As a first approach we performed a clinical field study to compare 2D and 3D video endoscopy in laparoscopic interventions in urological surgery.

The influence of LCD shutter glasses on spatial perception in stereoscopic visualization

For a general establishment of stereoscopic visualisation systems in clinical routine a fundamental analysis of the influence of technical, optical and physiological parameters onto visual spatial perception seems to be necessary to achieve an optimisation of the existing devices. As one important component of these systems we perform studies on the influence of LCD Shutter glasses on the individual binocular perception. The developed shutter system, the initial experiments and their results are presented.

Using human factor analysis and VR simulation techniques for the optimization of the surgical worksystem

The introduction of minimal invasive operating techniques into the conventional surgical worksystem, requires a fundamental analysis of the resulting problems. Based on ergonomic investigations, concerning working postures, static holding work and task sequence analysis, bottlenecks of workplace design can be demonstrated. As a contribution to the optimization of workplace design, the development of a CAD-based VR simulation environment with an integrated anthropometric man-model, as a very useful tool for anthropometric system design is presented.

A desktop image processing system for computer-assisted orthopedic surgery (DISOS)

Within the framework of our research activities in orthopedic surgery a PC-based imageprocessing system for the processing of x-ray computer-tomographic data has been developed. The system is used as a work platform for the investigations concerning interaction modalities, strategies and techniques for an efficient planning and execution of surgical interventions in orthopedic surgery by means of individual templates. The method of individual templates has been presented in /1,2/ and will not be subject of this paper.

Development, manufacturing and validation of a single-leaflet mechanical heart valve prosthesis

A new single-leaflet mechanical heart valve prosthesis was developed and evaluated in vitro and in vivo. The basic design of the closing body is S-shaped, with the leading and trailing edges parallel to the flow direction. This closing body has an additional cross camber and a fixed axis of rotation. The bearing pins of the disc are guided within oblong through bores of the housing, resulting in a self-cleaning effect. The valve housing has a nozzle-shaped configuration in order to avoid flow separation at the inlet and to reduce the associated pressure loss. Development and optimization studies were carried out by means of upscale valve models. After manufacturing of the closing body from pyrolytic carbon and of the orifice from titanium final hydrodynamic evaluation was carried out according to ISO 5840. The results were comparable or better than for bileaflet valves, the cavitation threshold was above a left ventricular dP/dt of 5500 mmHg/sec. The excellent in vitro results were confirmed by in vivo animal studies in calves and sheep with implantation times of 12 months and six months, respectively. These studies were carried out at the RWTH Aachen and at the NIH, Washington. Without any anticoagulation treatment the valves showed excellent results, confirming the concept of a valve design based on advanced engineering techniques.

The effect of storage temperature on the stability of frozen erythrocytes

A systematic study on the stability of frozen erythrocytes was performed. Washed and concentrated erythrocytes were mixed with an equal volume of cryoprotective solution containing 24% (w/w) hydroxyethyl starch (HES) and 60 mmol/liter NaCl according to an established protocol. Volumes of 250 microliters of this mixture were filled into polypropylene tubes and cooled to -196 degrees C with a rate of 293 degrees C/min by immersion in liquid nitrogen. The storage temperature was then varied from -10 to -75 degrees C and could be identified as the predominant factor influencing hemolysis kinetics. The effect of storage temperature on the frozen erythrocytes after thawing was evaluated by measuring the hemolysis in a dilute, isotonic NaCl solution (saline stability). A strong time dependence was found within the temperature range studied and could be described by an exponential kinetic law. A stability prediction was made for storage temperatures lower than those examined. Temperature ranges of qualitatively different hemolysis kinetics were identified and compared to devitrification behavior of intra-and extracellular solutions. The intracellular solution was simulated by a concentrated mixture of dried erythrocytes and water. The devitrification behavior was studied using DSC techniques. A rapidly frozen mixture was annealed at selected temperatures which fall into the range of storage temperatures for frozen erythrocytes. This paper tentatively interprets the devitrification data with respect to the means for cell damage during storage. The results are reviewed with respect to the design of a safe storage procedure.

The effect of left ventricular dP/dt on the in vitro dynamics of the Björk-Shiley Convexo-Concave mitral valve

Left ventricular (LV) dP/dt is considered an important hemodynamic factor influencing the dynamics of mechanical heart valve prostheses. LV dP/dt is dependent on patient factors including age, cardiac activity, health, and medication. The objective of this study was to determine the effect of LV dP/dt on the closing dynamics of mechanical heart valve prostheses in the mitral position. Eight instrumented 29 mm Björk-Shiley Convexo-Concave (BSCC) heart valves were tested in the pulse duplicator of the Helmholtz Institute. The valves had miniature strain gages mounted at the base of the outlet strut to measure impact loads at closure. Closing velocities were measured with a "light gate" device which was triggered by the closing leaflet. Physiologic pressure and flow waveforms were generated by a computer-controlled hydraulic drive unit. LV dP/dt was varied from 500 to 4000 mmHg/s simulating a wide range of physiologic conditions. It was found that the closing velocity was almost linearly related to LV dP/dt. At 4000 mmHg/s, closing velocities ranged from 1.5 to 2.0 m/s. Impact loads increased monotonically with LV dP/dt and closing velocity. In some valves, impact loads reached 2800 g at LV dP/dt of 4000 mmHg/s, and closing velocities of 2.0 m/s.

Concept, realization, and first in vitro testing of an intraarterial microaxial blood pump

Intravascular operating microaxial pumps have been clinically introduced (Hemopump 21; Hemopump 14) and have proven to be useful tools for cardiac assist. Due to device-related complications that are associated with the drive concept of an extracorporeal motor and a flexible drive shaft cable, a new pump concept is presented and has been refined in the development process. The cable is replace by a proximally attached drive unit and an extracorporeal power supply. In addition to ongoing hydrodynamic studies of the flow inside the pump and improvements of the overall hydraulic performance, a microelectric motor was realized and integrated. In vitro tests revealed the feasibility of such a concept.

Hydraulic refinement of an intraarterial microaxial blood pump

Intravascularly operating microaxial pumps have been introduced clinically proving to be useful tools for cardiac assist. However, a number of complications have been reported in literature associated with the extra-corporeal motor and the flexible drive shaft cable. In this paper, a new pump concept is presented which has been mechanically and hydraulically refined during the developing process. The drive shaft cable has been replaced by a proximally integrated micro electric motor and an extra-corporeal power supply. The conduit between pump and power supply consists of only an electrical power cable within the catheter resulting in a device which is indifferent to kinking and small curvature radii. Anticipated insertion difficulties, as a result of a large outer pump diameter, led to a two-step approach with an initial 6,4mm pump version and a secondary 5,4mm version. Both pumps meet the hydraulic requirement of at least 2.5l/min at a differential pressure of 80-100 mmHg. The hydraulic refinements necessary to achieve the anticipated goal are based on ongoing hydrodynamic studies of the flow inside the pumps. Flow visualization on a 10:1 scale model as well as on 1:1 scale pumps have yielded significant improvements in the overall hydraulic performance of the pumps. One example of this iterative developing process by means of geometrical changes on the basis of flow visualization is illustrated for the 6.4mm pump.

Fuzzy control concept for a total artificial heart

The development of an electromechanically driven total artificial heart (Helmholtz-TAH) was initiated in 1990. Anatomical fitting, biocompatibility, and automatic physiologic adaptation of pump output are the basic requirements that characterize the overall TAH concept. For evaluation of these features, a TAH labtype was developed. It provides most features of the conceptual artificial heart and supports in vitro testing of energy conversion, pump behavior, structural parts, sensors, and control concepts. A fuzzy controller has been implemented for adaptation of the pump rate to body perfusion demand by left pump chamber filling detection. This controller will be an important element of a future extensive TAH control system. The implementation is supported by a professional fuzzy control development tool that allows on-line and real time optimization of control strategies for dynamic processes. The first experiments proved the feasibility and the advantages of this fuzzy control concept. The first in vitro test results are presented.