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Biomedical subjects

R Beyar

Publications and source records attributed to R Beyar.

At least 91 records · Page 5Linked to original sources
PubMed1993

Intramyocardial fluid transport effects on coronary flow and left ventricular mechanics.

An integrated left ventricular (LV) model is used to solve, simultaneously and interactively, the LV mechanics, the coronary blood flow and capillary and interstitial fluid and mass transport, and to analyze the LV behavior under normal as well as pathological conditions. Accounting for the interstitial fluid mass balance in a LV flow-mechanical model allows to determine the LV wall volume in terms of the prevailing mechanical and flow conditions; it allows to uniquely define the flow-mechanical relationship and study pathologies, such as myocardial edema, which are directly related to changes in the myocardial fluid transport and content.

Biological Transport↗
PubMed1993

Effect of interconnecting collagen fibres on left ventricular function and intramyocardial compression.

OBJECTIVE: The aim was to study the effects of the collagen mesh that interconnects the myocardial fibres on left ventricular mechanics and intramyocardial pressure. METHODS: An earlier model which integrates a symmetrical left ventricular geometry and transmural muscle fibre structure with muscle fibre mechanics was expanded to include radial stiffness generated by dynamically stretched radial collagen fibres. The calculated end systolic pressure-volume relationship (ESPVR) was compared to left ventricular pressure and volume data from six open chest dogs, obtained over a wide load range. Midwall intramyocardial pressure measurements by flat intramyocardial transducer in six different dogs were also used. RESULTS: Consistent with the experiments, inclusion of radial stiffness yielded an ESPVR that was more curvilinear than the collagen-free model, and modified global left ventricular function in that the end systolic volume increased. A diastolic suction effect, manifested by a negative pressure with a steep diastolic pressure-volume relationship at low end systolic volumes, was predicted. The intramyocardial pressure was higher than the left ventricular pressure at the end of isovolumetric relaxation, when radial stretch is maximal and fibre stresses are relaxed. This is attributed to the radial fibre stress component. Intramyocardial pressure was only weakly dependent on left ventricular cavity pressure under wide load manipulations at constant contractility. The experiments also confirmed model predictions that (1) peak intramyocardial pressure is insensitive to load, (2) intramyocardial pressure is markedly higher than left ventricular pressure at the end of isovolumetric relaxation, and (3) intramyocardial pressure continues to rise during ejection towards a maximum value near end ejection. CONCLUSIONS: The transverse radial stiffness due to radial collagen interconnections between myocardial fibrils affects the global systolic left ventricular function, the diastolic suction effect, and the mechanism of systolic coronary compression.

Animals↗
PubMed1993

Small apex-to-base heterogeneity in radius-to-thickness ratio by three-dimensional magnetic resonance imaging.

Reported large base-to-apex differences in endocardial area ejection fraction may suggest large variability in myocardial function and load. To test ventricular load heterogeneities, we measured the ratio of radius of curvature to wall thickness (R/T), as a stress index reflecting myocardial load. End-diastolic (ED) and end-systolic (ES) magnetic resonance cross-sectional images were obtained in 15 open-chest dogs at 5 levels from base to apex, from which 4 three-dimensional thick disks were generated from adjacent image planes. The average R/T for each disk was calculated by planar and three-dimensional methods, using both midwall and endocardial radii of curvature. R/T was normalized to the apical value to quantify the relative changes in myocardial load. Normalized R/T using the midwall three-dimensional approach was 1.08, 1.11, 1.06, and 1.0 for ED (P = NS) and 1.25, 1.013, 1.08, and 1.0 for ES (P < 0.02), base to apex, respectively, while the other methods yielded higher values. Therefore, R/T calculated by the three-dimensional midwall approach shows only small apex-to base variations at ED (< 11%) and ES (< 25%), which is substantially less than the variability in area ejection fraction (102%). This suggests only small base-to-apex load heterogeneities, in spite of large changes in the area ejection fraction, an index reflecting specific ventricular geometry rather than local myocardial function.

Animals↗
PubMed1993

An optical device to measure the dynamics of apex rotation of the left ventricle.

Systolic counterclockwise rotation of the left ventricular apex with respect to the base has been defined as left ventricular (LV) twist or torsion. If rotation of the base during systole is small, we hypothesized that the dynamics of twist can be well characterized through the measurement of apical rotation alone. A device was designed to measure apical rotation in a simpler, more direct fashion, providing continuous high-fidelity dynamic measurements. The device consists of a light source, a position-sensitive diode, and a small rotating mirror that is coupled to the apex of the heart by a wire. As the wire rotates, apical rotation (measured in degrees) can be calculated from the position of the deflected light beam. The timing of apical rotation was compared with simultaneous recordings of electrocardiogram, LV pressure, and LV diameter measurements. An initial clockwise rotation (untwist) of 4 +/- 2 degrees (SD) occurred during isovolumic contraction followed by counterclockwise rotation (twisting) through ejection, reaching maximum apical rotation of -15 degrees just before the end of systole. Rapid untwisting during isovolumic relaxation was shown with near-complete dissipation of twist by the first one-third of the diastolic filling period. Caval occlusion caused a downward and leftward shift of the pressure-apical rotation loops, and more twist/untwist was seen to occur during the respective isovolumic contraction and relaxation periods. We conclude that this device provides precise timing and definition of rapid changes during isovolumic contraction and relaxation, confirms results obtained by more laborious methods, and provides an easy method to measure the dynamics of apical rotation continuously during interventions such as load changes.

Animals↗
PubMed1993

Ventricular interaction and septal deformation: a model compared with experimental data.

Diastolic ventricular interaction is associated with septal shift and deformation, the consequences of which have not been fully assessed. A model was therefore developed to describe the mechanisms involved in interaction between the ventricles under different loading conditions. We assumed a circular cardiac minor-axis geometry surrounded by a pericardial membrane with the left ventricle (LV) and septum described by three layers. To define the equilibrium condition, we required the net force-balance at the right ventricular (RV)-LV intersection points to equal zero. The model was tested with and without consideration of bending forces associated with a change of curvature of a thick-walled structure. Model results were compared with data from animal experiments subjected to aortic and pulmonary constriction. LV and RV end-diastolic pressures as well as pericardial pressure were measured. In six dogs, septal segment length was measured using sonomicrometry, and in seven dogs, endocardial curvature was measured using echocardiography. Model and experimental results show that 1) with severe RV loading, septal inversion occurs at a negative transseptal gradient, and 2) the end-diastolic septal segment length continues to shorten after septal inversion during pulmonary constriction. Model simulation suggests that bending moments account for the septal curvature at zero transseptal pressure. In addition, the model predicts the shift in the pressure-area relationship of each ventricle by a change in loading of the opposite ventricle and predicts that large transmural gradients in stress and strain are associated with septal inversion. Thus the model and the experimental data agree and describe the important factors that modulate diastolic septal mechanics during acute differential ventricular loading.

Animals↗
PubMed1993

Determinants of midwall circumferential segmental length of the canine ventricular septum at end diastole.

The ventricular septum attaches to the insertion points of the ventricular free walls, separates, and is shared by both the left (LV) and right (RV) ventricles. Changes in the transseptal pressure gradient (PTS, the difference between LV and RV pressures) will change the stress in the septum and, therefore, the length of the septal segment (Ls). However, since most of the septal myocardial fibers are continuous with those of LV free wall and the septum appears to be an integral part of LV, one might also expect that if the LV transmural pressure (Plvtm; the difference between LV and pericardial pressure) is raised, Ls would increase even in the absence of any change in PTS. Therefore, we hypothesized that at end diastole, Ls depends on both Plvtm and PTS. To test this hypothesis, we measured Ls (sonomicrometry), LV and RV pressures (micromanometers), and pericardial pressure (flat liquid-containing balloon) in seven anesthetized open-chest dogs. Plvtm was increased through volume loading, whereas PTS was maintained constant at 10, 5, 0, -5, -10, and -15 mmHg by adjusting the degree of constriction of the pulmonary artery or aorta. These procedures were performed first with pericardium reapproximated and then after the pericardium had been opened widely. At each controlled PTS level, Ls increased linearly with the increase in Plvtm. Both the slope and the intercept of this Ls-Plvtm relation were affected by PTS in a nonlinear fashion. (ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

A conical model to describe the nonuniformity of the left ventricular twisting motion.

The systolic contraction and fiber shortening in the left ventricle (LV) produces torsional moments in the myocardium, resulting in a gradient of angular displacements about the long axis. This is manifested as a counterclockwise rotation of the apex relative to the base, when viewed from the apex. Recent studies with magnetic resonance imaging (MRI), using noninvasive magnetic tags, have revealed three important properties of the LV twist: (a) The angle of twist (i.e., the angular rotation of a slice relative to the basal slice) is consistently higher at the endocardium as compared to the epicardium; (b) The twist increases towards the apex; and (c) Straight MRI-tagged radial lines at end-diastole (ED) are slightly curved at end-systole (ES), implying a nonlinear transmural variation of the twist. The present study suggests that the geometry of the LV at ES can be represented by a thick-walled hollow cone, and that the transmural twist patterns from ED to ES can be described using the continuum mechanics approach and a small strain analysis of an isotropic cone subjected to external torque. The predicted results are compared with the noninvasive MRI measurements of transmural twist in eight human volunteers. Given the epicardial angles of twist of each slice, the predicted endocardial angles of twist are in good correlation with the experimental findings (r = 0.86, slope = 1.09, SEE = 4.1 degrees). In addition, the model reliably describes the changes in the twist magnitude from apex to base (no significant difference from experimental values, P = 0.2), and predicts the curvilinear pattern at ES of the originally straight ED radial lines. Thus, the conical model with uniform properties of the LV, reliably predicts the nonuniformity of the twist patterns, implying that the LV twist is strongly affected by LV geometry.

Adult↗
PubMed1992

Myocardial contrast echocardiography: influence of ischaemia and hyperaemia in an animal model.

To evaluate changes in myocardial contrast echocardiography during ischaemia and hyperaemia, contrast studies were performed in 16 open chest dogs. Time-intensity curves were generated using videodensitometry after contrast injections to demonstrate ischaemic and non-ischaemic areas of interest during a wide range of coronary blood flow levels. For each time-intensity curve, the peak contrast intensity (PCI), washout halftime (T1/2) and area under the curve (AUC) were calculated. PCI and AUC decreased significantly only with severe ischaemia (90% or more reduction in flow), and increased significantly with hyperaemia of more than 2.5 times baseline flow. Both ischaemia and hyperaemia were found to prolong the T1/2. There was only a moderate linear correlation between the magnitude of hyperaemia and myocardial contrast echocardiographic parameters. There was significantly less increase in myocardial contrast echocardiographic parameters during hyperaemia in segments supplied by a stenosed coronary artery.

Albumins↗
PubMed1991

Vital organ perfusion during assisted circulation by manipulation of intrathoracic pressure.

BACKGROUND: We have previously shown, in dogs with severe cardiac depression, that modest cyclic increases in intrathoracic pressure, starting synchronously with left ventricular isovolumic contraction, significantly increase aortic flow and pressure. However, little is known of changes in vital organ perfusion during this technique of assisted circulation. METHODS AND RESULTS: We studied regional organ flow using radioactive labeled microspheres in 13 20-25-kg mongrel dogs. In the control group, after chemical induction of cardiac depression with verapamil and propranolol, coronary flow fell from 129.1 +/- 14.4 to 51.6 +/- 11.3 ml/100 g/min (p less than 0.005) and continued to decline over a 14-minute time period (flow was 32.2 +/- 11.5 ml/100 g/min at 7 minutes and 20.7 +/- 9.5 ml/100 g/min at 14 minutes [n = 6]; all p less than 0.05). In the intervention group, regional blood flow was evaluated before and after the induction of cardiac depression and also during assisted circulation using 400-msec, 20-25-mm Hg intrathoracic pressure increases delivered by a circumthoracic pneumatic vest, starting synchronously with left ventricular isovolumic contraction. In the intervention group, coronary flow fell from 119 +/- 26.7 to 47.9 +/- 13.1 ml/100 g/min 1 minute after the induction of cardiac depression (p less than 0.005). With the initiation of assisted circulation, coronary flow increased to 55.8 +/- 19.2 ml/100 g/min at 7 minutes and fell to 23.1 +/- 15.9 ml/100 g/min on termination of assisted circulation at 14 minutes (p less than 0.05 and p = NS versus control group flows at 1 and 14 minutes, respectively). During assisted circulation, cerebral, renal, and small intestinal flows also increased (all p less than 0.05 versus flows during myocardial depression). No significant increase in hepatic flow was observed. CONCLUSIONS: In the canine model, manipulation of intrathoracic pressure appears to be an effective, short-term, noninvasive means of not only increasing aortic pressure but also increasing vital organ perfusion during cardiogenic shock. Further studies are needed to assess the usefulness of this technique of assisted circulation in humans.

Animals↗
PubMed1991

Regional three-dimensional geometry and function of left ventricles with fibrous aneurysms. A cine-computed tomography study.

BACKGROUND: To assess the extent and nature of the dysfunction surrounding aneurysms of the left ventricle (LV), we examined the parameters of local and global three-dimensional shape, size, and function of LVs of eight patients with histologically confirmed anterior fibrous aneurysms. METHODS AND RESULTS: Three-dimensional reconstructions of each LV were made from 10-12 short-axis fast cine-angiographic computed tomography (cine-CT) slices encompassing the entire heart at end diastole and end systole. Regional three-dimensional wall thickness, thickening, motion, curvature, and stress index were calculated for 84 elements encompassing the entire LV. The aneurysmal border was defined by a sharp decrease in end-diastolic wall thickness and separated the LV into an aneurysmal zone and a normal zone that was further divided into adjacent normal (AN) and remote normal (RN) zones. As expected, thickening was negligible in both the aneurysmal and the border zones. Although both the AN and the RN zones had normal wall thickness (1.05 +/- 0.20 and 1.09 +/- 0.20 cm, respectively), thickening was depressed in the AN (0.22 +/- 0.08 cm) but not the RN (0.44 +/- 0.19 cm) zones. The size of the dysfunction zone (defined as less than 2 mm thickening) was found to be considerably greater than the anatomic size of the aneurysm (60.9 +/- 13.7% versus 33.6 +/- 7.6% of the left ventricular endocardial area, respectively; p less than 0.001). In addition, the AN zone had a smaller curvature and a higher stress index than the RN zone. CONCLUSIONS: LVs with fibrous aneurysms are characterized by a relatively large region of nonfunction that encompasses the thin aneurysmal area and its transitional border zone, a normally functioning remote zone, and an intermediate region of normal wall thickness but with reduced function, which may be attributed to its low curvature and high stress index.

Algorithms↗
PubMed1991

Evaluation of contractile state by maximal ventricular power divided by the square of end-diastolic volume.

BACKGROUND: Maximal ventricular power (PWRmax) reflects contractile state and has the potential to be noninvasively determined. However, its sensitivities to preload, afterload resistance, and inotropic state are incompletely defined. The present study determines these dependencies and proposes a novel power-based contractile index that is little altered by load. METHODS AND RESULTS: Seven open-chest, autonomically blocked dogs were instrumented with a proximal aortic flow probe, central aortic and ventricular micromanometers, and a conductance catheter for ventricular chamber volume. Preload was transiently reduced by left atrial hemorrhage, and afterload was increased by intra-aortic balloon inflation. Inotropic state was pharmacologically altered by lidocaine, dobutamine, propranolol, or verapamil. PWRmax was highly preload sensitive, altering 1.7 +/- 0.1-fold a given percent change in end-diastolic volume (EDV). This preload dependence was reduced by dividing PWRmax by EDV but was virtually eliminated when PWRmax was divided by EDV2. This latter index also displayed little change in response to as much as 60% increases in afterload resistance. PWRmax/EDV2 varied directly with inotropic state, correlating to both the slope (Ees) of the end-systolic pressure-volume relation (PWRmax x 1,000/EDV2 = 0.31 x Ees - 0.04, r = 0.82, p less than 0.001) and the slope (A) of the dP/dtmax-EDV relation (PWRmax x 1,000/EDV2 = 0.025 x A + 0.02, r = 0.86, p less than 0.001). PWRmax values determined from the product of ventricular pressure and flow versus central aortic pressure and flow were nearly identical over a broad loading range, indicating that PWRmax may be noninvasively assessed (i.e., without requiring left ventricular chamber pressure). CONCLUSIONS: PWRmax divided by EDV2 provides a measure of contractile function that is little influenced by loading conditions and has potential for noninvasive clinical use.

Animals↗
PubMed1990

The slope of the end-systolic pressure-volume relationship compared with the global end-systolic pressure-volume ratio in humans.

The slope of the end-systolic pressure-volume relationship (Emax), which is generated clinically by load manipulation, as well as the "absolute" peak systolic pressure end-systolic volume ratio (denoted as pressure-volume ratio), have been suggested as indices defining left ventricular function. This study represents an attempt to determine the relationship between these two indices by studying 20 patients (16 with coronary artery disease and 4 with normal coronary arteries) undergoing cardiac catheterization. Left ventriculography was performed three times in each patient: (1) in the control baseline state, (2) after rapid intravenous infusion of 250-300 cc of saline, and (3) after sublingual administration of 5 mg isosorbide dinitrate. Emax was approximated by linear regression using the peak left ventricular pressure (replacing end-systolic pressure) and the smallest left ventricular (end-systolic) volume for these three different loads. Acute ischemia with typical chest pain and ECG changes developed in 4 patients during saline loading. The pressure-volume ratio showed no change with load manipulation in patients who did not demonstrate ischemia. In the 4 patients who developed acute ischemia, the pressure-volume ratio dropped from 4.4 +/- 1.3 to 2.9 +/- 0.9 mmHg/ml (p less than 0.001). In all of the patients, the pressure-volume ratio, but not the Emax, correlated with the ejection fraction (r = 0.6; p less than 0.05). In addition, the Emax line demonstrated a markedly nonphysiological Vo. There was no correlation between Emax and pressure-volume ratio.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1990

Analysis and prediction of left ventricular performance under load changes during cardiac catheterization.

The applicability of a computer model, which relates the transmural mechanical distribution in the left ventricle (LV) to its global function at different loading conditions, was evaluated in patients with normal to near normal LV function undergoing cardiac catheterization. Left ventriculography and measurements of aortic and LV pressures were performed at baseline conditions and repeated following rapid volume expansion with intravenous infusion of 250 to 300 ml of physiologic saline and also after sublingual isosorbide-dinitrate (ISDN) administration. Twenty patients (18 men and 2 women, average age = 53 years) underwent coronary angiography and left ventriculography. Sixteen patients had coronary artery disease with one- to three-vessel involvement and 4 had normal coronary arteries. The measured input data into the model included the end-diastolic LV volume and wall thickness, aortic pressure, heart rate, and the peripheral resistance. The model parameters of myocardial contractility and arterial system capacitance for the control baseline conditions were estimated so that an accurate match was obtained between the predicted and the measured end-systolic (ES) volume and pressure. Using these parameters, model predictions for the two load perturbations were compared to the measurements. An excellent correlation was found between the predicted and measured LV ES volumes and peak-systolic pressures (PSP) (R2 greater than 0.994). In four patients, who developed ischemic symptoms during saline injection, the prediction of end-systole volumes were lower than the measured values, suggesting an actual reduction in contractility during acute ischemia. Therefore, the model is sensitive to contractility changes. The model predicts global LV performance, under different loading conditions, including stroke work, peak developed wall stress, velocity of fiber shortening, and myocardial oxygen consumption.

Cardiac Catheterization↗
PubMed1990

Force interval relationship (FIR) related to the global function of the left ventricle: a computer study.

A model which relates the left ventricular (LV) geometry, structure and sarcomere properties to its global function, recently proposed by the authors, is extended to account for contractility changes which are a function of the heart rate, prematurity of the beat and calcium transients within the cell. To characterise LV function and relate it to fibre function under varying rhythm conditions, a model of muscle force restitution, based on calcium kinetics, was used to calculate the maximum fibre stress at the optimum sarcomere length sigma o as the parameter which depends on the heart rate, the test pulse interval TPI, the action potential duration APD and the restitution time constant. The global LV force interval relationship FIR was then calculated, and by comparing the calculated FIR to the experimental measurement (in dogs) at the ventricular level, the constants of the restitution of force at the fibre level were derived. Based on these constants, the LV function under ejecting conditions at various rhythm disturbances was calculated and related to the local, distributed parameters. This approach provides a tool to describe ventricular function as well as transmural distribution of stress and sarcomere length at a wide variety of loading and rhythm conditions based on given 'muscle level' parameters.

Biomechanical Phenomena↗
PubMed1990

Mechanical pathophysiology of some heart diseases: a theoretical model study.

Sarcomere dynamics are related to the global left ventricular (LV) function in some representative pathological states, by using a theoretical model which combines sarcomere function, LV fibrous structure and geometry with the haemodynamic loading conditions. The analysis shows that pressure (concentric) hypertrophy due to hypertension or aortic stenosis is associated with an increase of the normal endocardial-to-epicardial gradient(s) of oxygen demand, which may be one of the causes for the development of endocardial fibrosis. The analysis also indicates that sarcomere shortening is relatively normal in compensated volume (eccentric) hypertrophy. Mitral stenosis demonstrates a case of decreased LV function, secondary to a chronic decrease in LV end diastolic volume, with sarcomeres that operate at their lowest length range. Conversely, the sarcomere function is depressed in cardiomyopathy; the heart's pumping function is maintained by appropriate adjustment mechanisms. However, the sarcomeres show minimal shortening and function at their highest length range with low (or zero) functional reserve. The study thus provides a quantitative tool that relates global LV function to local sarcomere dynamics in various pathological states.

Heart↗
PubMed1990

Effect of hyper- and hypovolaemia on regional myocardial oxygen consumption.

STUDY OBJECTIVE - The purpose of the investigation was to study the effect of preload on coronary blood flow and myocardial oxygen consumption in subendocardial and subepicardial regions of left ventricular free wall. DESIGN - Ventricular volume in anaesthetised open chest dogs was altered over a range of 20% to produce hypovolaemia (10-15% exsanguination) or hypervolaemia (colloid infusion), allowing measurements of regional blood flow and oxygen consumption with varying preloads. beta Adrenergic blockade was used to limit changes in inotropy, and heart rate was kept constant by pacing at 150 beats.min-1. SUBJECTS - 9 mongrel dogs of either sex weighing 24.9 (SEM 4.1) kg were studied. MEASUREMENTS and RESULTS - Left ventricular volume was calculated from ultrasonic measurements of long and short axis end diastolic diameters and wall thickness. Regional myocardial blood flow was estimated using radioactive microspheres, and oxygen consumption in each region was determined from microspectrophotometric measurements of oxygen saturations in small arteries and veins. Hypervolaemia increased subepicardial blood flow from 66.8(6.9) (normovolaemic) to 114.1(13.5) ml.min-1.100 g-1, and regional oxygen consumption from 4.08(0.57) to 6.44(1.08) ml.min-1.100 g-1. Values in the subendocardium were similar, except for oxygen consumption, which increased less than in the subepicardium. Left ventricular end diastolic volume, pressure, and output were each increased in hypervolaemia, but not dP/dt and systolic aortic pressure. Hypovolaemia reduced blood pressures without reducing end diastolic volume. CONCLUSIONS - Augmented flow work produced by increased preload (even in the absence of changes in pressure work) increases myocardial oxygen supply equally in the subepicardium and the subendocardium, while oxygen extraction and consumption are preferentially augmented in the subepicardium.

Animals↗