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At least 433 records · Page 24Linked to original sources
PubMed1993

Accurate systolic wall thickening by nuclear magnetic resonance imaging with tissue tagging: correlation with sonomicrometers in normal and ischemic myocardium.

OBJECTIVES: This study examined whether the correlation of systolic wall thickening (%WT) by nuclear magnetic resonance (NMR) imaging with wall thickening by sonomicrometry (SM) is improved by using a three-dimensional volume element model of the left ventricular wall. BACKGROUND: Left ventricular wall obliquity with respect to the imaging plane causes overestimation of wall thickness by planar imaging techniques. Wall thickness perpendicular to the endocardial surface can be accurately calculated by three-dimensional reconstruction of left ventricular wall segments. METHODS: Sonomicrometers were placed transmurally in 11 dogs (left anterior descending artery territory) with an imaging marker, visible on NMR imaging, sewn to the epicardial crystal. Two adjacent NMR short-axis image planes were radially segmented by four perpendicular spin-saturated planes (tags), dividing the myocardium into eight volume elements, one of which contained the sonomicrometer crystal pair. Left ventricular thickness and thickening were calculated by four methods: 1) linear = distance between epicardium and endocardium at midpoint in the segment with the sonomicrometer; 2) planar = area of that segment divided by the mean of the endocardial and epicardial arc lengths; 3) biplanar = average of wall thicknesses calculated by the planar method from the segment with sonomicrometers and the corresponding segment located in the adjacent short-axis imaging plane; and 4) three-dimensional = volume of the element with the sonomicrometers divided by the mean of the endocardial and epicardial surface areas. RESULTS: Regressions for all methods using pooled data from control periods and during ischemia: Linear %WT = 0.59 + 1.31 SM%WT (r = 0.71, SEE = 0.28, p < 0.0002) Planar %WT = 1.43 + 1.62 SM%WT (r = 0.87, SEE = 0.19, p < 0.0001) Biplanar %WT = 2.09 + 1.46 SM%WT (r = 0.90, SEE = 0.15, p < 0.0001) Three-dimensional %WT = 0.19 + 1.49 SM%WT (r = 0.95, SEE = 0.10, p < 0.0001) CONCLUSIONS: Nuclear magnetic resonance imaging with tissue tagging allows accurate noninvasive assessment of systolic wall thickening. The three-dimensional volume element approach, by accounting for obliquity between the image plane and the left ventricular wall, provides the strongest correlation between NMR imaging and percent systolic wall thickening by sonomicrometer crystals.

Animals↗
PubMed1993

Cardiac motion can alter proximal isovelocity surface area calculations of regurgitant flow.

OBJECTIVES: This study addressed the hypothesis that motion of the surface containing a regurgitant orifice relative to the Doppler ultrasound transducer can cause differences between actual flow rate and calculations based on the proximal flow convergence technique. BACKGROUND: In vitro studies quantitating regurgitant flow rate by proximal flow convergence have been limited to stationary orifices. Clinically, however, valve leaflets generally move relative to the ultrasound transducer during the cardiac cycle and can move at velocities important relative to the measured color aliasing velocities. The transducer therefore senses the vector sum of actual flow velocity toward the orifice and orifice velocity relative to the transducer. This can cause potential overestimation or underestimation of true flow rate, depending on the direction of surface motion. METHODS: The hypothesis was explored computationally and tested by pumping fluid at a constant flow rate through an orifice in a plate moving at 0 to 8 cm/s (velocities comparable to those described clinically for mitral and tricuspid annulus motion toward an apical transducer). RESULTS: Surface motion in the same direction as flow caused overestimation of the aliasing radius and calculated flow rate. Surface motion opposite to the direction of flow (typical for mitral and tricuspid regurgitation viewed from the apex or esophagus) caused underestimation of actual flow rate. The underestimation was greater for lower aliasing velocities (36 +/- 11% for 10 cm/s vs. 23 +/- 6% for 20 cm/s). Correcting for surface motion provided excellent agreement with actual values (y = 0.97x + 0.10, r = 0.99, SEE = 0.17 liters/min). CONCLUSIONS: Physiologic motion of the surface containing a regurgitant orifice can cause substantial differences between actual flow rate and that calculated by the proximal flow convergence technique. Los aliasing velocities used to optimize that technique can magnify this effect. Such errors can be minimized by using higher aliasing velocities (compatible with the need to measure the aliasing radius) or eliminated by correcting for surface velocity determined by an M-mode ultrasound scan.

Blood Flow Velocity↗
PubMed1995

Nature of flow acceleration into a finite-sized orifice: steady and pulsatile flow studies on the flow convergence region using simultaneous ultrasound Doppler flow mapping and laser Doppler velocimetry.

OBJECTIVES: This study investigated the proximal centerline flow convergence region simultaneously by both color Doppler and laser Doppler velocimetry. BACKGROUND: Although numerous investigations have been performed to test the flow convergence method, to our knowledge there has yet been no experimental study using reference standard velocimetric techniques to define precisely the hydrodynamic factors involved in the accelerating flow region during steady and pulsatile flow. METHODS: Using an in vitro model that allows velocity measurements by laser Doppler velocimetry with simultaneous comparison with color Doppler results, we studied the centerline flow acceleration region proximal to orifices of various sizes (0.08 to 2.0 cm2). RESULTS: Agreement between theory and experimental velocities was good for large flow rates through small orifices only, and only at distances > 1.2 cm from the orifice. Changing the orifice shape from circular to slitlike produced no significant changes in velocity profiles. Constraining the proximal side walls caused a significant increase in proximal velocities at distances > 0.7 cm for the largest orifice only (2.0 cm2). Calculated flow rates agreed well with actual flow rates, with functional dependence on proximal distance and orifice size. Velocity profiles for pulsatile flow were similar to steady state flow profiles and could be integrated to calculate stroke volumes, which followed actual flow volumes well, although with general overestimation (y = 1.22x + 0.164, r = 0.92), most likely due to the use of all available proximal velocities. CONCLUSIONS: The accelerating proximal flow region responds to several hydrodynamic factors that can affect flow quantitation using the flow convergence method in the clinical situation.

Blood Flow Velocity↗
PubMed1994

Balancing the circulation: theoretic optimization of pulmonary/systemic flow ratio in hypoplastic left heart syndrome.

OBJECTIVES: This study examined the effects of the pulmonary (QP)/systemic (QS) blood flow ratio (QP/QS) on systemic oxygen availability in neonates with hypoplastic left heart syndrome. BACKGROUND: The management of neonates with hypoplastic left heart syndrome is complex and controversial. Both before and after surgical palliation and before heart transplantation, a univentricle with parallel pulmonary and systemic circulations exists. It is generally assumed that balancing pulmonary and systemic blood flow is best to stabilize the circulation. METHODS: We developed a mathematical model that was based on the simple flow of oxygen uptake in the lungs and whole-body oxygen consumption to study the effect of varying the QP/QS ratio. An equation was derived that related the key variables of cardiac output, pulmonary venous oxygen saturation and the QP/QS ratio to systemic oxygen availability. RESULTS: The key findings are 1) as the QP/QS ratio increases, systemic oxygen availability increases initially, reaches a maximum and then decreases; 2) for maximal systemic oxygen availability, the optimal QP/QS ratio is < or = 1; 3) the optimal QP/QS ratio decreases as cardiac output or percent pulmonary venous oxygen saturation, or both, increase; 4) the critical range of QP/QS, where oxygen supply exceeds basal oxygen consumption, decreases as cardiac output and percent pulmonary venous oxygen saturation decrease; 5) the relation between oxygen availability and QP/QS is very steep when QP/QS approaches this critical value; and 6) the percent oxygen saturation of systemic venous blood is very low outside the critical range of QP/QS and high within the critical range. CONCLUSIONS: This analysis provides a theoretic basis for balancing both the pulmonary and systemic circulation and suggests that evaluating both systemic arterial and venous oxygen saturation may be a useful way to determine the relative pulmonary and systemic flows. When high systemic arterial and low systemic venous oxygen saturation are present, pulmonary blood flow should be decreased; conversely, when both low systemic arterial and venous oxygen saturation are present, more flow should be directed to the pulmonary circulation.

Cardiac Output↗
PubMed1994

A new approach for the quantification of complex lesion morphology: the gradient field transform; basic principles and validation results.

OBJECTIVES: This report describes the basic principles and the results from clinical evaluation studies of a new algorithm that has been designed specifically for the quantification of complex coronary lesions. BACKGROUND: Currently used edge detection algorithms in quantitative coronary arteriography, such as the minimum cost algorithm, are limited in the precise quantification of complex coronary lesions characterized by abruptly changing shapes of the obstruction. METHODS: The new algorithm, the gradient field transform, is not limited in its search directions and incorporates the directional information of the arterial boundaries. To evaluate its accuracy and precision, 11 tubular phantoms (sizes 0.6 to 5.0 mm), were analyzed. Second, angiographic images of 12 copper phantoms with U-shaped obstructions were analyzed by both the gradient field transform and the minimum cost algorithm. Third, 25 coronary artery segments with irregularly shaped obstructions were selected from 19 routinely acquired angiograms. RESULTS: The plexiglass phantom study demonstrated an accuracy and precision of -0.004 and 0.114 mm, respectively. The U-shaped copper phantoms showed that the gradient field transform performed very well for short, severe obstructions, whereas the minimum cost algorithm severely overestimated the minimal lumen diameter. From the coronary angiograms, the intraobserver variability in the minimal lumen diameter was found to be 0.14 mm for the gradient field transform and 0.20 mm for the minimum cost algorithm. CONCLUSIONS: The new gradient field transform eliminates the limitations of the currently used edge detection algorithms in quantitative coronary arteriography and is therefore particularly suitable for the quantification of complex coronary artery lesions.

Algorithms↗
PubMed1994

A compliant tubular device to study the influences of wall strain and fluid shear stress on cells of the vascular wall.

PURPOSE: Cellular constituents of the blood vessel wall are continuously subjected, in vivo, to both mechanical and hemodynamic forces, which elicit structural and biologic responses. We have developed a compliant tubular system, the vascular simulating device (VSD), that reproduces these forces, while supporting the attachment and the experimental manipulation of endothelial and smooth muscle cells. METHODS: The VSD consists of a compliant silicone rubber tube coupled to a pump system, which permits the simultaneous application of known levels of pressure and flow, to vascular wall cells cultured on the inner surface of the tube. Seeded cells can be monitored visually under phase contrast or fluorescent optics, as well as harvested and analyzed for biologic responses. RESULTS: The elastic modulus and compliance of the silicone rubber tube are similar to those of canine and human arteries. Endothelial and smooth muscle cells cultured on the lumenal surface of the tubes remain attached and viable after subjecting them to physiologic pulsatile flow and cyclic strain. CONCLUSION: The VSD makes it possible to approximate, in vitro, those forces encountered by vascular wall cells, in vivo and therefore may make it possible to determine whether specific combinations of mechanical and hemodynamic forces are causally associated with specific vascular diseases.

Animals↗
PubMed1994

The O'Brien-Angell stentless valve. Early results of 100 implants.

From August 1991 to May 1993, 100 unselected consecutive patients in whom an aortic bioprosthesis was indicated underwent aortic valve replacement with the O'Brien-Angell stentless porcine xenograft (Bravo Cardiovascular Model 300). The indication was calcified aortic stenosis (AS) in 62 cases (isolated in 44, with associated cardiac lesions in 18), aortic insufficiency in 24 cases (isolated in 13 with associated cardiac lesions in 11), and redo operations in 14 cases. Forty-four percent of the patients were over 70 years of age. With the recommended supra-annular single running suture technique, the aortic cross-clamp times in isolated procedures ranged from 32 to 70 min, mean 39 min. Valve function was studied by echocardiography in 92 patients. Early post-operative transvalvular gradients were usually in the low range, below 15 mmHg in 68% of the cases. Isolated central valvular regurgitation was absent in 60.8%, "microscopic to trivial in 31.5% and mild to moderate in 1.2%. Perivalvular regurgitation was absent in 65.2%, microscopic to trivial in 26.2% and mild to moderate in 3.2%. Thirty-five of 40 patients with follow-ups exceeding 6 months had non-invasive controls. Transvalvular gradients showed a 30 to 40% reduction when compared to the immediate postoperative values. There were no new isolated central regurgitations but in two cases previously mild-to-moderate perivalvular leaks evolved to moderately severe leaks. Two valves were explanted, one for rupture of a Prolene 4/0 and one due to a slack Prolene 4/0 suture. We consider that the O'Brien-Angell stentless valve was easy to handle in all situations encountered.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1995

A regression model of ultrasound reflectivity from normal myocardium.

In this paper the potentialities of back-scattered ultrasound to detect changes in transmural myocardial wall structure have been assessed by using a second order regression model of integrated back-scatter. The model was tested through simulation studies and experimental measurements. The experimental results prove the proposed model attractive to detect physiological transmural changes in myofibre orientation from epicardial to endocardial left ventricular wall.

Biomedical Engineering↗
PubMed1995

A dynamic Fourier series for the compression of ECG using FFT and adaptive coefficient estimation.

In this article, a new ECG data compression technique is proposed. The method relies on modelling quasi-periodic ECG signals as a dynamic Fourier series. Fourier coefficients are continuously estimated using either an FFT algorithm or the adaptive least mean square algorithm. Results from simulated normal and pathological ECGs are presented and discussed. The merits of each of the above two methods are also illustrated. Furthermore, a comparison with other compression techniques is also discussed.

Algorithms↗
PubMed2005

Quantification of the dynamic behavior over time of narrow-band components present in heart rate variability by means of the instantaneous amplitude and frequency.

The statistical properties of the time- and frequency-domain characteristics of heart rate variability are known to vary over time. A method is presented to compute the time-varying spectral contents of the characteristic narrow-band frequency components of heart rate variability by means of the instantaneous amplitude and frequency at an optimal time resolution. The instantaneous frequency may show oscillatory, but also irregular periods in time. An index of the instantaneous bandwidth is computed to discriminate between oscillatory and irregular periods and to correct the instantaneous amplitude and frequency for irregular periods.

Algorithms↗
PubMed2005

Parametrical modelling of a premature ventricular contraction ECG beat: comparison with the normal case.

The aim of this paper is to analyse a parametrical Gaussian kernel based model. The proposed model is tested on two types of electrocardiogram (ECG) beats, the normal case beat and the premature ventricular contraction (PVC) one. Basically, the model is constituted of N Gaussians where their corresponding parameters are estimated by optimising a specific criterion. The modelling technique has been validated using MIT/BIH databases. As a result of this study, we show that a normal beat can be modelled using 18 parameters and only 15 parameters are needed to reconstruct the PVC one.

Computer Simulation↗
PubMed2005

Phase response characteristics of sinoatrial node cells.

In this work, the dynamic response of the sinoatrial node (SAN), the natural pacemaker of the heart, to short external stimuli is investigated using the Zhang et al. model. The model equations are solved twice for the central cell and for the peripheral cell. A short current pulse is applied to reset the spontaneous rhythmic activity of the single sinoatrial node cell. Depending on the stimulus timing either a delay or an advance in the occurrence of next action potential is produced. This resetting behavior is quantified in terms of phase transition curves (PTCs) for short electrical current pulses of varying amplitude which span the whole period. For low stimulus amplitudes the transition from advance to delay is smooth, while at higher amplitudes abrupt changes and discontinuities are observed in PTCs. Such discontinuities reveal critical stimuli, the application of which can result in annihilation of activity in central SAN cells. The detailed analysis of the ionic mechanisms involved in its resetting behavior of sinoatrial node cell models provides new insight into the dynamics and physiology of excitation of the sinoatrial node of the heart.

Action Potentials↗
PubMed2005

Development of a reliable automatic speed control system for rotary blood pumps.

BACKGROUND: Axial blood pumps have been very successfully introduced into the arena of prolonged clinical support. However, they do not offer inherent load-responsive mechanisms for adjusting pumping performance to venous return and changes in physiologic requirements of the patient. To provide for these adjustments we developed an algorithm for demand-responsive pump control based on a reliable suction detection system. METHODS: A PC-based system that analyzes pump performance based on available flow, heart rate and short-term performance history was developed. The physician defines levels of "desired flow" at rest and during exercise, depending on heart rate. In case this desired flow cannot be maintained due to limited venous return, the maximal available flow level is determined from an analysis of the actual pump data (flow, speed and power consumption). An expert system continuously checks the flow signal for any indication of suction. Periodic speed variations then adapt pump performance to the patient's condition. RESULTS: First, stability and functionality were proven under various settings in vitro. The algorithms were then tested in 15 patients in intensive care, in the standard ward, and during bicycle exercise. The system reacted properly to demand changes, at exercise level, in response to coughing and at various Valsalva maneuvers. Suction could also be successfully prevented during severe arrhythmia and in patients with critical cardiac geometry. Exercise tests showed decreases in pulmonary arterial pressure (-22 +/- 9.9%) and pulmonary capillary wedge pressure (-42 +/- 18.54%), and an increase in pump flow (19 +/- 9.5%) and workload (8 +/- 6.1%), all when compared with constant-speed pumping. CONCLUSIONS: A closed-loop control system equipped with an expert system for reliable suction detection was developed that improves response to change in venous return for rotary pump recipients. The system was robust, stable and safe under a wide range of everyday living conditions.

Algorithms↗
PubMed2005

Flow-dependent changes in Doppler-derived aortic valve effective orifice area are real and not due to artifact.

OBJECTIVES: We sought to determine whether the flow-dependent changes in Doppler-derived valve effective orifice area (EOA) are real or due to artifact. BACKGROUND: It has frequently been reported that the EOA may vary with transvalvular flow in patients with aortic stenosis. However, the explanation of the flow dependence of EOA remains controversial and some studies have suggested that the EOA estimated by Doppler-echocardiography (EOA(Dop)) may underestimate the actual EOA at low flow rates. METHODS: One bioprosthetic valve and three rigid orifices were tested in a mock flow circulation model over a wide range of flow rates. The EOA(Dop) was compared with reference values obtained using particle image velocimetry (EOA(PIV)). RESULTS: There was excellent agreement between EOA(Dop) and EOA(PIV) (r2 = 0.94). For rigid orifices of 0.5 and 1.0 cm2, no significant change in the EOA was observed with increasing flow rate. However, substantial increases of both EOA(Dop) and EOA(PIV) were observed when stroke volume increased from 20 to 70 ml both in the 1.5 cm2 rigid orifice (+52% for EOA(Dop) and +54% for EOA(PIV)) and the bioprosthetic valve (+62% for EOA(Dop) and +63% for EOA(PIV)); such changes are explained either by the presence of unsteady effects at low flow rates and/or by an increase in valve leaflet opening. CONCLUSIONS: The flow-dependent changes in EOA(Dop) are not artifacts but represent real changes in EOA attributable either to unsteady effects at low flow rates and/or to changes in valve leaflet opening. Such changes in EOA(Dop) can be relied on for clinical judgment making.

Aortic Valve↗
PubMed2003

Experimental analysis of heart rate variability of long-recording electrocardiograms in normal subjects and patients with coronary artery disease and normal left ventricular function.

The heart rate signal contains valuable information about cardiac health, which cannot be extracted without the use of appropriate computerized methods. This paper presents an analysis of various electrocardiograms, the aim of which is to categorize them into two distinct groups. Group A represents young male subjects with no prior occurrence of coronary disease events and Group B represents middle-aged male subjects who have symptomatic coronary artery disease without myocardial infarction and whose 12-lead ECGs do not contain any abnormalities, thus wrongly indicating a normal subject. Electrocardiographic recordings are approximately 2h in length and acquired under conditions that favor the stationarity of collected data. Linear and nonlinear characteristics are studied by applying several techniques including Fourier analysis, Correlation Dimension Estimation, Approximate Entropy, and the Discrete Wavelet Transform. The small variations of the diagnostic information given by each one of the methods as well as the slightly different conclusions among similar studies indicate the necessity of further investigation, combined use, and complementary application of different approaches.

Adult↗
PubMed2004

Numerical investigation of the non-Newtonian blood flow in a bifurcation model with a non-planar branch.

The non-Newtonian fluid flow in a bifurcation model with a non-planar daughter branch is investigated by using finite element method to solve the three-dimensional Navier-Stokes equations coupled with a non-Newtonian constitutive model, in which the shear thinning behavior of the blood fluid is incorporated by the Carreau-Yasuda model. The objective of this study is to investigate the influence of the non-Newtonian property of fluid as well as of curvature and out-of-plane geometry in the non-planar daughter vessel on wall shear stress (WSS) and flow phenomena. In the non-planar daughter vessel, the flows are typified by the skewing of the velocity profile towards the outer wall, creating a relatively low WSS at the inner wall. In the downstream of the bifurcation, the velocity profiles are shifted towards the flow divider. The low WSS is found at the inner walls of the curvature and the lateral walls of the bifurcation. Secondary flow patterns that swirl fluid from the inner wall of curvature to the outer wall in the middle of the vessel are also well documented for the curved and bifurcating vessels. The numerical results for the non-Newtonian fluid and the Newtonian fluid with original Reynolds number and the corresponding rescaled Reynolds number are presented. Significant difference between the non-Newtonian flow and the Newtonian flow is revealed; however, reasonable agreement between the non-Newtonian flow and the rescaled Newtonian flow is found. Results of this study support the view that the non-planarity of blood vessels and the non-Newtonian properties of blood are an important factor in hemodynamics and may play a significant role in vascular biology and pathophysiology.

Arteries↗
PubMed2005

A numerical assessment of wall shear stress changes after endovascular stenting.

This theoretical/numerical study aims at assessing the haemodynamic changes induced by endovascular stenting. By using the classical one-dimensional linear pressure waves theory in elastic vessels, we first show that the modulus of the reflection coefficient induced by an endovascular prosthesis is most likely small since it is proportional to the stent-to-wavelength ratio. As a direct consequence, the wall motion of the elastic (stented) artery can be prescribed a priori and the coupled fluid-structure problem does not have to be solved for assessing the haemodynamic changes due to stenting. Several 2D axisymetric calculations are performed to solve the unsteady incompressible Navier-Stokes equations on moving meshes for different types of (stented) arteries. The numerical results suggest that endovascular stenting increases the systo-diastolic variations of the wall shear stress (by 35% at the middle of the stent, by almost 50% in the proximal transition region). Additional calculations show that over-dilated stents produce less haemodynamic perturbations. Indeed, the increase of the amplitude of the wall shear stress variations over the cardiac cycle is only 10% when the stent radius is equal to the radius of the elastic artery at systole (instead of being equal to the mean artery radius).

Elasticity↗
PubMed2006

A volumetric model for growth of arterial walls with arbitrary geometry and loads.

Stress and deformation in arterial wall tissue are factors which may influence significantly its response and evolution. In this work we develop models based on nonlinear elasticity and finite element numerical solutions for the mechanical behaviour and the remodelling of the soft tissue of arteries, including anisotropy induced by the presence of collagen fibres. Remodelling and growth in particular constitute important features in order to interpret stenosis and atherosclerosis. The main object of this work is to model accurately volumetric growth, induced by fluid shear stress in the intima and local wall stress in arteries with patient-specific geometry and loads. The model is implemented in a nonlinear finite element setting which may be applied to realistic 3D geometries obtained from in vivo measurements. The capabilities of this method are demonstrated in several examples. Firstly a stenotic process on an idealised geometry induced by a non-uniform shear stress distribution is considered. Following the growth of a right coronary artery from an in vivo reconstructed geometry is presented. Finally, experimental measurements for growth under hypertension for rat carotid arteries are modelled.

Animals↗