Abstract: The need for smaller, more efficient ventricular assist devices that can be used in a more chronic setting have led to exploration of mechanical circulatory support in the pediatric population. The pediatric Jarvik 2000 heart (child size), under development, was implanted in six juvenile sheep and studied for both acute fit and chronic performance evaluation. Daily hemodynamic measurements of cardiac output and pump output at varying pump speeds were taken. In addition, plasma free hemoglobin, lactic acid dehydrogenase, and platelet activation from blood samples were determined at baseline, after implantation, and twice a week thereafter. The measured flow through the outflow graft at increasing speeds from 10,000 rpm to 14,000 rpm with an increment of 1,000 rpm were 1.47 +/- 0.43, 1.89 +/- 0.52, 2.36 +/- 0.61, 2.80 +/- 0.73, and 3.11 +/- 0.86 (L/min). The baseline plasma free hemoglobin was 11.95 +/- 4.76 (mg/dL), with subsequent mean values being <30 mg/dL at postimplantation and weekly postimplantation measurements. Both lactic acid dehydrogenase and platelet activation showed an acute increase within the first week after implantation with subsequent return to baseline by 2 weeks after surgery. Our initial animal in vivo experience with the pediatric Jarvik 2000 heart shows that a small axial flow pump can provide partial to nearly complete circulatory support with minimal adverse effects on blood components.

Abstract: A miniature Maglev blood pump based on magnetically levitated bearingless technology is being developed and optimized for pediatric patients. We performed impeller optimization by characterizing the hemodynamic and hemocompatibility performances using a combined computational and experimental approach. Both three-dimensional flow features and hemolytic characteristics were analyzed using computational fluid dynamics (CFD) modeling. Hydraulic pump performances and hemolysis levels of three different impeller designs were quantified and compared numerically. Two pump prototypes were constructed from the two impeller designs and experimentally tested. Comparison of CFD predictions with experimental results showed good agreement. The optimized impeller remarkably increased overall pump hydraulic output by more than 50% over the initial design. The CFD simulation demonstrated a clean and streamlined flow field in the main flow path. The numerical results by hemolysis model indicated no significant high shear stress regions. Through the use of CFD analysis and bench-top testing, the small pediatric pump was optimized to achieve a low level of blood damage and improved hydraulic performance and efficiency. The Maglev pediatric blood pump is innovative due to its small size, very low priming volume, excellent hemodynamic and hematologic performance, and elimination of seal-related and bearing-related failures due to adoption of magnetically levitated bearingless motor technology, making it ideal for pediatric applications.

Abstract: The CentriMag centrifugal blood pump is a newly developed ventricular assist device based on magnetically levitated bearingless rotor technology. A combined computational and experimental study was conducted to characterize the hemodynamic and hemocompatibility performances of this novel blood pump. Both the three-dimensional flow features of the CentriMag blood pump and its hemolytic characteristics were analyzed using computational fluid dynamics (CFD)-based modeling. The hydraulic pump performance and hemolysis level were quantified experimentally. The CFD simulation demonstrated a clean and streamlined flow field in the main components of the CentriMag blood pump. The predicted results by hemolysis model indicated no significant high shear stress regions in the pump. A comparison of CFD predictions and experimental results showed good agreements. The relatively large gap passages (1.5 mm) between the outer rotor walls and the lower housing cavity walls provide a very good surface washing through a secondary flow path while the shear stresses in the secondary flow paths are reduced, resulting in a low rate of hemolysis ([Normalized Index of Hemolysis] NIH = 0.0029 +/- 0.006) without a decrease of the pump's hydrodynamic performance (pressure head: 352 mm Hg at a flow rate of 5.0 L/min and a rotational speed of 4,000 rpm).

Abstract: The understanding of erythrocyte deformation under conditions of high shear stress and short exposure time is central to the study of hemorheology and hemolysis within prosthetic blood contacting devices. A combined computational and experimental microscopic study was conducted to investigate the erythrocyte deformation and its relation to transient stress fields. A microfluidic channel system with small channels fabricated using polydimethylsiloxane on the order of 100 mum was designed to generate transient stress fields through which the erythrocytes were forced to flow. The shear stress fields were analyzed by three-dimensional computational fluid dynamics. Microscopic images of deforming erythrocytes were experimentally recorded to obtain the changes in cell morphology over a wide range of fluid dynamic stresses. The erythrocyte elongation index (EI) increased from 0 to 0.54 with increasing shear stress up to 123 Pa. In this shear stress range, erythrocytes behaved like fluid droplets, and deformed and flowed following the surrounding fluid. Cells exposed to shear stress beyond 123 Pa (up to 5170 Pa) did not exhibit additional elongation beyond EI=0.54. Two-stage deformation of erythrocytes in response to shear stress was observed: an initial linear elongation with increasing shear stress and a plateau beyond a critical shear stress.

Abstract: BACKGROUND: Cardiac remodeling has been shown to have deleterious effects at both the global and local levels. The objective of this study is to investigate the role of strain in the initiation of structural and functional changes of myocardial tissue and its relation to alteration of calcium-handling proteins during cardiac remodeling after myocardial infarction. METHODS: Sixteen sonomicrometry transducers were placed in the left ventricular free wall of 9 sheep to measure the regional strain in the infarct, adjacent, and remote myocardial regions. Hemodynamic, echocardiographic, and sonomicrometry data were collected before myocardial infarction, after infarction, and 2, 6, and 8 weeks after infarction. Regional myocardial tissues were collected for calcium-handling proteins at the end study. RESULTS: At time of termination, end-systolic strains in 3 regionally distinct zones (remote, adjacent, and infarct) of myocardium were measured to be -14.65 +/- 1.13, -5.11 +/- 0.60 (P < or = .05), and 0.92 +/- 0.56 (P < or = .05), respectively. The regional end-systolic strain correlated strongly with the abundance of 2 major calcium-handling proteins: sarcoplasmic reticulum Ca2+ adenosine triphosphatase subtype 2a (r2 = 0.68, P < or = .05) and phospholamban (r2 = 0.50, P < or = .05). A lesser degree of correlation was observed between the systolic strain and the abundance of sodium/calcium exchanger type 1 protein (r2 = 0.17, P < or = .05). CONCLUSIONS: Regional strain differences can be defined in the different myocardial regions during postinfarction cardiac remodeling. These differences in regional strain drive regionally distinct alterations in calcium-handling protein expression.

Abstract: OBJECTIVES: Currently available therapies for acute and chronic lung diseases have not been effective and have various problems associated with the technologies used. We present a novel active mixing pump-lung with the goal of providing total respiratory support to ambulatory patients. METHODS: The pump-lung is based on the concept of active mixing oxygenation within a constrained vortex. The rotation of hollow-fiber membranes disrupts the concentration boundary layer, increasing gas exchange efficiency, and simultaneously pumps the blood. Consequently, the amount of membranes required to achieve gas transfer sufficient for total respiratory support is considerably small. A series of studies, including computational design, experimental bench testing, and in vivo animal experiments, have been performed to implement this concept into a viable artificial pump-lung device. RESULTS: A series of pump-lung prototypes with a membrane surface area of 0.17 to 0.5 m2 were designed and characterized in vitro with bovine blood, demonstrating extremely high gas exchange efficiency. The prototype with a gas exchange surface area of 0.5 m2 was evaluated in calves. The device provided oxygen transfer of approximately 115 mL/min for respiratory support of an animal for up to 5 days. CONCLUSIONS: Progress to date suggests a high likelihood of success for an extracorporeal shorter-term lung that can be switched in and out like dialysis devices. Our device is unique in that it incorporates an integrated pumping and active mixing principle for excellent gas transfer and eliminates the need of the native right ventricle's ability to power blood through the artificial and natural lungs.

Abstract: Over the past several decades, blood-soluble drag reducing polymers (DRPs) have been shown to significantly enhance hemodynamics in various animal models when added to blood at nanomolar concentrations. In the present study, the effects of the DRPs on blood circulation were tested in anesthetized rats exposed to acute hemorrhagic shock. The animals were acutely resuscitated either with a 2.5% dextran solution (Control) or using the same solution containing 0.0005% or 5 parts per million (ppm) concentration of one of two blood soluble DRPs: high molecular weight (MW=3500 kDa) polyethylene glycol (PEG-3500) or a DRP extracted from Aloe vera (AVP). An additional group of animals was resuscitated with 0.0075% (75 ppm) polyethylene glycol of molecular weight of 200 kDa (PEG-200), which possesses no drag-reducing ability. All of the animals were observed for two hours following the initiation of fluid resuscitation or until they expired. We found that infusion of the DRP solutions significantly improved tissue perfusion, tissue oxygenation, and two-hour survival rate, the latter from 19% (Control) and 14% (PEG-200) to 100% (AVP) and 100% (PEG-3500). Furthermore, the Control and PEG-200 animals that survived required three times more fluid to maintain their blood pressure than the AVP and PEG-3500 animals. Several hypotheses regarding the mechanisms underlying these observed beneficial hemodynamic effects of DRPs are discussed. Our findings suggest that the drag-reducing polymers warrant further investigation as a potential clinical treatment for hemorrhagic shock and possibly other microcirculatory disorders.

Abstract: Biologically active heart valves (tissue engineered and recellularized tissue-derived heart valves) have the potential to offer enhanced function when compared to current replacement value therapies since they can possibly remodel, and grow to meet the needs of the patient, and not require chronic medication. However, this technology is still in its infancy and many fundamental questions remain as to how these valves will function in vivo. It has been shown that exposing biologically active tissue constructs to pulsatile pressures and flows during in vitro culture produces enhanced extracellular matrix protein expression and cellularity, although the ideal hydrodynamic conditioning regime is as yet unknown. Moreover, in vitro organ-level studies of living heart valves aimed at studying the remodeling processes require environments that can accurately reproduce in vivo hemodynamics under sterile conditions. To this end, we have developed a system to study the effects of subjecting biologically active heart valves to highly controlled pulsatile pressure and flow waveforms under sterile conditions. The device fits inside a standard incubator and utilizes a computer-controlled closed loop feedback system to provide a high degree of control. The mean pressure, mean flow rate, driving frequency, and shape of the pulsatile pressure waveform can be changed automatically in order to simulate both physiologic and nonphysiologic hemodynamic conditions. Extensive testing and evaluation demonstrated the device's ability to subject a biologically active heart valve to highly controlled pulsatile waveforms that can be modulated during the course of sterile incubation.

Abstract: BACKGROUND: Pathophysiology of long-term continuous flow left ventricular assist is not well described. With many of these devices becoming available, it is important to examine for possible pathologic effects. In this study we examined the relationship between diminished pulsatility and pathologic changes in renal cortical arteries. METHODS: Twenty-nine calves were implanted with various continuous flow left ventricular assist systems in a left ventricle-descending thoracic aorta bypass configuration. Pulsatility was quantified by pulse pressure and pulsatility index. Pathologic changes of the renal cortex arteries were described and evaluated by medial thickness, medial/vascular cross-sectional area ratio, and smooth muscle cell count, to quantify hypertrophy or hyperplasia. Seven calves, which underwent a sham-implant, were used as controls. RESULTS: Systolic arterial pressure, pulse pressure, and pulsatility index were significantly lower and diastolic pressure was significantly higher than before implant in pump-implanted animals. Twenty-three of 29 pumpimplanted calves (79.3%) had medial smooth muscle cell hypertrophy in renal cortex arteries, whereas none of sham-implanted calves had any abnormal lesions. When the pump-implanted calves were grouped according to the presence of smooth muscle cell hypertrophy, there was a clear trend toward lower pump flow rate in calves with lesions. Renal function was within the normal range in all calves. CONCLUSIONS: There appears to be a relationship between smooth muscle cell hypertrophy in renal cortex arteries and continuous flow left ventricular assist. Furthermore, although the pathologic changes are likely multifactorial, these lesions appear to be related to lower pump assist rates.

Abstract: Continuous flow ventricular assist devices (CFVADs) are thought to be the next generation of circulatory assist devices. With many now in various stages of development or clinical trial, it is important that the physiologic aspects of these pumps be critically analyzed. In this study, 15 calves were divided into two groups. One group received a CFVAD, and the other a sham implant. Two additional animals were used in an acute study to examine aortic blood flow patterns from a CFVAD. Tissue perfusion was measured on all animals before surgery and then weekly thereafter. Before surgery, there was no difference in hemodynamics or tissue perfusion between studied animals. Postoperatively, CFVAD animals had statistically significant increased diastolic pressure. Significantly decreased pulse pressure, pulse index, and tissue perfusion were also observed in CFVAD animals. Results from the flow pattern studies suggested that at moderate levels of pump support (40-75%), the amount of blood flow distal to the outflow graft anastomosis decreased approximately 25% because of increased regurgitant blood flow in the aorta. These results suggest that the diminished tissue perfusion is likely due to changes in aortic hemodynamics and provide some insight into the distribution of flow from CFVADs.

Abstract: As continuous flow pumps become more prominent as long-term ventricular assist devices, the wide range of conditions under which they must be operated has become evident. Designed to operate at a single, best-efficiency, operating point, continuous flow pumps are required to perform at off-design conditions quite frequently. The present study investigated the internal fluid dynamics within two representative rotary fluid pumps to characterize the quality of the flow field over a full range of operating conditions. A Nimbus/UoP axial flow blood pump and a small centrifugal pump were used as the study models. Full field visualization of flow features in the two pumps was conducted using a laser based fluorescent particle imaging technique. Experiments were performed under steady flow conditions. Flow patterns at inlet and outlet sections were visualized over a series of operating points. Flow features specific to each pump design were observed to exist under all operating conditions. At off-design conditions, an annular region of reverse flow was commonly observed within the inlet of the axial pump, while a small annulus of backflow in the inlet duct and a strong disturbed flow at the outlet tongue were observed for the centrifugal pump. These observations were correlated to a critical nondimensional flow coefficient. The creation of a "map" of flow behavior provides an additional, important criterion for determining favorable operating speed for rotary blood pumps. Many unfavorable flow features may be avoided by maintaining the flow coefficient above a characteristic critical coefficient for a particular pump, whereas the intrinsic deleterious flow features can only be minimized by design improvement. Broadening the operating range by raising the band between the critical flow coefficient and the designed flow coefficient, is also a worthy goal for design improvement.

Abstract: The geometric configuration of the cannula connection to the left ventricular (LV) apex was studied with respect to several characteristics defining functionality and compatibility. The authors had previously determined, through in vivo studies in sheep, that the design of the cannula used with a dynamic blood pump for LV circulatory support can significantly affect the hemodynamics by improving both the bypass flow rate and the fluid dynamics within the ventricle. The tip of the cannula can aid in preventing wall to wall ventricular collapse, as well as septal shift, due to reduced LV pressure. Proper surgical placement of the cannula with respect to the endocardial surface of the LV can also be simplified by the tip geometry. To investigate the anatomic interaction and fluid dynamics of apical cannulation, transparent compliant casts of bovine LVs were fabricated for in vitro flow visualization. Two different heart geometries were cast, end systolic and end diastolic. The latter was fitted with a pericardial mitral valve and pressurized in a pulsatile fashion to simulate the wall movement of a beating heart. The internal flow and anatomy were visualized with fluorescent particle tracking velocimetry. These studies were performed with conventional cannula tips, as well as a novel, trumpet mouth cannula. The visualization clearly shows the dramatic differences in flow between the geometries tested, and strongly advocates a trumpet mouth design. This novel tip demonstrated excellent placement, beneficial stenting, and improved blood flow by reducing apical stasis and recirculation. Ongoing evaluation of these and future geometries include the application of in vitro endoscopy, quantitative velocimetry, and extension to dilated human ventricles.

Abstract: Nimbus Inc. (Rancho Cordova, CA) and the University of Pittsburgh have completed the second year of development of a totally implanted axial flow blood pump under the National Institutes of Health Innovative Ventricular Assist System Program. The focus this year has been on completing pump hydraulic development and addressing the development of the other key system components. Having demonstrated satisfactory pump hydraulic and biocompatibility performance, pump development has focused on design features that improve pump manufacturability. A controller featuring full redundancy has been designed and is in the breadboard test phase. Initial printed circuit layout of this circuit has shown it to be appropriately sized at 5 x 6 cm to be compatible with implantation. A completely implantable system requires the use of a transcutaneous energy transformer system (TETS) and a diagnostic telemetry system. The TETS power circuitry has been redesigned incorporating an improved, more reliable operating topography. A telemetry circuit is undergoing characterization testing. Closed loop speed control algorithms are being tested in vitro and in vivo with good success. Eleven in vivo tests were conducted with durations from 1 to 195 days. Endurance pumps have passed the 6 month interval with minimal bearing wear. All aspects of the program continue to function under formal quality assurance.

Abstract: An important consideration for clinical application of rotary blood pump based ventricular assist is the avoidance of ventricular collapse due to excessive operating speed. Because healthy animals do not typically demonstrate this phenomenon, it is difficult to evaluate control algorithms for avoiding suction in vivo. An acute hemodynamic study was thus conducted to determine the conditions under which suction could be induced. A 70 kg calf was implanted with an axial flow assist device (Nimbus/UoP IVAS; Nimbus Inc., Rancho Cordova, CA) cannulated from the left ventricular apex to ascending aorta. On initiation of pump operation, several vasoactive interventions were performed to alter preload, afterload, and contractility of the left ventricle. Initially, dobutamine increased contractility and heart rate ([HR] = 139; baseline = 70), but ventricular collapse was not achievable, even at the maximal pump speed of 15,000 rpm. Norepinephrine decreased HR (HR = 60), increased contractility, and increased systemic vascular resistance ([SVR] = 24; baseline = 15), resulting in ventricular collapse at a pump speed of 14,000 rpm. Isoproterenol (beta agonist) increased HR (HR = 103) and decreased SVR (SVR = 12), but ventricular collapse was not achieved. Inferior vena cava occlusion reduced preload, and ventricular collapse was achieved at speeds as low as 11,000 rpm. Esmolol (beta1 antagonist) decreased HR (HR = 55) and contractility, and ventricular collapse was achieved at 11,500 rpm. Episodes of ventricular collapse were characterized initially by the pump output exceeding the venous return and the aortic valve remaining closed throughout the cardiac cycle. If continued, the mitral valve would remain open throughout the cardiac cycle. Using these unique states of the mitral and aortic valves, the onset of ventricular collapse could reliably be identified. It is hoped that the ability to detect the onset of ventricular collapse, rather than the event itself, will assist in the development and the evaluation of control algorithms for rotary ventricular assist devices.

Abstract: An axial-flow ventricular assist device (VAD) under development at the authors' facility is intended for use as a long-term implantable device. At high speeds axial-flow VADs can collapse the native ventricle and damage the heart muscle, lung tissue, and blood. A prototype algorithm was developed to maintain physiologic perfusion to the vital organs while preventing ventricular collapse, through analysis of the electrical current waveform of the motor. The premise of the control algorithm is that the hemodynamics of the patient are reflected in the shape of this waveform. This approach is intended to eliminate the need for invasive sensors, thus effectively using the pump itself as a transducer. The control algorithm regulates the speed of the pump by comparing the motor-current waveform with reference waveforms using a matched filter. The matched filter was evaluated by its classification and differentiation performance. Thus far, the authors have been able to classify the waveforms into one of the four physiologic regions (below, within, or above the optimal range, and ventricular suction) with over 90% reliability. Ongoing work is directed toward improving the detection of ventricular suction, as this condition must be strictly avoided.

Abstract: Nimbus and the University of Pittsburgh (UOP) have continued the development of a totally implanted axial flow blood pump under the National Institutes of Health (NIH) Innovative Ventricular Assist System (IVAS) program. This 62 cc device has an overall length of 84 mm and an outer diameter of 34.5 mm. The inner diameter of the blood pump is 12 mm. It is being designed to be a totally implanted permanent device. A key achievement during the past year was the completion of the Model 2 pump design. Ten of these pumps have been fabricated and are being used to conduct in vitro and in vivo experiments to evaluate the performance of different materials and hydraulic components. Efforts for optimizing the closed loop speed control have continued using mathematical modeling, computer simulations, and in vitro and in vivo testing. New hydraulic blade designs have been tested using computational fluid dynamics (CFD) and flow visualization. A second generation motor was designed with improved efficiency. To support the new motor, a new motor controller fabricated as a surface mount PC board has been completed. The program is now operating under a formal QA system.

Abstract: Several earlier studies have indicated that bileaflet mechanical heart valves behave irregularly at low cardiac output and low pulse rate conditions, and that their hydrodynamic performances are generally inadequate. The authors conducted in vitro experiments in a pulsatile mock circulatory loop to compare the performance of the St. Jude Medical (SJM) valve and a long body bileaflet prosthesis recently introduced by Medical Carbon Research Institute (MCRI) (Austin, TX). The new MCRI mechanical heart valve model was designed with emphasis on improved hydrodynamic efficacy by introducing a long body with parallel leaflets and by leaflets increasing the flow area. Experimental studies were conducted on five test valves (MCRI 19 mm, MCRI 25 mm, SJM 19 mm, SJM 23 mm, and SJM 29 mm) with cardiac outputs of 2.0, 2.5, 3.0, and 3.5 L/min at a pulse rate of 40 beats/min, and 3.5, 4.0, 4.5, and 5.0 L/min at a pulse rate of 70 beats/min. Transvalvular pressure drop and closure volume were assessed by measuring the instantaneous ventricular and aortic pressures and aortic flow. The leaflet motions of the tested valves were observed by direct video recording using a charge coupled device camera while the flow measurements were being conducted. Testing under simulated physiologic ventricular and aortic pressure waveforms, the results of this study show that the MCRI bileaflets remained fully open during the entire ejection phase, even at very low cardiac output conditions (2.0 L/min). The closure volume (defined as percentage of forward flow volume) increased with decreasing cardiac output, as reported earlier by others. Comparative results also indicate that the MCRI design has nearly a two size pressure drop advantage over the SJM, with significantly smaller closure volume.

Abstract: The purpose of this investigation was to establish a correlation between mechanical heart valve (MHV) cavitation and transient pressure (TP) signals at MHV closure. This correlation may suggest a possible method to detect in vivo MHV cavitation. In a pulsatile mock flow loop, a study was performed to measure TP and observe cavitation bubble inception at MHV closure under simulated physiologic ventricular and aortic pressures at heart rates of 70, 90, 120, and 140 beats/min with corresponding cardiac outputs of 5.0, 6.0, 7.5, and 8.5 L/min, respectively. The experimental study included two bileaflet MHV prostheses: 1) St. Jude Medical 31 mm and 2) Carbomedics 31 mm. High fidelity piezo-electric pressure transducers were used to measure TP immediately before and after the valve leaflet/housing impact. A stroboscopic lighting imaging technique was developed to capture cavitation bubbles on the MHV inflow surfaces at selected time delays ranging from 25 microseconds to 1 ms after the leaflet/housing impact. The TP traces measured 10 mm away from the valve leaflet tip showed a large pressure reduction peak at the leaflet/housing impact, and subsequent high frequency pressure oscillations (HPOs) while the cavitation bubbles were observed. The occurrence of cavitation bubbles and HPO bursts were found to be random on a beat by beat basis. However, the amplitude of the TP reduction, the intensity of the cavitation bubble (size and number), and the intensity of HPO were found to increase with the test heart rate. A correlation between the MHV cavitation bubbles and the HPO burst was positively established. Power spectrum analysis of the TP signals further showed that the frequency of the HPO (cavitation bubble collapse pressures) ranged from 100 to 450 kHz.

Abstract: An in vitro experimental study was performed to investigate the mounting compliance effect on the occluder closing dynamics and the transient pressure at the closing of the mitral Medtronic Hall (MH) mechanical heart valve (MHV). The closing velocity and the transient pressure were simultaneously measured at heart rates of 70, 90, 120, and 140 beats/minute with cardiac outputs of 5.0, 6.0, 7.5, and 8.5 liters/minute, respectively. The experiment was conducted under simulated physiologic ventricular and aortic pressures in a pulsatile mock flow loop. The characteristics of the transient pressure were investigated by detailed mapping of the transient pressure field in the atrial chamber using high frequency pressure transducers. Simultaneous measurements of the occluder closing velocity and the transient pressure around the seat stop of the MH showed that the transient pressure generated on the inflow side dropped below the vapor pressure of liquid during the occluder's sudden deceleration at closing. The amplitude of the transient pressure reduction (TR) was proportional to the occluder approaching velocity. The development of the transient pressure in the rigid and flexible mountings were significantly different. In the rigid mounting (RM), the pressure was reduced below the liquid's vapor pressure and maintained below -350 mmHg for approximately 180 microseconds. Strong signals of high frequency pressure oscillations (HPO) were recorded in the transient pressure traces. The timing of the HPO was found to be consistent with that of the cavitation bubble collapse as observed by others. In the flexible mounting (FM), TR also occurred, but recovered quickly and was followed immediately by a positive pressure spike. Relatively weak HPO appeared in the transient pressure trace. The mapping of the transient pressure field showed that both the transient pressure reduction (on the major orifice side) or rise (on the minor orifice side) as well as the HPO were locally generated near the valve occluder surface. The transient pressure attenuated with distance away from the occluder surface. The HPO were detectable as far as 40 mm away from the occluder surface. The rigid mounting pressure signals showed characteristically two occurrences of high frequency pressure oscillations. The HPO with smaller amplitude occurred first after the initiation of the TR, followed by a burst of strong HPO at about 450 microseconds. It is believed that they were the result of the collapse of cavitation bubbles. The strong HPO did not appear in the flexible mounting signals. The study indicated that the mounting compliance played a significant role in the MHV cavitation inception and the subsequent bubble growth. It also suggested the possibility of detecting the cavitation by using a high frequency pressure transducer positioned in the atrial chamber.

Abstract: The effect of gravitational field on the asynchronous closure of four different types of bileaflet heart valves (BHV) were investigated in an in vitro mock circulation system. The experimental study involved the 29 mm St. Jude Medical Standard, 29 mm CarboMedics, 29mm Edwards-Duromedics and 29 mm Edwards-Tekna BHVs. The valves were tested in the mitral position on an inclined 45 degrees plane of the pulsatile mock flow loop. The test valves were orientated with their pivotal axis horizontal so that the gravitational vectors on the two leaflets were clearly disparate. Using a specially designed laser optical system, the time difference at which the closing leaflets made their first contacts with the valve housing was measured. The closing velocity of each leaflet was measured separately by the laser sweeping technique (LST). The experiments were conducted under physiologic ventricular and aortic pressures at the heart rates of 70, 90, and 120 beats/minute with the corresponding flow rates of 5.0, 6.0, and 7.5 liters/minute. Asynchronous closing motions were registered in all four tested BHVs. The leaflet closing motions were random in nature, but indicated clear dependency on the orientations with respect to the gravitational field. The lower leaflet which makes a generally level swing to close, was assisted initially by the gravity and was always found to close earlier than the upper leaflet, which swing upward to close. The initial closure of the upper leaflet was against the gravity which delayed the leaflet motion. Depending on the BHV designs, the time delays between the two leaflets were found to vary from beat to beat but to follow certain probability distributions. In general, the leaflet/housing contact time between the two valve leaflets exhibited the clear trend of decreasing delay time at closure with the increase of the heart rate. For each of the valves tested, the average impact velocity of the first closing leaflet was found always smaller than that of the second closing leaflet at all three heart rates tested. The impact velocities of both the BHV leaflets were found to increase with the heart rate. The difference in the closing velocities between the two leaflets decreases with the increase of the heart rate and is generally proportional to the impact time delay between the two leaflets.

Abstract: The maximum left ventricular pressure slope (dP/dt) value has been used by several investigators as the criterion for studying mitral valve closure. In this article, the relationship between the ventricular pressure slope (dP/dt) and the leaflet closing behavior of bileaflet mechanical heart valves (BMV) is investigated. Two current BMVs, the St. Jude Medical 29 mm and CarboMedics 29 mm, installed in the mitral position of a mock circulatory pulsatile flow loop were used as the study model. Under simulated physiologic pressures and flow conditions, the experiment was conducted at 70, 90, and 120 beats/min with corresponding flow rates of 5.0, 6.0, and 7.5 liters/min, respectively. A laser sweeping technique was used to monitor the leaflet closing motion within the last 3 degrees excursion at valve closure. A modified dual beam laser sweeping technique system was used to register the difference of leaflet/housing impact time between the two BMV closing leaflets in asynchronous closure. Common BMV asynchronous closures were found in both BMVs at all three heart rates tested. The second closing leaflet was found to always close at higher velocity than the first. Simultaneous measurements of the ventricular pressure (Pv) and the leaflet closing time showed that Pv exhibited three stage characteristics. In the first stage, Pv gradually increased as the ventricle was filled. A sudden rise of Pv occurred immediately after closing of the first leaflet. The maximum dp/dt occurred in the third stage after closure of both BMV leaflets. The BMV closing behavior and the corresponding Pv pattern were found to depend strongly upon valve type and heart rate. The time averaged ventricular pressure slope (dp/dt) values at 70, 90, and 120 beats/min were about 40, 70, and 150 mmHg/sec for the St. Jude Medical valve and 40, 105, and 205 for the CarboMedics valve during the first closing stage. The maximum dp/dt values were 2670, 4350, and 5000 mmHg/sec for the St. Jude Medical valve and 1210, 2530, and 3210 mmHg/sec for the CarboMedics valve at the three heart rates tested, respectively. The study showed that the left ventricular pressure patterns (dP/dt) at valve closure were the result of valve operation under given driving conditions. The dp/dtmax cannot be used as the criterion for studying BMV closure.

Abstract: The rapid deceleration of mechanical heart valve leaflets or discs at closure, and their rebound after impact between the leaflets or discs and housing may produce conditions that favor cavitation inception. Precision measurements of the disc closing velocity using a laser sweeping technique (LST) were made on two Medtronic Hall (Med Hall) mitral valve models, the 29 mm Standard and D-16 Med Hall valves. The experiment was carried out in a pulsatile mock flow loop (PFL) by installing the tested valve in the mitral position of the PFL. Tests were conducted under physiologic pressures at heart rates of 70, 90, and 120 beats/min with flow rates of 5, 6, and 7.5 l/min, respectively. For each valve tested, the experimental series was carried out to include both a flexible mounting and a rigid mounting mechanism. The time history of the disc closing velocity during the last 3 degrees before final closure was measured. Results show that the disc approached the valve housing at a near constant velocity of approximately 1.3, 1.5, and 2.0 m/sec for the Med Hall Standard valve, and 1.9, 2.5, and 2.6 m/sec for the Med Hall D-16 valve at 70, 90, and 120 beats/min, respectively. After impact, the valve of disc rebounded with velocities that depended on the mounting modalities and heart rates. In the rigid mounting, the disc rebound velocities of the Med Hall Standard and D-16 valves were about 60%-70% and 70%-80% of their respective approaching velocities. For the flexible mounting, the rebound velocities were 15%-25% and 20%-30% of their respective approaching velocities. The results demonstrate that a slight modification in the seat stop geometry of Med Hall valves can significantly affect disc closure behavior.