A bovine model for detecting high intensity transient signals originating from mechanical heart valves

In patients with mechanical heart valves (MHVs), transcranial Doppler methods commonly detect high intensity transient signals (HITS) representing microemboli. These microemboli, which are presumably gaseous, may cause stroke and cognitive deterioration. A bovine model was therefore developed for studying the relationship between mitral MHV induced HITS and potential etiogenic factors. We placed an 18 mm, 4 MHz Doppler probe in the brachiocephalic artery to detect MHV induced microbubbles at baseline (rest) and under 9 other conditions. To elucidate the gas composition (CO2 or N2) of the microbubbles, we administered 1%, 3%, and 5% CO2, and 100% O2. To determine effect of the heart rate, we paced the heart at 120, 160, and 180 bpm. To alter the myocardial contractility, we gave dobutamine and esmolol. Two independent, blinded observers counted the HITS from recorded doppler spectra. HITS were defined by an initial unidirectional spectral deviation, a signal power of >8 dB relative to the background power, and lack of a cyclic appearance. The electrocardiogram, aortic and LV pressures, and LV dP/dt were obtained telemetrically. The calves were studied 4 to 6, 8 to 10, and 12 to 14 weeks postoperatively, after which the animals were sacrificed at an approximate 4 month study duration, and a postmortem evaluation of the heart and the main viscera was performed. In all, 27 HITS recordings were made in 10 calves. Myocardial contractility was the only factor to significantly affect HITS frequency; the heart rate and blood gas concentrations had minimal effect on HITS frequency. Our bovine model will be useful for assessing valve designs, as well as the mechanism of HITS, the composition of the microemboli, and their possible pathophysiologic effects on the kidneys and brain.     

Relationship between exercise-induced heart rate increase and the formation of microbubbles and high-intensity transient signals in mechanical heart valve implanted patients

Introduction

The formation and collapse of vapor-filled bubbles near a mechanical heart valve is called cavitation. Microbubbles can be detected in vivo by doppler ultrasonography (USG) as HITS (high intensity transient signals) in cranial circulation. We investigated the relationship between exercise induced heart rate increase and HITS formation in cranial circulation.

Material and methods

Thirty-nine mechanical heart valve implanted (8 aortic valve replacement (AVR) + mitral valve replacement (MVR), 9 AVR, 22 MVR) patients aged 18-80 years old were included in our study. Microbubbles were counted in the left ventricular cavity via transthoracic echocardiography at rest per cardiac cycle. Afterwards transcranial Doppler USG was performed and HITS were counted in each patient''s middle cerebral artery at 5 min duration. Subsequently an exercise test according to the Bruce protocol was performed. After achieving maximal heart rate, microbubbles in the left ventricle and HITS were counted again.

Results

Microbubbles in the left ventricle and transcranial HITS increased after exercise significantly compared to resting values (15.79 ±10.91 microbubbles/beat vs. 26.51 ±18.00 microbubbles/beat, p < 0.001; 6.13 ±8.07 HITS/5 min vs. 13.15 ±15.87 HITS/5 min, p = 0.001). There was a significant correlation between microbubbles and HITS counts after peak exercise (r = 0.55, p < 0.001).

Conclusions

In our study, we found that the microbubbles were increasing as the heart rate increased and more HITS were propelled to the cerebral circulation. As previously shown, HITS can alter cognitive functions. Therefore heart rate control is essential in mechanical heart valve patients to protect neurocognitive functions.     

Transient, three-dimensional flow field simulation through a mechanical, trileaflet heart valve prosthesis

Thromboembolic complications are one of the major challenges faced by designers and researchers in development of artificial organs with blood-contacting devices such as heart valve prostheses, especially mechanical valves. Besides increasing the thrombogenic potential, these valves change the hydrodynamic performance of the heart. In this study, the flow through a trileaflet, mechanical heart valve prosthesis was modeled with transient computational fluid dynamics to analyze flow patterns causing thrombus formations on valves. The valve was simulated under conditions of a test rig (THIA II), which was specially designed to analyze different valves with respect to thrombosis. The main goal of this study was to mimic the exact conditions of the test rig to be able to compare numerical and experimental results. The boundary conditions were obtained from experimental data as leaflet kinematics and pressure profiles. One complete cycle of the valve was simulated. Numerical flow and pressure results were analyzed and compared with experimental results. Shear stress and shear rates were determined with respect to thrombogenic potential, especially in the pivot regions, which seem to be the main influence for activation and deposition of thrombocytes. Approximately 0.7% of the blood volume moving through the fluid domain of the valve was exposed to shear rates high enough to cause platelet activation. However, shear rates of up to 20,000 s?1 occurred in pivot regions. The pressure differences between the simulation and experimental data were approximately 2.5% during systole and increased up to 25% during diastole. The presented method, however, can be used to gain more information about the flow through different heart valve prostheses and, thus, improve the development process.     

Detection and classification of gaseous microemboli during pulsatile and nonpulsatile perfusion in a simulated neonatal CPB model

We compared the effects of perfusion modes (pulsatile vs. nonpulsatile) on gaseous microemboli delivery using the Emboli Detection and Classification (EDAC) Quantifier at postpump, postoxygenator, and postarterial filter sites in a simulated pediatric cardiopulmonary bypass (CPB) model. The mock loop was subjected to five different pump flow rates of equal 100 ml/min intervals, ranging from 400 to 800 ml/min. When the target pump flow rate was achieved, 5 cc air was introduced into the venous line. The EDAC system recorded gaseous microemboli counts simultaneously at three locations in 5-minute intervals. Regardless of the type of perfusion mode, when the pump flow rate was increased, more gaseous microemboli were generated at postpump site. Compared with nonpulsatile flow, pulsatile flow did deliver significantly more gaseous microemboli at postpump site, but there was no difference between two groups at postoxygenator and postarterial filter sites. Capiox Baby-RX hollow-fiber membrane oxygenator significantly reduced the gaseous microemboli counts in both groups at all five pump flow rates with either pulsatile flow or nonpulsatile flow in this model. Our results suggest that using this novel EDAC system, we could detect the size of gaseous microemboli, as small as 10 microm, and the percentage of detected gaseous microemboli, <40 microm, was about 90% in total gaseous microemboli counts at any flow rate with pulsatile or nonpulsatile flow.     

DeBakey Surgitool mechanical heart valve prosthesis, explanted at 32 years

Contemporary mechanical heart value prostheses are expected to last "just about forever" or the patient's lifeline. They do however still suffer complications, some of which necessitate premature explantation. Complications today are mainly related to patient compliance with anticoagulant medication, infection and hemorrhage. The DeBakey Surgitool mechanical heart valve was the first such device to have Pyrolyte components. We present a DeBakey surgitool mechanical heart valve that was in place for 32 years! It was explanted for dysfunction related to tissue overgrowth and not to its related components. With good patient compliance, this mechanical heart valve prosthesis is an example of good prosthetic valve durability.     

Comparison of two different blood pumps on delivery of gaseous microemboli during pulsatile and nonpulsatile perfusion in a simulated infant CPB model

The purpose of this study was to compare two different blood pumps (Jostra roller pump vs. Medos deltastream DP1 rotary pump) on delivery of gaseous microemboli during pulsatile and nonpulsatile perfusion in a simulated infant cardiopulmonary bypass (CPB) model. The Jostra and Medos pump were used in parallel pattern. The circuit was primed with lactated ringer's solution (700 ml) and the postfilter pressure was maintained at 100 mm Hg. Three transducers (postpump, postoxygenator and postfilter sites) of the Emboli Detection and Classification (EDAC) Quantifier were inserted into the CPB circuit to detect and classify gaseous microemboli. Trials were conducted at flow rates ranging from 500 to 1250 ml/min (250 ml/min increments). The arterial filter purge line was kept open during all trials. After injecting 20 ml air into the venous line, 2-minute segments of data were recorded simultaneously through three transducers. This entire process was repeated six times for each unique combination of blood pump, flow rate and perfusion mode, yielding a total of 96 experiments. Independent of perfusion mode and flow rate, Medos pump delivered less gaseous microemboli than Jostra pump at the postpump site, but only at 1,250 ml/min of pump flow rate the differences reached statistical significance (p < 0.01). There was no difference in delivery at the postfilter site. Compared with nonpulsatile flow, pulsatile flow transferred significantly more gaseous microemboli at the postpump site at 1,250 ml/min of pump flow rate in both groups (p < 0.01). The majority of gaseous microemboli were trapped by the membrane oxygenator. The results of this study confirm that rotary pump could deliver less gaseous microemboli than roller pump at the postpump site when a fixed volume air was introduced into the venous line. Pulsatile flow could transfer more gaseous microemboli at the postpump site, no matter which blood pump was used. Only few gaseous microemboli appeared at the postfilter site at high flow rates with an open arterial filter purge line.     

Cavitation potential of mechanical heart valve prostheses

Just like technical check valves, the function of mechanical heart valve prostheses may presumably also lead to cavitation effects during valve closure. Due to the waterhammer effect, cavitation may primarily occur in the mitral position leading to high mechanical loading of the valve itself and of corpuscular blood elements. Ten different types of commercial mechanical heart valves were investigated in the mitral position of a pulsatile mock loop, to detect cavitation thresholds under physiologically similar conditions by cinematographic techniques. Almost all these valve prostheses show cavitation up to a ventricular pressure gradient of 5000 mmHg/s. The threshold depends on valve type and size and is sometimes within the physiological range below 2000 mmHg/s. Visible cavitation bubbles with a diameter of up to 1.8 mm and a collapse time of less than 0.1 ms suggest that vapour cavitation may play an important role for material and blood damage in mechanical heart valve prostheses.     

Automatic Detection of Microemboli During Percutaneous Coronary Interventions

The objective of this study was to develop an analysis method for the automatic detection of intracoronary microemboli triggered high intensity signals (HITS) during percutaneous coronary interventions (PCI). The recorded ultrasonic Doppler velocity spectra from an intracoronary ultrasonic guide-wire were decomposed into 13 wavelet scales applying the continuous wavelet transform. From 7 wavelet scales which were most suitable for a differentiation between HITS and pulsatile flow, envelopes were calculated and combined to improve the HITS-to-background noise ratio. For different intensity thresholds the resulting number of HITS was automatically counted and compared with the number estimated by experienced observers. In a first validation trial HITS were detected within a simplified in vitro model with a sensitivity of 89.2% and a positive predictive value of 87.6%. In a following clinical study 211 HITS from 18 patients during PCI were counted manually by the observers. With the developed wavelet-based method 189 HITS were correctly detected (sensitivity of 89.6%, positive predictive value of 85.5%). The introduced new method automatically detects intracoronary HITS for the first time with a reliable accuracy. This may support further studies evaluating the incidence and consequences of coronary microembolization during coronary interventions.     

Autopsy-determined causes of death following heart valve replacement

The pathological findings and the principal cause of death were reviewed in 275 patients with 345 heart valve prostheses. Rheumatic fever necessitated valve replacement in 73% of these patients. Tissue valves bore significantly fewer and scantier thrombi than did the mechanical prostheses. Patients with these two groups of valves showed no significant difference with regard to the incidence of infarction of systemic organs. Patients with prostheses showed infarcts of the spleen, brain, and heart more often than did nonoperated controls with valvular disease. Among the mechanical valves, patients with Starr-Edwards valves showed the highest incidence of fatal thromboembolism. Prosthesis-related problems formed the biggest single cause of death in the 275 patients with valve prostheses. The mechanical valve group, which formed 82% of the total, had prosthesis-related problems as the prime cause of death; in the tissue valve group this complication ranked third in importance after unknown causes and diseases unrelated to valve surgery. Analysis of the valve-related causes of death showed that thrombosis and infection of the prosthesis were more important in the mechanical valves, whereas structural failure was more common with the tissue valves. Prosthesis-related fatal complications were present in 13% of the patients who died within 30 days postoperatively and in 61% of those who died later.     

The capability of trapping gaseous microemboli of two pediatric arterial filters with pulsatile and nonpulsatile flow in a simulated infant CPB model

The study objective was to test the capability of Medtronic Affinity and Terumo Capiox pediatric arterial filters to trap gaseous microemboli in a simulated infant cardiopulmonary bypass (CPB) model. The filters were used in parallel pattern. The circuit was primed with lactated ringer's solution (700 ml) and postfilter pressure was maintained at 100 mm Hg using a Hoffman clamp. Trials were conducted at flow rates ranging from 500 to 1,250 ml/min. After introducing 20 ml air into the venous line via an 18-G needle, 2-minute segments of data were recorded. This entire process was repeated 6 times for each unique combination of arterial filter, flow rate and perfusion mode, yielding a total of 96 experiments. More than 80% of gaseous microemboli were trapped by the two pediatric arterial filters. With increased flow rates and pulsatile mode, more gaseous microemboli passed through the arterial filters. There were no differences in terms of the percentage of gaseous microemboli trapped and pressure drops between Medtronic Affinity and Terumo Capiox pediatric arterial filters. Results demonstrated that Medtronic Affinity and Terumo Capiox pediatric arterial filters could trap the majority of gaseous microemboli in this particular setting of an open arterial filter purge line in a simulated infant CPB circuit with pulsatile and nonpulsatile flow.     

Characteristics of pulsatile blood flow through the curved bileaflet mechanical heart valve installed in two different types of blood vessels: velocity and pressure of blood flow

The aim of this study was to investigate the flow fields of blood flowing through the curved bileaflet mechanical heart valve. A numerical analysis was carried out with the fluid-structure interaction between the blood flow and the motion of leaflets in two different types of blood vessels (type A, with sinus blood vessel, and type B, without sinus blood vessel). When the leaflet was fully opened, a fluttering phenomenon was detected in association with the blood flow, and recirculation flows were observed in the sinus region of the blood vessel for type A. During the closing phase, regurgitation was formed between the ring and the edge of the each leaflet for both types. When the leaflet came into contact with the valve ring at the end of the closing phase, rebound of the leaflet occurred. In consideration of the entire domain, the pressure drop occurs mainly in the valve region. The present results showed tendencies similar to those obtained by previous experiments for blood flow and contribute to the development of the curved bileaflet mechanical heart valve prostheses.     

Frequency analysis of high-intensity transient signals of transcranial Doppler ultrasound in patients supported with a left ventricular assist device

Left ventricular assist devices (LVADs) have become more important than ever for patients with terminal heart disease. The chronic shortage of donors for heart transplantation may enhance the importance of LVADs as destination therapy. To prevent cerebral complications, the evaluation of high-intensity transient signals (HITS) of transcranial Doppler (TCD) ultrasound is important. In the present study, HITS were measured on seven occasions in patients supported with LVADs. The high-frequency counts in HITS were greater than the low-frequency counts (47 ± 42 vs. 9 ± 9 in 3 min, P = 0.025), and declined further with oxygen delivery to the patient. The high frequency of signals may imply air microemboli, and the counts may increase with time. Presented in part at the 2007 Joint Congress of the 45th Annual Conference of the Japanese Society for Artificial Organs and the 2nd Annual Conference of the International Federation for Artificial Organs, Osaka     

Gaseous microemboli detection in a simulated pediatric CPB circuit using a novel ultrasound system

The objective of this study was to evaluate the effect of flow rate and perfusion mode on the delivery of gaseous microemboli in a simulated pediatric cardiopulmonary bypass (CPB) circuit with an open arterial filter purge line using a novel ultrasound detection system. The circuit was primed with 450 ml fresh, heparinized bovine blood plus 200 ml Lactated Ringer's solution (total volume 650 ml, corrected Hct 25%). After the injection of 5 ml air into the venous line, an Emboli Detection and Classification (EDAC) Quantifier (Luna Innovations, Inc., Roanoke, VA) was used to simultaneously record microemboli counts at postpump, postoxygenator, and postarterial filter sites. Trials were conducted at four different flow rates (500, 750, 1,000, 1,250 ml/min) and two perfusion modes (pulsatile, nonpulsatile). Microemboli counts uniformly increased with increasing pump flow rates. In all trials, the majority of gaseous microemboli detected in the simulated pediatric CPB circuit were <20 microm in diameter. At the lowest flow rate tested (500 ml/min), all microemboli (>10 microm) were cleared from the circuit by the oxygenator and arterial filter. Clearance efficiency was decreased at higher flow rates (750-1,250 ml/min). Over 98% of microemboli detected at the postoxygenator site were <40 microm in diameter. In general, pulsatile flow delivered more microemboli to the circuit at postpump and postoxygenator sites than nonpulsatile flow. The results of this study confirmed that entrained air from the venous line could be delivered to the systemic circulation (as represented by our pediatric pseudo patient) at flow rates from 750 to 1,250 ml/min, despite the presence of an arterial filter and open arterial filter purge line. All of the microemboli distal to the arterial filter were smaller than the conventional detectable level of 40 microm.     

In vitro hemolysis by mechanical heart valve prostheses with tilting disc.

Hemolytic and subhemolytic blood damage by mechanical heart valve prostheses have been observed in both clinical and in vitro investigations. A direct comparison between these studies is not possible. Nevertheless the transfer of some in vitro results to the behaviour of the valve in situ may be performed considering the similarity principle. This requires the use of dimensionless similarity numbers such as the plasma's hemoglobin concentration (PHb) or others, instead of dimensioned parameters. To evaluate the in vitro hemolysis of valve prosthesis a test chamber filled with human banked blood was used. An artificial ventricle ensuring an oscillatory flow through the valve was also used. The rise of PHb was evaluated in terms of a similarity number, called the lysis number. This number describes the probability of destroying a single red blood cell participating once in the hemolytic process under consideration. The lysis number, a Bj?rk-Shiley valve (TAD 29), was found to be in the order of 2 x 10(-4). From this, the survival time of erythrocytes in patients with an artificial heart valve was estimated. It was found to be in the order of 20 d of T50 Cr in agreement with clinical results.     

In vitro squeeze-flow phenomena at the instant of mechanical valve closure

Cavitation bubble formation associated with mechanical valve closure has been investigated in vitro, and the region of bubble formation has been correlated with large negative pressure transients. The region of cavitation bubbles forms in valve designs where leaflets interact with seat stops. It has been postulated that the fluid is squeezed between the leaflet and the seat stop and radially propelled at high velocities, resulting in further pressure reduction below the vapor pressure of fluid and initiating cavitation bubble formation. We conducted in vitro experiments to visualize and detect the presence of squeeze-flow phenomena associated with valve closure of mechanical heart valves. The closing dynamics were studied by simulating a single closing event of the leaflet with the valve mounted at the mitral position. Squeeze flow was detected at the instant of valve closure, when the valve leaflet interacts with the valve seat stop. The use of a high-speed video camera at 1000 frames per second with strobe light at 16000 pulses per second enabled the visualization of cavitation bubbles and its radial motion from the valve's seat stop due to the squeeze-flow effect. Vapor cavitation bubbles were observed to collapse within 0.5 ms after inception. In mechanical valves without seat stops in the major orifice region, bubbles of duration longer than that of the cavitation bubbles were observed. These microbubbles were present for 4 s before collapsing and are believed to be air bubbles whose presence in vivo has been detected with ultrasound imaging.     

Phonocardiogram spectral analysis simulator of mitral valve prostheses.

Spectral analysis of sounds produced in vitro by mitral valve prostheses placed in a specially designed flow simulator has been carried out using a short-time Fourier representation of the recorded signal. Time variations of power spectra are displayed as a three-dimensional plot. Sounds produced by three types of valves, namely ball and cage, tilting disk and porcine valves, were analysed. Each valve type produced a characteristic spectrogram, and, for a given valve, spectrograms were reproducible to within a margin of 5 dB. The simulator may be used to detect structural deficiencies and functional abnormalities of prosthetic heart valves. In addition to quantifying the noise level of mechanical valves, the system may be used for quality control purposes to identify faulty valves.     

Artificial valves “up to date” in Japan

We have many choices when selecting artificial valves for valve replacement surgery. It is necessary to know the characteristics of the various prosthetic valves to make an appropriate decision for each valvular heart disease patient. In this review paper, we describe the features and benefits of the artificial valves available in Japan. Standard and new generation bioprostheses and mechanical prostheses are reviewed. The new technology of the catheter-delivery heart valve is also mentioned in this paper.     

In vitro hydrodynamics of a decellularized pulmonary porcine valve,compared with a glutaraldehyde and polyurethane heart valve

BACKGROUND: Hydrodynamic performance of a decellularized pulmonary porcine valve was evaluated with a computer versatile pulse duplicator and compared to glutaraldehyde fixated stentless porcine bioprosthesis and a polyurethane heart valve. METHODS: Decellularized pulmonary porcine matrices (Group I, n = 5) were treated chemically to become cell-free collagen matrices. The findings of this heart valve were compared with aortic glutaraldehyde treated porcine prostheses (Group II, n = 5) and polyurethane three leaflet valve prostheses (Group III, n = 1). Measurements were performed in 0.9% saline test fluid at room temperature. Measurements compared were closing time, closing volume, systemic pressure difference and energy losses. Each valve was measured 6 times with 70 beats/minute, a stroke volume of 70 ml corresponds to a cardiac output of 4.9 L/minute. RESULTS: Group I and group III showed no significant differences between parameters. The measured closing time was significantly different (p < 0.001) between group I and II, respectively 24.333 and 53.600 ms and group II and III respectively 53.600 and 24.000. Difference in closing volume was significant (p < 0.05) between groups II and I respectively 3.67 and 0.68 ms and group II and III respectively 3.67 and 0.71. Systolic mean pressure gradient was 18.25 +/- 1.04 mm Hg in group II which was significantly different (p < 0.001) from groups I and III, respectively 10.65 +/- 0.29 mm Hg and 7.70 +/- 0.30 mm Hg. CONCLUSIONS: Decellularized pulmonary porcine valves showed the same excellent performance as polyurethane valve prosthesis, which are superior to the investigated glutaraldehyde fixed xenograft.     

Axial flow pump support in a Marfan patient with an aortic mechanical heart valve: case report and literature review

The presence of a mechanical heart valve in the aortic position is usually considered a contraindication for the use of cardiac assist devices. Only a few cases with the combination of mechanical circulatory support and valve prostheses have been reported in the literature to date, and the experience is even more limited in the new generation of miniaturized axial flow pumps. We present a case report of a patient with a mechanical aortic heart valve who was successfully supported with a continuous flow pump and discuss the literature available on this problem. Further on, the patient was weaned from his ventricular assist device after 456 days of support.     

脉动流下机械心脏瓣膜瓣阀开启状态的评价

背景：体外机械型人工心脏瓣膜(机械瓣)性能的评价涉及心输出量、反流量、有效瓣口面积、跨瓣压差,以及应力场、流场和成穴现象等。 目的：对3种机械瓣的瓣阀开启状态进行可视性观察和评价。 方法：用脉动流模拟循环装置系统,维持系统整个状态不变,在模拟心搏出量4 L/min、模拟心率75次/min和收缩时间占其循环周期46.2%的条件下,分别将久灵双叶瓣、Carbomedics双叶瓣和C-L侧倾碟瓣置于主动脉瓣位,将高速摄像机置于模拟循环装置动脉腔的正上方,观察10个连续模拟心动周期中瓣阀开启状态。利用自编图像处理软件包,捕获瓣阀开启角度最大的1幅图像,作为计算该只瓣膜在1个心动周期中最大开放面积和开启角度的基准。 结果与结论：脉动流下,25 mm CarboMedics瓣、25 mm和23 mm久灵双叶瓣在开放到最大位时,可见瓣阀抖动现象,27 mm C-L侧倾碟瓣未见瓣阀抖动。用不同的计算方法测量上述瓣膜的瓣口面积显示,由厂家提供的瓣口实际面积最大,用Green公式计算的瓣膜开放面积次之,用Gorin公式计算的有效瓣口面积最小。根据三角形定理计算的瓣阀开放角度,久灵双叶瓣和CarboMedics瓣的两个瓣阀的开放角度不一致,并均小于瓣膜固有的开放角度;C-L侧倾碟瓣的开放角度也未达其固有的开放角度。提示机械型人工心脏瓣膜双叶瓣的瓣阀开放不同步,瓣阀有抖动现象;瓣阀在脉动周期中呈不完全性开启。