Biomedical subjects

T Arts

Publications and source records attributed to T Arts.

At least 73 records · Page 4Linked to original sources

PubMed1990

Noise reduction in estimating cardiac deformation from marker tracks.

Deformation of the cardiac wall is measured by using optical or radiopaque markers attached to the wall. When digitized images are used, the accuracy of the measurement of a marker position is limited by pixel resolution and the size of the marker. The spatial accuracy is improved by singular value decomposition (SVD) filtering. This filtering procedure is based on the assumption that displacements of markers are mutually related because they are embedded in a common continuum. In a computer stimulation with 48 markers in 51 video frames, the accuracy of the measurement of a marker position improved from 0.14 to 0.045 (SD) pixel. In an open-chest animal experiment, with markers on the surface of the heart, the deformation patterns were extracted more clearly using SVD filtering, while mutually related high-frequency components were not suppressed. In a 50-frame sequence of 256 X 256 video images of a 45 mm X 35 mm deforming surface with 50 markers of 8 pixels in diameter, the marker position resolution improves from 0.1 to 0.03 (SD) pixel (6 microns). Strain is determined with an accuracy of 0.002 over a distance of 30 pixels (6 mm).

Animals↗

PubMed1990

Redistribution of myocardial fiber strain and blood flow by asynchronous activation.

Hearts of 11 anesthetized open-chest dogs were paced from the right atrium (RA), right ventricular outflow tract (RVOT), and left ventricular apex (LVA). Maps of the sequence of electrical activation (192 electrodes), fiber strain (video technique), and blood flow (microsphere technique) in the epicardial layers were obtained from a 15- to 20-cm2 area of the anterior left ventricular wall. Electrical asynchrony in this area was 10 +/- 5 (RA), 52 +/- 12 (RVOT), and 30 +/- 16 ms (LVA, mean +/- SD, P less than 0.05 for RVOT and LVA compared with RA). Epicardial fiber strain during the ejection phase was uniformly distributed during RA pacing. However, during ventricular pacing it ranged from 13 +/- 33% (RVOT) and 23 +/- 29% (LVA) of the value during RA pacing in early-activated regions to 268 +/- 127% (RVOT) and 250 +/- 130% (LVA) of this value in late-activated regions. Epicardial blood flow ranged from 81 +/- 22% (RVOT) and 79 +/- 23% (LVA) in early-activated regions to 142 +/- 42% (RVOT) and 126 +/- 22% (LVA) in late activated regions. In all above values P less than 0.05 compared with RA. During RVOT pacing, gradients of epicardial electrical activation time, fiber strain, and blood flow pointed in the same direction. Compared with RVOT pacing, during LVA pacing all gradients were opposite in direction, and the gradients of electrical activation time and blood flow appeared to be smaller. These results indicate that timing of electrical activation is an important determinant for the distribution of fiber strain and blood flow in the left ventricular wall.

Animals↗

PubMed1990

Mean transit time for the assessment of myocardial perfusion by videodensitometry.

The intrinsic limitations of coronary arteriography to predict the physiological effects of coronary obstructions are well known. Therefore, more direct assessments of the functional significance of coronary stenoses are becoming increasingly important. Study of contrast passage by electrocardiogram-triggered digital radiography has been proposed as a way of assessing changes in myocardial perfusion. The main problems in this approach are the limited time for motionless image acquisition, the potential alteration of vascular volume between different states, and the changing flow pattern induced by contrast agents. This has led to empiric substitution of mean transit time (Tmn) by other time parameters and to representation of vascular volume by maximal contrast intensity (Dmax). To avoid these problems, intact dogs were studied during almost motionless image acquisition of 20-25 consecutive paced heart beats obtained with synchronous radiographic pulses. In this way, unequivocal and reproducible determination of Tmn was possible. Constant and maximal vascular volume was created by continuous infusion of dipyridamole, and it was proved that coronary flow in this model was not influenced by contrast injections. Flow in the circumflex artery was measured by a ring mounted and calibrated Doppler probe. In each dog, flow in the circumflex artery was varied by a balloon occluder in 12 small steps (range, 0-174 +/- 42 ml/min). Inverse appearance time (1/Tapp), Dmax, Dmax/Tapp, inverse time of maximal intensity (1/Tmax), and 1/Tmn were calculated and the relations of these parameters to measured flow were investigated. Tmn proved to be the most reliable parameter for this purpose (r = 0.97 +/- 0.02; mean +/- SD), followed by Tmax (r = 0.93 +/- 0.04). Dmax failed to represent vascular volume but, in fact, showed a moderate correlation with flow (r = 0.78 +/- 0.22), as did Tapp (r = 0.64 +/- 0.18, 0.75 +/- 0.27, and 0.59 +/- 0.26 for the three definitions of Tapp used in this study). Dmax/Tapp correlated better with flow than either component separately. Our results indicate that the mean transit time calculated by videodensitometry can be used to accurately assess changes in myocardial perfusion strictly according to the original principles of indicator dilution theory.

Animals↗

PubMed1990

Determination of tissue motion velocity by correlation interpolation of pulsed ultrasonic echo signals.

Correlation interpolation is introduced as a method to determine the displacement of moving biological tissue on the basis of a sequence of ultrasonic echo signals. The echo signal is sampled along the echo depth with approximately 4 samples per average high frequency period. Sampling in time occurs with the pulse repetition frequency. The necessary information is extracted from a crosscorrelation function between successive signals, which is modelled using four parameters. The parameters are estimated from five calculated correlation sums and the shift with maximum correlation is determined. In contrast to existing techniques, the performance of this method is determined mainly by the number of samples used, while the ratio of the number of samples in depth and time is irrelevant. Using 64 samples at a signal-to-noise power ratio of 10, the standard deviation of the error in the determination of the shift in depth is 0.08 sampling intervals. As in many other methods, the width of the aliasing interval equals the mean frequency period.

Echocardiography, Doppler↗

PubMed1989

Discrepancies between myocardial blood flow and fiber shortening in the ischemic border zone as assessed with video mapping of epicardial deformation.

Myocardial function around the border of ischemia was investigated in eight open-chest dogs using video mapping of epicardial deformation. With this method, 40-60 white markers attached to the left ventricular epicardium were traced in time automatically. Before and 5-10 min after coronary artery occlusion, blood flow and epicardial deformation were determined in 30-40 regions with a spatial resolution of about 5 mm. Epicardial deformation was expressed as subepicardial fiber shortening and surface area decrease during the ejection phase. The latter indicates local contribution to stroke volume. The absolute values of these variables were normalized relative to the central ischemic (= 0%) and remote non-ischemic area (= 100%). The 50% contour line of a variable was defined as its border. The average distance between the borders of perfusion and function was not significantly different from zero, due to considerable variation in this distance both within one heart (+/- 5.7 mm) and between mean distances for different hearts (+/- 4.4 mm). The width of the transition zone (distance between the 20% and 80% contour lines) of surface area decrease and subepicardial fiber shortening was significantly larger (20.5 and 15.0 mm, respectively) than those of transmural and subepicardial blood flow (8.5 and 9.5 mm, respectively). The present results demonstrate that in a 20-mm zone around the border of ischemia, major discrepancies are present between perfusion and deformation.

Animals↗

PubMed1989

Dynamics of left ventricular wall and mitral valve mechanics--a model study.

The relation between global left ventricular pumping characteristics and local cardiac muscle fiber mechanics is represented by a mathematical model of left ventricular mechanics in which the mitral valve papillary muscle system is incorporated. The wall of the left ventricle is simulated by a thick-walled cylinder. Transmural differences in fiber orientation are incorporated by changing the direction of material anisotropy across the wall. The cylinder is free to twist. The upper end of the cylinder is covered by a thin, flexible sheet, representing the base of the left ventricle. The mitral valve is incorporated in this sheet. The tips of the mitral leaflets are connected by chordae tendineae to the papillary muscles which are attached to the bottom of the cylinder. Canine cardiac cycles were simulated for various end-diastolic values of left ventricular volume (25-120 ml, control 60 ml), left atrial pressure (0-2.7 kPa, control 0.22 kPa) and aortic pressure (5-11 kPa, control 11 kPa). In this wide range of preload and afterload mechanical loading of the muscle fibers appeared to be distributed quite evenly (SD: +/- 5% of control value) over all muscular structures of the left ventricle, including the papillary muscles.

Animals↗

PubMed1988

Effects of pyruvate on post-ischemic myocardial recovery at various workloads.

In the present study the hemodynamic and metabolic effects of pyruvate (5 mM), added as cosubstrate to glucose (11 mM) perfused, transiently ischemic, isolated working rat hearts, were evaluated. During 2 h of normoxic perfusion pyruvate improved functional stability, prevented depletion of glycogen and triacylglycerol stores, and increased non-esterified fatty acid (NEFA) levels, even at relatively high workloads. The elevated NEFA levels are in line with the notion that pyruvate competes with endogenously produced fatty acids for oxidative energy production. After 45 min of global ischemia pyruvate was found (a) to affect markedly the relative contribution of ATP, ADP and AMP to the total adenine nucleotide content and (b) to stimulate the degradation of glycogen and to enhance the accumulation of lactate, suggesting enhanced anaerobic ATP production. After restoration of flow pyruvate reduced the incidence of fibrillation and markedly improved recovery of cardiac output at both normal and high workload. Pyruvate did neither attenuate the release of lactate dehydrogenase, a marker for cell death, nor improve the conservation of the total adenine nucleotide and ATP content of hearts reperfused for 30 min. The latter findings indicate that hemodynamic recovery during reperfusion in the presence of pyruvate is neither related to the absolute tissue content of ATP nor to a reduction of irreversible cell damage, and suggest that pyruvate exerts its advantageous hemodynamic effects rather by improving the condition of reversibly damaged cells during reperfusion.

Animals↗

PubMed1988

Deformation of the dog aortic valve ring during the cardiac cycle.

Changes in strain in the line of aortic valve leaflet attachment (aortic ring) were measured during the cardiac cycle by means of an inductive technique. To that purpose coils were sutured to each commissure and base point of the aortic ring, when the animals were on a cardiopulmonary bypass. After bypass and stabilization of the hemodynamic variables changes in the aortic strain were measured at aortic pressures ranging from 4 to 20 kPa. Aortic pressure at the level of the commissure points and left ventricular pressure were measured to assess transvalvular pressure. Commissure strain appeared to depend on aortic and transvalvular pressure throughout the cardiac cycle. At an aortic pressure of 10 kPa (75 mm Hg), the derivative of commissure strain to aortic pressure was found to be 1.9 x 10(-5) +/- 1.2 x 10(-5) Pa-1 (mean +/- SD). During the ventricular ejection phase commissure strain was 0.04 +/- 0.03 higher than during ventricular filling. Maximum variations in basal strain during the cardiac cycle ranged from 0.03 to 0.15. During the ejection phase the basal segments adjoining the myocardium shortened whereas the segment close to the non-contracting anterior mitral valve leaflet lengthened. Strain between a base and a commissure point of the aortic ring were synchronous with the cardiac cycle, but no specific pattern could be found.

Animals↗

PubMed1988

On-line measurement of aortic valve ring deformation during the cardiac cycle.

An electronic device is described for the measurement of relative changes in segment length within the aortic valve ring during the cardiac cycle. The technique is based on the principle of magnetic induction. A magnetic field generated in one coil induces a voltage in another coil. From the amplitude of this voltage the strain between both coils can be determined because the strength of the magnetic field decreases with distance. In vitro, over a range of 5-25 mm, strains less than or equal to 0.20 strain units can be measured with an accuracy of 0.008 strain units. The frequency response is 0-150 Hz (-3 dB). By varying the generator and receiver assignment at a frequency of 2 kHz and multiplexing the signals of six coils, six strains can be measured simultaneously. As an example, simultaneous recordings of commissure strains in the aortic valve and left ventricular and ascending aortic pressures, as obtained in open-chest dogs, are shown.

Animals↗

PubMed1988

Wall shear rate in arterioles in vivo: least estimates from platelet velocity profiles.

Velocity profiles, as determined in vivo in rabbit mesenteric arterioles with fluorescently labeled platelets as natural flow markers, were used to calculate least estimates of the actual wall shear rate in these microvessels (17-32 micron diam). The fit of the velocity data points described the profile as close to the wall as 0.5 micron. To satisfy the no-slip condition, a thin layer of fluid with a steep velocity gradient near the wall was assumed. Least estimates of wall shear rate, as calculated from the fitted platelet-velocity profiles and using the mean velocity gradient in this layer of fluid, ranged from 472 to 4,712 s-1 with a median value of 1,700 s-1. Red blood cell center-line velocities varied between 1.3 and 14.4 mm/s (median 3.4). The wall shear rates were at least 1.46-3.94 (median 2.12) times higher than expected on the basis of a parabolic velocity distribution but with the same volume flow in the vessel. Considerable spatial differences in wall shear rate might exist even within a short segment of a vessel.

Animals↗

PubMed1986

Methods to measure blood flow velocity of red blood cells in vivo at the microscopic level.

Several methods to measure red blood cell velocity in microvessels by electronic means are discussed. Signals are generated by the red blood cells present in the microscopic image of the microvessels. These signals can be converted to obtain an output signal proportional to the actual red blood cell velocity. The method of spatial filtering by interlacing gratings is discussed in terms of a filter with an input signal. Adaptation of optical factors that might improve the velocity measurement is obtained by a mathematical analysis. Different methods of correlation are presented. The temporal correlation (dual slit and video window) and spatial correlation methods are discussed in relation to factors influencing the quality of the correlogram, the peak of which is proportional to red blood cell velocity. The conversion of red blood cell velocity to volume flow is put in perspective.

Animals↗

PubMed1986

Mapping of epicardial deformation using a video processing technique.

A method has been developed to measure deformation of the canine epicardium during the cardiac cycle simultaneously in a number (eight) of small regions (1 X 1 cm2). Approximately 50 white markers (diameter 1.5 mm) are attached to the epicardium and their motion is recorded on tape by a video camera. Marker positions are detected by computer processing of the digitized images. In each region the three deformation parameters are calculated from the displacements of all markers in that region by means of a least-squares criterium. In the experimental situation in the center of the area of the epicardium analyzed the accuracy of measuring circumferential strain, base-to-apex strain and shear is +/- 0.005, +/- 0.005 and +/- 0.002 rad, respectively. The method has been applied in an experiment in which local ischemia of the left ventricular wall was induced by occluding the anterior descending branch of the left coronary artery. Healthy and ischemic regions could clearly be distinguished by the differences in deformation.

Animals↗

PubMed1986

Gradients in fiber shortening and metabolism across ischemic left ventricular wall.

Blood flow, metabolism, and fiber shortening in various layers of left ventricular wall were studied during the initial 5 min of ischemia. In open-chest dogs (n = 51) ischemia was induced by coronary arterial stenosis (median value of mean perfusion pressure distal to stenosis 3.3 kPa). Epicardial deformation measurements with an inductive technique allowed estimation of fiber shortening in inner (eendo,est) and outer layers (eepi) of left ventricular free wall during the ejection phase. The decrease of eendo,est occurred within a few seconds after onset of stenosis, whereas eepi started to decrease 30 s later. After 1 min, eendo,est diminished to zero concomitantly with a reduction of blood flow and creatine phosphate content in the inner layers by 68 and 46%, respectively. In contrast a 60% reduction of eepi was associated with a decrease in blood flow of only 32% and no significant decrease in creatine phosphate in the outer layers. H+ and inorganic phosphate were released simultaneously into the local venous blood starting within 1 min of ischemia. During the initial 5 min of ischemia the content of ATP and glycogen remained unchanged across the ischemic wall. Present results indicate that the decrease of fiber shortening in the inner layers is associated with severe metabolic dearrangements, as reflected by the depletion of creatine phosphate. They also indicate that, during coronary arterial stenosis, impaired fiber shortening in the outer layers may result from the impairment of mechanical function in the inner layers, rather than from metabolic disturbances in the outer layers themselves.

Animals↗

PubMed1986

Velocity profiles of blood platelets and red blood cells flowing in arterioles of the rabbit mesentery.

Velocity profiles were determined in rabbit mesenteric arterioles (diameter 17-32 micron). A good spatial resolution was obtained by using the blood platelets as small and natural markers of flow, providing for the first time in vivo detailed, quantitative information about the shape of the velocity profiles in microvessels. In some experiments red blood cell velocity profiles were recorded as well. Easy detection of the cells of interest could be achieved by labelling them selectively with a fluorescent dye and visualizing them by intravital fluorescence video microscopy, using flashed illumination. Pairs of flashes were given with a short, preset time interval between both flashes, yielding in one TV picture two images of the same cell displaced over a certain distance for the given time interval. Velocity and mean radial position of cells, flowing within an optical section around the median plane of the vessel, were determined. The shape of the velocity profiles of platelets and red blood cells was similar. The profiles were flattened as compared to a parabola, both in systole and diastole. Vessel diameter did not change measurably during the cardiac cycle. As an index of the degree of blunting of the profiles, the ratio of the maximal and mean velocity of the profile was used, which is 2 for a parabola and 1 for complete plug flow. The index ranged from 1.39 to 1.54 (median 1.50), and increased with vessel diameter. Calculations showed that the blunting of the profiles cannot be explained by an influence of the finite depth of the optical section.

Animals↗

PubMed1985

Dynamic capacitance of epicardial coronary arteries in vivo.

The dynamic capacitance of epicardial coronary arteries (i.d. greater than or equal to 0.4 mm) in vivo was assessed from the volume stiffness and volume of these arteries. The volume stiffness was derived from the pressure wave front velocity as determined in dogs by measuring the delay time between the pressure pulses recorded proximal and distal to a segment of the anterior descending branch of the left coronary artery. The pressure pulse was generated elsewhere in the arterial system during diastole. The volume of the epicardial coronary arteries was calculated from the lengths and diameters as measured in araldite casts, making corrections for in-vitro/in-vivo differences in dimensions. The dynamic capacitance of the right coronary artery, and the anterior descending and circumflex branches of the left coronary artery at an arterial pressure of 13.3 kPa and a frequency between 7 and 30 Hz was found to be 0.0024 +/- 0.0013, 0.0062 +/- 0.0028 and 0.0079 +/- 0.0035 mL/kPa (mean +/- SD), respectively. The total capacitance of the epicardial coronary arteries was calculated to be (0.007 mL/kPa)/100 g, which is small as compared to the total capacitance of the coronary vasculature, including the intramyocardial compartment, which is in the order of (0.5 mL/kPa)/100 g [1].

Animals↗

PubMed1985

Interaction between intramyocardial pressure (IMP) and myocardial circulation.

In our concept of the interaction between intramyocardial pressure (IMP) and myocardial perfusion, IMP is defined as the hydrostatic pressure in the soft tissue surrounding the myocardial fibers. In a mathematical model of the mechanics of the left ventricle the latter definition results in values for IMP equal to left ventricular pressure in the inner layers of the wall, and a continuous decrease across the wall to zero in the outer layers. Modulation of coronary artery flow during the cardiac cycle is predominantly due to compression of the coronary vasculature by the IMP during the systolic phase of the cardiac cycle, resulting in back-squeezing components of this flow. In a mathematical model of the dynamics of the coronary circulation, containing a large capacitance at the level of the coronary microvasculature, the modulations of coronary artery flow were found to be similar to those found in animal experiments in open-chest dogs.

Animals↗

PubMed1985

A comparison of ascorbate and glucose transport in the heart.

Multiple indicator-dilution experiments were done to compare the transcapillary exchange of tracer amounts of L-[14C]ascorbate and D-[3H]glucose (against an intravascular reference 131I-albumin) in Ringer-perfused (5 mM glucose) isolated rabbit hearts. The indicator-dilution curves for the two were virtually superimposed over the first 40-80 s. Estimates of the capillary permeability-surface area products, PSc, were the same, 2.3 +/- 0.7 (SD) ml X g-1 X min-1 (n = 18), in accord with the coincidence of their instantaneous extractions. The similarity of glucose and ascorbate permeabilities is explained by the similarity in molecular weights and passive diffusivity, their lipophobic nature, and the paucity of carrier-mediated endothelial transport for either molecule. The data were analyzed via a model composed of aggregates of spatially distributed capillary-tissue units (capillary blood, interstitium, myocytes) accounting for the heterogeneity of regional flows. The interstitial volumes in this preparation are enlarged, 0.30 +/- 0.04 ml/g. There is substantial entry into myocardial cells, the cell permeability-surface area products being approximately 2-3 ml X g-1 X min-1 for ascorbate and glucose. The estimated volumes of interstitial and intracellular space, 0.30 and 0.47 ml X g-1 X min-1, reflect interstitial edema and are very close to measured values, giving reassurance concerning the methods of modeling analysis.

Animals↗

PubMed1985

Polarographic measurement of ascorbate washout in isolated perfused rabbit hearts.

To study the myocardial washout of ascorbate, the applicability of polarographic detection of ascorbate ions by a platinum electrode (sensitive area 0.03 mm2) was investigated, in both a calibration setup (sampling flow along the electrode: 100 microliter X s-1) and isolated, retrogradely perfused rabbit hearts. In the calibration setup at pH 7.4, the sensitivity of the electrode was 70 microA/mol. This sensitivity increased moderately with increasing pH (13%/unit pH) and increasing sampling flow rate (14% at an increase from 100 to 150 microliter X s-1). In the isolated hearts, ascorbate infused into the aorta was detected in a right ventricular drain by the electrode as well as by the use of 14C-labeled ascorbate. Both recorded time courses were similar except for a scaling factor dependent on flow velocity. During continuous infusion the arteriovenous difference of ascorbate was 2 +/- 2% (SD), indicating a relatively low consumption of ascorbate by the isolated heart. We conclude that polarographic measurement of ascorbate in the coronary effluent of an isolated rabbit heart can be performed on-line and relatively easily.

Animals↗