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  • 1
    Electronic Resource
    Electronic Resource
    Mechanical Factors in the Degeneration of Porcine Bioprosthetic Valves: An Overview (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of cardiac surgery 4 (1989), S. 0 
    Details
    ISSN: 1540-8191
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract Predilection of certain sites of the porcine bioprosthetic valve (PBV) leaflets to calcification and tissue disruption has provoked suggestions that design factors and mechanical stresses may be major reasons leading to degeneration. In recent years, computer based numerical models of PBVs have shown a close association between sites of leaflet calcification and disruption and sites of leaflet stress concentration. These numerical models have also provided a means through which methodical design optimization can be carried out. Increasing stent flexibility, for instance, was shown to lead to an overall reduction of mechanical stresses on the PBV leaflets. Reducing the stent height, on the other hand, was accompanied by an undesirable increase of overall leaflet stresses. Despite encouraging work in this field, more research is needed to further elucidate means by which the structural integrity of bioprosthetic valves can be preserved through a minimization of the adverse effects of mechanical stresses.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1540-8191.1989.tb00294.x
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  • 2
    Electronic Resource
    Electronic Resource
    Leukocytes, Platelets, and Surface Microstructure of Spontaneously Degenerated Porcine Bioprosthetic Valves (1988)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of cardiac surgery 3 (1988), S. 0 
    Details
    ISSN: 1540-8191
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The microstructure of 33 spontaneously degenerated porcine bioprosthetic valves was assessed by scanning electron microscopy in order to gather insight regarding the degenerative process. Twenty-four mitral and 9 aortic valves were removed from 32 patients. The duration of insertion was 7.7 ± 2.4 years (mean ± SD). All valves showed denudation of endothelial cells and exposure of the subendothelial fibrous tissue. Fibroblas±like cells were occasionally seen. Platelet deposits were observed on 22 of 33 valves (67%). Leukocytes were observed on the surface of 27 of 33 valves (82%). Mononuclear leukocytes were the most common category of cells (66%). Crystalline material was present on the surface of some leukocytes, suggesting that they may serve as a nidus for calcification. Transmission electron microscopy showed leukocytes in the process of phagocytizing collagen fibers. Macrophages, by exerting their scavenger function seem to contribute to destruction of the collagen framework of the valves. Whether the observed lymphocytes and plasma cells reflect an immunological involvement is unclear.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1540-8191.1988.tb00246.x
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  • 3
    Electronic Resource
    Electronic Resource
    RELATION OF INTRAMYOCARDIAL PRESSURE TO CORONARY PRESSURE AT ZERO FLOW (1986)
    Oxford, UK : Blackwell Publishing Ltd
    Clinical and experimental pharmacology and physiology 13 (1986), S. 0 
    Details
    ISSN: 1440-1681
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: 1. The purpose of this study was to determine the extent to which coronary pressure at zero coronary flow (Pf=0) may relate to extravascular compressive forces determined by direct measurements of left ventricular intramyocardial pressure.2. Studies were performed in nine open-chest anaesthetized dogs in which the anterior descending coronary artery was cannulated and perfused from the carotid artery. Coronary pressure was measured at the tip of the cannula.3. Intramyocardial pressure was measured with a 1 mm diameter micromanometer inserted directly into the subepicardium. The atrioventricular node was obliterated by cautery and the heart was electrically paced. Long diastolic pauses, sufficient to allow coronary flow to reach zero, were produced by the cessation of electrical pacing.4. In the autoregulated coronary bed, Pf=0, 47 mmHg (s.e.m. = 9), exceeded subepicardial pressure at zero flow, 23 mmHg (s.e.m. =2; P 〈 0.001). During maximal vasodilatation with adenosine, Pf=0, 16 mmHg (s.e.m. = 11), was not significantly different from subepicardial pressure at zero flow, 21 mmHg (s.e.m. =4). These observations indicate that, in addition to coronary vasomotor tone, diastolic myocardial tissue pressure is important in the genesis of Pf=0.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1440-1681.1986.tb00928.x
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  • 4
    Electronic Resource
    Electronic Resource
    High flow attenuates relaxation by acetylcholine in isolated perfused canine femoral arteries (1988)
    Springer
    Heart and vessels 4 (1988), S. 14-18 
    Details
    ISSN: 1615-2573
    Keywords: Endothelium ; Flow ; Acetylcholine ; Nitroprusside
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The effect of changes in flow on endothelial-dependent and -independent dilation of isolated perfused arteries was determined. Femoral arteries from ten dogs were excised and mounted in a vessel bath containing a physiological salt solution (PSS) at 37°C, bubbled with 95% O2-5% CO2. The vessel outer diameter was measured using an ultrasonic crystal dimension system. Arteries were perfused with PSS at a steady flow of 1 ml/s at 90 mmHg. Tone was induced in the arteries by the addition of phenylephrine to the vessel bath. Acetylcholine (six dogs) and sodium nitroprusside (four dogs) were added to the perfusate in a cumulative fashion and changes in vessel diameter were recorded until maximum vasodilation was achieved. Dose-response relationships to the agents were determined with each vessel perfused at 1 and 4 ml/s. Sensitivity to each agent was measured as the ED50 value calculated from the respective dose-response relationships. Sensitivity to acetylcholine, but not sodium nitroprusside, was significantly decreased in arteries perfused at 4 ml/s versus 1 ml/s [acetylcholine ED50: 1 ml/s=(0.79±0.31)×10−8 mol/l, 4 ml/s=(1.55±0.60) × 10−8 mol/l,P〈0.05; sodium nitroprusside ED50: 1 ml/s=(2.57±0.12)×10−7 mol/l, 4 ml/s=(2.69±0.60)×10−7 mol/l, not significant]. We conclude that high flow decreases sensitivity to acetylcholine in canine femoral arteries and suggest that vascular reactivity to this agent may be modified by changes in shear on the vascular endothelium.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02058682
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  • 5
    Electronic Resource
    Electronic Resource
    Coronary extravascular compression influences systolic coronary blood flow (1986)
    Springer
    Heart and vessels 2 (1986), S. 140-146 
    Details
    ISSN: 1615-2573
    Keywords: Coronary extravascular compression ; Systolic coronary flow ; LV subepicardial pressure ; Intracoronary isoproterenol ; Maximal vasodilation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The purpose of this study was to determine whether a relation exists between systolic coronary blood flow and systolic coronary extravascular compressive forces. Studies were performed in seven open-chest dogs in which the left anterior descending coronary artery was cannulated and perfused from the left carotid artery. Pressure within the left ventricular subepicardium, a measure of coronary extravascular compression in the subepicardium, was measured with a 1-mm-diameter catheter-tip micromanometer inserted directly into the myocardium in the region of perfusion. Systolic extravascular compressive forces were augmented by a local intracoronary injection of 1µg isoproterenol. Measurements were made in the presence of coronary vasomotor tone and were repeated following local maximal vasodilatation with adenosine. In the regulated coronary bed, systolic coronary flow decreased (18±4 vs −2±4 ml/min,P〈0.01) as intramyocardial pressure increased (127±5 vs 222±12 mmHg,P〈0.001). Similarly, in the maximally vasodilated coronary bed, systolic coronary flow decreased (103±16 vs 38±11 ml/min,P〈0.001) as intramyocardial pressure increased (112±6 vs 204±16 mmHg,P〈0.001). These observations indicate that an augmentation of coronary extravascular compressive forces during systole is accompanied by a diminution of systolic coronary flow irrespective of coronary vasomotor tone.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02128139
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  • 6
    Electronic Resource
    Electronic Resource
    Estimation of reynolds shear stresses during pulsatile flow in the region of aortic valves (1985)
    Springer
    Annals of biomedical engineering 13 (1985), S. 17-23 
    Details
    ISSN: 1573-9686
    Keywords: Reynolds stresses ; Velocity ; Aortic valve
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract Some investigators have attempted to estimate the Reynolds shear stress on the basis of a single component of velocity. The purpose of this investigation is to determine the validity of such estimates in a complex flow field, such as occurs in the cardiovascular system in the region of the aortic valve. Turbulent velocities were obtained in an in vitro pulse duplicating system with a two-channel laser Doppler anemometer. Velocities were measured in the region of two stenotic natural aortic valves and a normal stent mounted porcine bioprosthetic valve. Constants of proportionality between the Reynolds shear stress, averaged over ejection, and the Reynolds normal stress were determined. The constants of proportionality depended uppn the local conditions, namely, whether the valves were stenotic or normal bioprosthetic. There was wide scatter of data. This suggests that any estimate of the Reynolds shear stress, based upon a single axial velocity in a complex flow field, such as occurs in the cardiovascular system, is likely to be inaccurate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02371247
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  • 7
    Electronic Resource
    Electronic Resource
    Evaluation of left ventricular function during ejection (1992)
    Springer
    Annals of biomedical engineering 20 (1992), S. 127-138 
    Details
    ISSN: 1573-9686
    Keywords: Ventricular performance ; Ventricular contractility ; Cardiac function ; Cardiac energy ; Heart ; Power
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract Several ejection indices of left ventricular performance are described. The most sensitive is the rate of change of power, especially when measured at the time of peak left ventricular wall tension. When measured at peak tension, the rate of change of power reflects force-velocity-length relations and, therefore, is theoretically appealing. The rate of change of power itself incorporates terms shown to be of functional significance. Among these is the rate of change of flow. This expression is more readily measured than the rate of change of power and can be approximated noninvasively by the measurement of blood acceleration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02368509
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  • 8
    Electronic Resource
    Electronic Resource
    Flow in a symmetrically branched tube simulating the aortic bifurcation: The effects of unevenly distributed flow (1980)
    Springer
    Annals of biomedical engineering 8 (1980), S. 159-173 
    Details
    ISSN: 1573-9686
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract The effects of unevenly distributed flow were investigated in a symmetrically branched tube with an angle of branching and branch-to-trunk area ratio that were comparable to the human descending aorta. Profiles of velocity were measured at the vertex of the bifurcation with a laser Doppler anemometer during pulsatile flow as well as steady flow. The mean Reynolds numbers were 500, 1000, and 1500. When flow in the branches was equal, reversals were present along the outer wall during the minimal phase of the flow cycle at a Reynolds number of 500. Such reversals were absent at higher Reynolds numbers. When flow in the branches was unequal, reversals occurred only in the branch with the lower flow. Such reversals occurred at all of the Reynolds numbers studied. Pulsatile flow separation, however, did not occur at any Reynolds number when the flow in the branches was equal. Pulsatile flow separation occurred in the partially occluded branch when flow in the branch was ≤8% of the total flow only at Reynolds numbers of 1000 and 1500. A prominent difference between pulsatile and steady flow was that reversals along the outer wall occurred during pulsatile flow at percentages that were prominently higher than the percentage of flow in the branch that produced reversals during steady flow. These observations may be pertinent to understanding the potential of characteristics of flow in the genesis of atherosclerosis in the region of the bifurcation of the aorta.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02364662
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