Dynamic finite element analysis of the aortic root from MRI-derived parameters

An understanding of aortic root biomechanics is pivotal for the optimisation of surgical procedures aimed at restoring normal root function in pathological subjects. For this purpose, computational models can provide important information, as long as they realistically capture the main anatomical and functional features of the aortic root.Here we present a novel and realistic finite element (FE) model of the physiological aortic root, which simulates its function during the entire cardiac cycle. Its geometry is based on magnetic resonance imaging (MRI) data obtained from 10 healthy subjects and accounts for the geometrical differences between the leaflet-sinus units. Morphological realism is combined with the modelling of the leaflets’ non-linear and anisotropic mechanical response, in conjunction with dynamic boundary conditions.The results show that anatomical differences between leaflet-sinus units cause differences in stress and strain patterns. These are notably higher for the leaflets and smaller for the sinuses. For the maximum transvalvular pressure value, maximum principal stresses on the leaflets are equal to 759, 613 and 603 kPa on the non-coronary, right and left leaflet, respectively. For the maximum aortic pressure, average maximum principal stresses values are equal to 118, 112 and 111 kPa on the right, non-coronary and left sinus, respectively.Although liable of further improvements, the model seems to reliably reproduce the behaviour of the real aortic root: the model's leaflet stretches, leaflet coaptation lengths and commissure motions, as well as the timings of aortic leaflet closures and openings, all matched with the experimental findings reported in the literature.     

Relation between regional electrical activation time and subepicardial fiber strain in the canine left ventricle

To determine the relation between regional electrical activation time and fiber strain, epicardial electrical activation and deformation were measured in six open-chest dogs at the left ventricular anterior free wall after 15 min of right atrial, left ventricular free wall, left ventricular apex, or right ventricular outflow tract pacing, when end-diastolic pressure was normal or elevated (volume-loading). Regional electrical activation was measured using a 192-electrode brush. Regional subepicardial fiber strain (e _f) was measured simultaneously in 16 regions, using optical markers which were attached to the epicardial surface and recorded on video. When relating regional e _f during the ejection phase to regional activation time, the best correlation was found when a hemodynamic time reference rather than an electrophysiological one is used. Using the moment of the maximum rate of change of left ventricular pressure as the time reference for electrical activation, regional electrical activation time (t _ea) and the degree of e _f during the ejection phase could be fitted by a linear regression equation e _f=a t _ea+b in which a=–3.46±0.73 s^–1 and b=–0.28±0.05. For electrical activation times ranging from -40 to -80 ms, fiber strain was estimated with an accuracy of ±0.026 (±SE) with this relation. During right atrial pacing, t _ea and e _f were on the average –48 ms and –0.10 respectively. On further investigation, the relation between e _f and t _ea appeared to be influenced by end-diastolic pressure. For normal (1.1 kPa) and elevated end-diastolic pressure (1.8 kPa), the slope of the linear regression line was –3.96 and –2.86 s^–1, respectively. Three conclusions may be drawn. Firstly, the time interval between the moment of regional electrical activation and the moment of the maximum rate of change of left ventricular pressure is an index of regional fiber strain. Secondly, it can be concluded from the above equations that electrical asynchrony of more than 30 ms causes non-uniformities in the degree of e _f of the order of mean e _f during pacing from the right atrium. Finally, differences in fiber strain during asynchronous electrical activation are less pronounced at larger filling pressures.     

A three-dimensional analysis of flow in the pivot regions of an ATS bileaflet valve

Bileaflet heart valves are currently the most commonly implanted type of mechanical prosthetic valve, because of their low transvalvular pressure drop, centralised flow and durability. However, in common with all mechanical heart valves, implanted bileaflet valves show an inherent tendency for blood clot formation at the valve site. Fluid dynamical phenomena associated with blood clotting are elevated blood shear stresses and regions of persistent blood recirculation, particularly when both occur together. Using three-dimensional CFD modelling, combined with enlarged scale experimental modelling, we investigated the blood flow through the ATS bileaflet valve during forward flow, with particular attention to the leaflet pivot regions. Recirculating regions were found both within and downstream of the valve housing ring. Qualitative assessment of the entire cardiac cycle suggested that recirculating blood within the housing ring will be washed away whilst the valve is closed, but as with all bileaflet valve designs recirculating blood downstream of the valve may have a residence time much longer than one cardiac cycle.     

Finite Element Investigation of Stentless Pericardial Aortic Valves: Relevance of Leaflet Geometry

Recent developments in aortic valve replacement include the truly stentless pericardial bioprostheses with single point attached commissures (SPAC) implantation technique. The leaflet geometry available for the SPAC valves can either be a simple tubular or a complex three-dimensional structure molded using specially designed molds. Our main objective was to compare these two leaflet designs, the tubular vs. the molded, by dynamic finite element simulation. Time-varying physiological pressure loadings over a full cardiac cycle were simulated using ABAQUS. Dynamic leaflet behavior, leaflet coaptation parameters, and stress distribution were compared. The maximum effective valve orifice area during systole is 633.5 mm² in the molded valve vs. 400.6 mm² in the tubular valve, and the leaflet coaptation height during diastole is 4.5 mm in the former, in contrast to 1.6 mm in the latter. Computed compressive stress indicates high magnitudes at the commissures and inter-leaflet margins of the tubular valve, the highest being 3.83 MPa, more than twice greater than 1.80 MPa in the molded valve. The molded leaflet design which resembles the native valve exerts a positive influence on the mechanical performance of the SPAC pericardial valves compared with the simple tubular design. This may suggest enhanced valve efficacy and durability.     

Perkutane transfemorale Valvuloplastie bei Patienten mit kalzifizierter Aortenstenose und deutlich erhöhtem Operationsrisiko: Klinischer Verlauf und Wertigkeit der Dopplersonographie zur Beurteilung des Therapieerfolges

Summary Percutaneous transluminal valvuloplasty (PTV) was performed in 24 patients (aged 67–86 years, mean: 76±5.7 years) with calcific aortic stenosis and high operative risk. The gradient between maximal left ventricular and aortic pressures (peak-to-peak gradient, PPPG) could be reduced by 52% from 73±21 to 34±12 mmHg (p<0.001). Peak pressure gradient (PPG), as assessed by continuous wave Doppler, could be reduced from 80±28 to 58±21 mmHg (p<0.001). Aortic valve area (AVA) as determined by Doppler and two dimensional echocardiography increased significantly from 0.39±0.14 to 0.61±0.3 cm² (p<0.05). Clinical symptoms were found to be improved in 5 of 8 patients with impaired ejection fraction and in 11 of 16 patients with normal ejection fraction during the first week after PTV. Complications due to the procedure were surgical revision of femoral artery puncture site in one patient and hemodynamic relevant pericardial effusion in another patient. Transmitral early (E) and late (L) diastolic filling integrals were measured by pulsed Doppler: the ratio E/L decreased significantly after PTV from 0.9±0.5 to 0.63±0.31 (p<0.03) indicating further reduction of left ventricular early diastolic filling. Ejection fraction, stroke volume and cardiac output did not significantly change immediately after PTV.The results indicate, that PTV can successfully reduce aortic pressure gradients and improve symptoms in patients with calcific aortic stenosis and high operative risk. Doppler echocardiography provides an adequate method to noninvasively evaluate the initial outcome of PTV and seems valuable for the assessment of long term results.

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Abkürzungsverzeichnis AKE prothetischer Aortenklappenersatz - AoP maximaler systolischer Aortendruck - AVA Aortenklappenöffnungsfläche - E Integral frühdiastolischer Füllungsgeschwindigkeiten - EF Ejektionsfraktion - HR Herzfrequenz - HZV Herzzeitvolumen - L Integral spätdiastolischer Füllungsgeschwindigkeiten - LVP maximaler systolischer linksventrikulärer Druck - NYHA New York Heart Association - PPG peak pressure gradient - PPPG peak-to-peak pressure gradient - PTV Perkutane transmfemorale Valvuloplastie - SEP Systolische Ejektionsperiode - SV Schlagvolumen     

Assessment of the influence of the compliant aortic root on aortic valve mechanics by means of a geometrical model

In recent years several researchers have suggested that the changes in the geometry and angular dimensions of the aortic root which occur during the cardiac cycle are functional to the optimisation of aortic valve function, both in terms of diminishing leaflet stresses and of fluid-dynamic behaviour. The paper presents an analytical parametric model of the aortic valve which includes the aortic root movement. The indexes used to evaluate the valve behaviour are: the circumferential membrane stress and the stress at the free edge of the leaflet, the index of bending strain, the bending of the leaflet at the line attachment in the radial and circumferential directions and the shape of the conduit formed by the leaflets during systole. In order to evaluate the role of geometric changes in valve performance, two control cases were considered, with different reference geometric configuration, where the movement of the aortic root was ignored. The results obtained appear consistent with physiological data, especially with regard to the late diastolic phase and the early ejection phase, and put in evidence the role of the aortic root movement in the improvement of valve behaviour.     

自体心包补片修补主动脉瓣环辅助主动脉瓣置换

背景：在主动脉置换过程中常遇到瓣环钙化、瓣周囊肿等特殊情况,这时一般应用特殊技术辅助主动脉瓣置换。 目的：观察自体心包补片修补主动脉瓣环辅助主动脉瓣置换治疗钙化性主动脉瓣狭窄并瓣环钙化的临床可行性。 方法：回顾性分析2009年1月至 2012年1月郑州大学第一附属医院42例钙化性主动脉瓣狭窄并瓣环钙化患者的临床资料,并通过统计学软件处理自体心包补片修补主动脉瓣环技术辅助主动脉瓣置换前后的主动脉瓣有效瓣口面积指数、最大跨瓣压差、血流峰值速度、左室射血分数等数据,分析自体心包补片修补主动脉瓣环技术辅助主动脉瓣置换的应用效果。 结果与结论：无置换中死亡病例,置换中主动脉阻断时间为52-88(63.0±18.1) min,体外循环时间为78-122(102.6±25.1) min,置换后1例患者出现急性肾功能衰竭,经床旁血透治疗后治愈。余患者无严重置换并发症。置换后住院天数为7-20(13.6±5.5) d。置换后多普勒超声心动图示：瓣膜功能良好,均未发现主动脉瓣周漏。置换后6个月的主动脉瓣有效瓣口面积指数、最大跨瓣压差、血流峰值速度、左室射血分数均有显著改善,与置换前比较差异均有显著性意义(P < 0.05)。证实对置换适应证合适的特殊换瓣患者,自体心包补片修补主动脉瓣环辅助主动脉瓣置换可取得满意的外科治疗效果,且操作安全简单,是一项可行的技术。     

Effects of exercise training on plasma catecholamines and blood pressure in labile hypertensive subjects

Summary Plasma catecholamine concentrations (norepinephrine, NE; epinephrine, E) were measured along with heart rate (HR) and blood pressure (BP) at rest in supine (20 min) and standing (10 min) positions and in response to cycle ergometer exercise (5 min; 60% estimated maximal aerobic power) in 12 hypertensive patients before and after 20 weeks of aerobic training on cycle ergometer (six males, one female) or by jogging (five males). In a control group of labile hypertensive patients (five males, two females), estimated maximal aerobic power as well as HR and BP at rest in the supine and standing positions and in response to exercise were not modified from the first to the second evaluation (43±4 vs 43±5 ml·kg^–1·min^–1). In comparison estimated maximal aerobic power significantly increased in both training groups (cycle: 38±4 to 43±4; jogging: 38±3 to 46±4 ml·kg^–1·min^–1). However HR and BP were not modified following training, except for small reductions in systolic (18.9 to 18 kPa: 142 to 135 mmHg) and diastolic pressures (13.3 to 12 kPa: 100 to 90 mmHg) (p<0.05) at standing rest in the cycle group. Changes in plasma E and NE concentrations at rest and in response to exercise were small and not consistent: plasma NE was lower at standing rest following cycle training, (559±95 vs 462±108 pg·ml^–1) but a similar reduction was observed in the control group (428±45 vs 321±28 pg·ml^–1); plasma E was lower at rest following cycle training (29±7 vs 12±8 pg·ml^–1), but was higher in response to exercise (137±24 vs 419±113 pg·ml^–1). These results are in accordance with previous reports which do not clearly demonstrate that physical training in hypertensive patients lowers BP and the activity or reactivity of the sympathetic system.     

Surface strains in the anterior leaflet of the functioning mitral valve

The mitral valve (MV) is a complex anatomical structure whose function involves a delicate force balance and synchronized function of each of its components. Elucidation of the role of each component and their interactions is critical to improving our understanding of MV function, and to form the basis for rational surgical repair. In the present study, we present the first known detailed study of the surface strains in the anterior leaflet in the functioning MV. The three-dimensional spatial positions of markers placed in the central region of the MV anterior leaflet in a left ventricle-simulating flow loop over the cardiac cycle were determined. The resulting two-dimensional in-surface strain tensor was computed from the marker positions using a C ⁰ Lagrangian quadratic finite element. Results demonstrated that during valve closure the anterior leaflet experienced large, anisotropic strains with peak stretch rates of 500%–1000%/s. This rapid stretching was followed by a plateau phase characterized by relatively constant strain state. We hypothesized that the presence of this plateau phase was a result of full straightening of the leaflet collagen fibers upon valve closure. This hypothesis suggests that the MV collagen fibers are designed to allow leaflet coaptation followed by a dramatic increase in stiffness to prevent further leaflet deformation, which would lead to valvular regurgitation. These studies represent a first step in improving our understanding of normal MV function and to help establish the principles for repair and replacement. © 2002 Biomedical Engineering Society. PAC2002: 8719Hh, 8719Uv, 8719Rr     

Bioprosthetic Heart Valve Leaflet Deformation Monitored by Double-Pulse Stereo Photogrammetry

A double-pulse stereo photogrammetry technique has been developed for the dynamic assessment of the leaflet deformation of bioprosthetic heart valves under simulated physiological conditions. By using a specially designed triggering technique, which takes the advantage of the field transfer mechanisms of the charge coupled device camera, two consecutive images separated by a time interval as short as 5 ms were captured. This made it possible to investigate the realistic leaflet deformation during the valve opening and closing processes which typically last 25–45 ms. This technique was applied to assess a newly developed pericardial valve leaflet in a physiological pulse flow loop. Quantitative leaflet deformations of the valve opening and closing were generated from sequences of digital images. The results can later be applied to finite element analysis of bioprosthetic heart valve leaflet stress and strain during a complete cardiac cycle. © 2002 Biomedical Engineering Society. PAC02: 8719Hh, 8768+z, 8719Rr, 8719Uv     

Development of an animal experimental model for a bileaflet mechanical heart valve prosthesis

The objective of this study was to develop a pre-clinical large animal model for the in vivo hemodynamic testing of prosthetic valves in the aortic position without the need for cardiopulmonary bypass. Ten male pigs were used. A composite valved conduit was constructed in the operating room by implanting a prosthetic valve between two separate pieces of vascular conduits, which bypassed the ascending aorta to the descending aorta. Prior to applying a side-biting clamp to the ascending aorta for proximal grafting to the aortic anastomosis, an aorta to femoral artery shunt was placed just proximally to this clamp. The heart rate, cardiac output, Vmax, transvalvular pressure gradient, effective orifice area and incremental dobutamine stress response were assessed. A dose-dependent increase with dobutamine was seen in terms of cardiac output, Vmax, and the peak transvalvular pressure gradient both in the native and in the prosthetic valve. However, the increment was much steeper in the prosthetic valve. No significant differences in cardiac output were noted between the native and the prosthetic valves. The described pre-clinical porcine model was found suitable for site-specific in-vivo hemodynamic assessment of aortic valvular prosthesis without cardiopulmonary bypass.     

Stress Variations in the Human Aortic Root and Valve: The Role of Anatomic Asymmetry

The asymmetry of the aortic valve and aortic root may influence their biomechanics, yet was not considered in previous valve models. This study developed an anatomically representative model to evaluate the regional stresses of the valve within the root environment. A finite-element model was created from magnetic-resonance images of nine human valve–root specimens, carefully preserving their asymmetry. Regional thicknesses and anisotropic material properties were assigned to higher-order elastic shell elements representing the valve and root. After diastolic pressurization, peak principal stresses were evaluated for the right, left, and noncoronary leaflets and root walls. Valve stresses were highest in the noncoronary leaflet (538 kPa vs right 473 kPa vs left 410 kPa); peak stresses were located at the free margin and belly near the coaptation surfaces (averages 537 and 482 kPa for all leaflets, respectively). Right and noncoronary sinus stresses were 21% and 10% greater than the left sinus. In all sinuses, stresses near the annulus were higher than near the sinotubular junction. Stresses vary across the valve and root, likely due to their inherent morphologic asymmetry and stress sharing. These factors may influence bioprosthetic valve durability and the incidence of isolated sinus dilatation. © 1998 Biomedical Engineering Society. PAC98: 8745Bp, 8710+e, 0270Dh     

Adaptation of human left ventricular volumes to the onset of supine exercise

Summary The purpose of this study was to measure the changes and rates of adaptation of left ventricular volumes at the onset of exercise. Eight asymptomatic subjects, in whom intramyocardial markers had been implanted 3–6 years previously during aortocoronary bypass surgery, exercised in the supine position at a constant workload of 73.6 W for 5 min. Six also exercised first at 16.4 W, and then against a workload which progressively increased by 8.2 W every 15s. Cardiac volumes were measured by computer assisted analysis of the motion of the implanted markers. In the constant workload test, cardiac output increased rapidly from 5.7±1 min^–1 to 10.3±1.9 1 min^–1 by 2 min and then increased more slowly to 10.8±2.0 1 min^–1 by 5 min. The cardiac output increase was mainly due to an increase in heart rate from 68±12 beats min^–1 to 120±16 beats min^–1 with minimal changes in stroke volume. The time constant for the early increase in cardiac output was 45 s and for heart rate, 35 s. With progressively increasing workloads, there was an almost linear increase of heart rate and cardiac output, but these increased at a slower rate than during the early phase of the constant load exercise test. In conclusion: (i) rapid changes in cardiac output during supine exercise were produced by changes in heart rate; (ii) changes in stroke volume provided minor adjustments to cardiac output; (iii) the end-diastolic volume was almost constant.     

Dynamic In Vitro Quantification of Bioprosthetic Heart Valve Leaflet Motion Using Structured Light Projection

Quantification of heart valve leaflet deformation during the cardiac cycle is essential in understanding normal and pathological valvular function, as well as in the design of replacement heart valves. Due to the technical complexities involved, little work to date has been performed on dynamic valve leaflet motion. We have developed a novel experimental method utilizing a noncontacting structured laser-light projection technique to investigate dynamic leaflet motion. Using a simulated circulatory loop, a matrix of 150–200 laser light points were projected over the entire leaflet surface. To obtain unobstructed views of the leaflet surface, a stereo system of high-resolution boroscopes was used to track the light points at discrete temporal points during the cardiac cycle. The leaflet surface at each temporal point was reconstructed in three dimensions, and fit using our biquintic hermite finite element approach (Smith et al., Ann. Biomed. Eng. 26:598–611, 2001). To demonstrate our approach, we utilized a bovine pericardial bioprosthetic heart valve, which revealed regions of complex flexural deformation and substantially different shapes during the opening and closing phases. In conclusion, the current method has high spatial and temporal resolution and can reconstruct the entire surface of the cusp simultaneously. Because it is completely noncontacting, this approach is applicable to studies of fatigue and bioreactor technology for tissue engineered heart valves. © 2001 Biomedical Engineering Society. PAC01: 8719Hh, 8780-y, 4262Be, 8719St     

应用于零维左心血液循环的二尖瓣模型的研究

提出一个可以准确合理地模拟二尖瓣动力学特性的瓣叶运动流阻模型。考虑影响二尖瓣瓣叶运动的跨瓣压差和血流推力,建立二尖瓣运动的控制方程,提出依赖于瓣叶打开角度θ的瓣叶运动流阻模型,把该模型应用于零维左心血液循环系统,得到血液动力学特性。在保持心输出量和反流分数一致的条件下,比较该模型、瞬态关闭的阶梯流阻模型和经验指定的时变流阻模型。结果发现,瓣叶运动流阻模型能反映瓣膜关闭过程中的血液动力学,如压差和流量的滞后性以及关闭流量,同时该模型可以通过调整单位转动惯量跨瓣压差影响系数K_p和血流影响系数K_b的大小,改变瓣膜打开过程和关闭过程所需时间,瓣膜打开和关闭时间分别为50.0和40.2 ms。该模型可弥补阶梯流阻模型中忽略瓣膜运动过程的瞬态关闭的缺点,同时也能避免时变流阻模型中关闭起始时间的不合理性。此模型较为合理准确地模拟二尖瓣关闭过程的动力学特性,且简单易控制。     

射血分数-压差比值评价伴左心功能不全的主动脉瓣狭窄程度

目的初步探讨射血分数-压差比值评价伴左心功能不全的主动脉瓣狭窄程度可行性。方法80例左心室收缩功能不全的主动脉瓣狭窄患者，其中男32例，女性48例，年龄38～85岁，平均年龄42岁。用彩色多普勒超声测量主动脉瓣口面积（AVA）、左心室射血分数（EF），Bernoulli方程计算主动脉瓣口跨瓣压差（△P），Simpson容积描记法计算射血分数压差比值即EFPR（EFPR=EF／△P），分析AVA与△P、EFPR之间的相关性；用ROC曲线比较△P、EFPR两参数评价主动脉瓣狭窄程度的敏感度和特异度。结果对主动脉瓣狭窄伴左心室收缩功能不全患者，用Simpson容积描记法计算ERPR估测AVA较Bemoulli方程计算的△P法更准确（r=0.9172对r=-0.6796，P〈0.001）；将EFPR小于1.0、△P大于10.7kda（80mmHg）来估测重度主动脉瓣狭窄伴左心功能不全时，EFPR和△uP的敏感度和特异度分别为87.5％、68.6％（P〈0.05）和98.3％、37.2％（P〈0.01），表明EFPR估测重度主动脉瓣狭窄伴左心功能不全患者的特异度及敏感度较高。结论EFPR能准确估测主动脉瓣狭窄患者主动脉瓣狭窄程度，特别对伴左心功能不全的重度主动脉瓣狭窄患者瓣膜狭窄程度的评价。EFPR较传统参数有更高敏感性和特异性。     

Exercise training mode affects the hemodynamic responses to lower body negative pressure in women

To determine if different exercise modes used to improve cardiovascular fitness result in differing cardiovascular responses to lower body negative pressure (LBNP) in exercise-trained women, seven chronically exercising female runners (RUN) and 11 swimmers (SWIM) of similar fitness levels maximal oxygen uptake, [ , mean (SEM) = 50 (2) and 45 (2) ml·kg^–1·min^–1, respectively; P > 0.05] underwent serial exposures to LBNP at pressures of 0, –1.3, –2.7 and –5.3 kPa (referenced to ambient barometric pressure). Forearm vascular resistance (venous occlusion plethysmography) increased with LBNP but did not differ between groups at any level of LBNP. At 0 and –1.3 kPa, the total peripheral resistance index (TPRI; impedance cardiography) was significantly (P < 0.05) higher in RUN than SWIM [1.118 (0.028) vs 0.787 (0.040) at 0 kPa and 1.245 (0.100) vs 0.840 (0.040) kPa·1·min^–1 m^–2 at –1.3 kPa]. At an LBNP of –2.7 kPa, stroke index (SI) was significantly higher in SWIM than RUN [57.8 (4.6) vs 41.9 (4.0) ml·beat^–1 · m^–2] while TPRI remained greater in RUN than SWIM. At –5.3 kPa, SWIM exhibited a higher cardiac index [3.232 (0.209) vs 2.447 (0.189) 1·min^–1·m^–2] and SI [49.4 (4.4) vs 31.0 (4.5) ml·beat^–1·m^–2] but reduced heart rate [71 (3) vs 83 (5)beats·min^–1] and TPRI [0.968 (0.043) vs 1.655 (0.128) kPa·1·min^–1 · m^–2]. Mean arterial pressure declined significantly at an LBNP of –5.3 kPa in both groups; pulse pressure was lower (P < 0.05) in RUN than SWIM at LBNP values of –2.7 and –5.3 kPa. These data suggest that: (1) female runners experience a greater increase in systemic vasoconstriction even though female swimmers can better maintain their cardiac index at high levels of LBNP, and (2) training mode appears to affect the pulse pressure responses to LBNP in exercise-trained women.     

Pannus formation in aortic valve prostheses in the late postoperative period

We present three patients who underwent repeat aortic valve replacement for prosthetic valve dysfunction caused by tissue ingrowth in the late postoperative period. These patients (three women aged 48–51 years, mean 49.3 ± 1.53 years) underwent operations for restriction of prosthetic valve leaflet movement by pannus in the left ventricular outflow tract. The interval from the previous operation ranged from 8.0 to 9.6 years (mean 9.6 ± 2.0 years). The symptoms of the patients were New York Heart Association functional class I, II, and IV in one patient each. Diagnosis was made by cinefluoroscopy in two patients and aortography in one patient. The operative procedures consisted of aortic valve replacement (n = 1) and aortic valve replacement with mitral valve replacement (n = 2). Pannus was found at the left ventricular aspect of the prosthetic valve in all patients. In two patients, the pannus directly restricted movement of the leaflet and also severely narrowed the inflow orifice of the prosthetic valve. In the other patient, the pannus had grown at a distance of 7mm from the valve and narrowed the left ventricular outflow tract circularly. The postoperative course was uneventful and all three patients were discharged in a good condition. One patient died of pneumonia 8 months after surgery and the other two patients have remained well and have been followed up for one and a half years. In conclusion, there may be a discrepancy between the clinical symptoms and the grade of subvalvular stenosis caused by pannus. Therefore, it is essential for satisfactory operative results that early diagnosis be made by various means.     

Numerical simulation of mechanical mitral heart valve closure

A computational fluid dynamic simulation of a mechanical heart valve closing dynamics in the mitral position was performed in order to delineate the fluid induced stresses in the closing phase. The pressure and shear stress fields in the clearance region and near the inflow (atrial) side of the valve were computed during the mitral heart valve closure. Three separate numerical simulations were performed. The atrial chamber pressure was assumed to be zero in all the simulations. The first simulation was steady flow through a closed mitral valve with a ventricular pressure of 100 mm Hg (1.3 kPa). In the second simulation, the leaflet remained in the closed position while the ventricular pressure increased from 0 to 100 mm Hg at a rate of 2000 mm Hg/s (simulating leaflet closure by gravity before the ventricular pressure rise – gravity closure). In the third case, the leaflet motion from the fully open position to the fully closed position was simulated for the same ventricular pressure rise (simulating the normal closure of the mechanical valve). Normal closure (including leaflet motion towards closure, and sudden stop in the closed position) resulted in a relatively large negative pressure transient which was not present in the gravity closure simulation. The wall shear stresses near the housing and the leaflet edge close to the inflow side were around 4000 Pa with normal closure compared to about 725 Pa with gravity closure. The large negative pressure transients and significant increase in wall shear stresses due to the simulation of normal closure of the mechanical valve is consistent with the previously reported increased blood damage during the closing phase. © 2001 Biomedical Engineering Society. PAC01: 8719Hh, 8780-y, 8719Uv, 8710+e     

Effects of inspiratory impedance on hemodynamic responses to a squat–stand test in human volunteers: implications for treatment of orthostatic hypotension

Recent studies in our laboratory demonstrated that spontaneous breathing through an inspiratory impedance threshold device (ITD) increased heart rate (HR), stroke volume (SV), cardiac output (Q), and mean arterial blood pressure (MAP) in supine human subjects. In this study, we tested the effectiveness of an ITD as a countermeasure against development of orthostatic hypotension, provoked using a squat-to-stand test (SST). Using a prospective, randomized blinded protocol, 18 healthy, normotensive volunteers (9 males, 9 females) completed two-counterbalanced 6-min SST protocols with and without (sham) an ITD set to open at 0.7 kPa (7-cm H₂O) pressure. HR, SV, Q, total peripheral resistance (TPR), and MAP were assessed noninvasively with infrared finger photoplethysmography. Symptoms were recorded on a 5-point scale (1=normal; 5=faint) of subject perceived rating (SPR). The reduction in TPR produced by SST (–35±5 %) was not affected by the ITD. Reduction in MAP with ITD during the transient phase of the SST (–3.6±0.5 kPa or –27±4 mmHg) was less (P=0.03) than that measured while breathing through a sham device (–4.8±0.4 kPa or –36±3 mmHg) despite similar (P<0.926) elevations in HR of 15±2 bpm. SV (+2±4 %) and Q (+22±5 %) with the ITD were higher (P<0.04) than SV (–8±4 %) and Q (+10±6 %) without the ITD. SPR was 1.4±0.1 with ITD compared to 2.0±0.2 with the sham device (P<0.04). This reduction in orthostatic symptoms with application of an ITD during the SST was associated with higher MAP, SV and Q. Our results demonstrate the potential application of an ITD as a countermeasure against orthostatic hypotension.