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Biomedical subjects

F C Yin

Publications and source records attributed to F C Yin.

At least 73 records · Page 4Linked to original sources
PubMed1989

Dependence of left ventricular twist-radial shortening relations on cardiac cycle phase.

Cardiac models have proposed tight coupling between the systolic twisting motion of the left ventricle about its longitudinal axis and muscle shortening. Whether a similar relationship holds during diastole is unknown. The present study determined the dynamic twist-radial shortening relationship throughout the cardiac cycle in six in situ canine left ventricles. Radiopaque markers (15-26) were implanted throughout the myocardial midwall in six canine left ventricles. Three-dimensional marker location was determined by computer analysis of biplane cineradiograms (60 frames/s), and the results were transformed to cardiac cylindrical coordinates. Mean chamber twist was defined as the gradient along the long axis of circumferential rotation relative to end diastole. Changes in chamber dimension were indexed by average radial shortening, normalized to span from 0 at end diastole to 1.0 at end systole. During systole, ventricular twist and radial shortening were linearly related with an average slope of -0.058 radians (r = 0.99). However, during early diastolic relaxation there was substantial untwist (48 +/- 20% of total) despite only an approximately 15% increase in mean radial dimension resulting in a much steeper twist-percent shortening relationship (-0.24 radians, r = 0.96). During most of the remainder of diastolic filling, the twist-shortening relation was shallower (-0.02, r = 0.91) than the corresponding systolic relation (P less than 0.05). Thus the twist-radial shortening relation depends on the phase of the cardiac cycle. These data suggest that models of chamber mechanics that incorporate twisting motion need to account for the matrix surrounding the muscles in addition to the shortening and lengthening of the muscle fibers.

Animals↗
PubMed1989

Arterial hemodynamic indexes in Marfan's syndrome.

Aortic impedance, wave reflection magnitude, and compliance were measured in patients with Marfan's syndrome during diagnostic cardiac catheterization. Impedance and wave reflections were calculated from standard Fourier series analysis of ascending aorta micromanometer pressure and electromagnetic flow records. Compliance was estimated by a method recently proposed that uses the area under the pressure-time curve assuming a two-element Windkessel model of the circulation. Measurements were made in the baseline state, during vasodilatation with nitroprusside, after beta-adrenergic receptor blockade with intravenous propranolol, and during vasodilatation after beta-blockade. Marfan's syndrome produces alterations from normal in some hemodynamic variables: during baseline conditions, the magnitude of wave reflection was higher than in normal patients. This was normalized by vasodilatation and further increased by beta-blockade. Despite the greatly dilated aortic root, the aortic characteristic impedance was in the normal range, suggesting increased aortic wall stiffness. The baseline total arterial compliance was greatly increased with nitroprusside and was reduced by beta-blockade. beta-Blockade did not decrease the maximum acceleration of blood into the ascending aorta. These are the first detailed measurements of hemodynamic indexes in this disease. The results suggest that acute beta-blockade is not necessarily beneficial in reducing hemodynamic loading on an already dilated aortic root.

Adult↗
PubMed1989

Aortic compliance in human hypertension.

Aortic compliance in normotensive and hypertensive Chinese subjects undergoing diagnostic cardiac catheterization was compared by using a newly described method that allows for determination of the pressure dependence of compliance if one assumes a value for the exponential coefficient of the pressure-volume relation of the large arteries. Under baseline conditions in the normotensive and hypertensive groups at mean aortic pressures of 96.3 and 128.6 mm Hg, aortic compliance averaged 1.47 and 0.80 ml/mm Hg, respectively. Compliance in the hypertensive group at a diastolic pressure of 99.4 mm Hg (which was nearly equal to the mean normotensive pressure) was 1.072 ml/mm Hg--still significantly lower than in the normotensive group. During nitroprusside infusion, however, the compliances in the hypertensive group increased to levels equal to or greater those in the normotensive group. Thus, these data confirm that aortic compliance is lower in hypertensive than in normotensive humans. They further demonstrate that the lower compliance cannot be attributed entirely to the elevated blood pressure, suggesting that excess smooth muscle tone may be partly responsible.

Adult↗
PubMed1989

Aortic impedance and hydraulic power in the chick embryo from stages 18 to 29.

Little is known about the hemodynamic properties of the rapidly expanding arterial bed during embryonic development. Using a servo-null pressure system and 20-MHz pulsed Doppler velocity meter, we recorded simultaneous dorsal aortic pressure and velocity waveforms. The waveforms were digitized at 3-msec intervals and subjected to Fourier analysis. We calculated hydraulic energy and the impedance spectrum to 10 Hz. From stages 18 to 29, heart rate (148 +/- 3 to 193 +/- 9 beats/min), systolic pressure (1.14 +/- 0.12 to 3.04 +/- 0.10 mm Hg), and mean dorsal aortic blood flow (21 +/- 2 to 214 +/- 19 mm3/min) increased. Peripheral vascular resistance (Z0: 30.4 +/- 4.8 to 6.4 +/- 0.7 dyne x sec/mm5), and the impedance moduli (Z1: 6.5 +/- 1.0 to 1.7 +/- 0.2 dyne x sec/mm5; Z2: 6.1 +/- 1.2 to 1.7 +/- 0.1 dyne x sec/mm5; Z3: 7.3 +/- 1.1 to 1.7 +/- 0.2 dyne x sec/mm5) decreased. Total hydraulic power increased from 48 +/- 7 to 2,606 +/- 96 nW, while the proportion of oscillatory energy increased from 29 +/- 2% to 65 +/- 4%. With development hydraulic load decreases, total external work increases and the dorsal aorta and embryonic vascular bed becomes more compliant. A greater proportion of total energy is expanded in pulsatile blood flow, suggesting that ventricular-arterial coupling is less efficient later in development.

Animals↗
PubMed1989

Constitutive relations and finite deformations of passive cardiac tissue II: stress analysis in the left ventricle.

We present a new approach for estimation of transmural distributions of stress and strain in the equatorial region of a passive left ventricle. We employ a thick-walled cylindrical geometry, assume that myocardium is incompressible, and use a three-dimensional constitutive relation that yields a material symmetry consistent with observed transmural variations in muscle fiber orientations. Moreover, we consider finite deformations including inflation, extension, twist, and transmural shearing and suggest a new method for determination of the requisite deformation parameters directly from experimental strain data. We show representative transmural distributions of stress and strain, and perform a parametric study to illustrate differing predictions of stress induced by varying boundary conditions, muscle fiber orientations, or modes of deformation. Our analysis can be used to guide and check future predictions of cardiac stresses, and to guide experimentalists by suggesting the accuracy of measurements essential for stress analysis in the heart.

Animals↗
PubMed1989

A theoretically-based experimental approach for identifying vascular constitutive relations.

We employ a structurally-motivated phenomenological formulation to identify biomechanical experiments which can be used to determine a vascular constitutive relation directly from data. Large deformations, nonlinear material behavior, load-dependent anisotropy, material heterogeneity and incompressibility are accounted for in the analysis. For purposes of illustration, we outline a procedure for studying elastic arteries wherein the behavior of the media and adventitia is considered separately. This general approach for identifying vascular constitutive relations can be applied to any vessel or airway, however, and should provide certain advantages over previous microstructural or purely phenomenological formulations.

Arteries↗
PubMed1988

Aging and aortic impedance in beagle dogs.

Previous studies of the effect of aging on aortic impedance have proved inconclusive. Studies in man could be affected by clinically inapparent atherosclerotic effects which cannot be separated from aging effects. Therefore, to more directly examine the role of aging, hemodynamic studies were carried out in awake, resting young and senescent non-atherosclerotic beagles before and after the administration of various vasoactive agents. On separate days graded infusions of a rapid acting beta-adrenergic agonist and antagonist (isoproterenol and propranolol), and alpha-adrenergic agonist and antagonist (phenylephrine and phenoxybenzamine), combined alpha- and beta-adrenergic antagonists (propranolol plus phenoxybenzamine), and a non-specific vasodilator (nitroprusside) were administered. Aortic impedance was calculated from high-fidelity pressure and flow signals using standard Fourier analysis methods. Under baseline conditions no age-associated differences in any of the hemodynamic parameters were determined to be likely. The only significant age-associated difference in response was to graded doses of isoproterenol wherein there was a larger increase in flow and a greater decrease in resistance in the young compared to the senescent dogs. There were no age-associated differences in response to any of the other drugs. Thus, these data are further evidence in an intact circulation for an age-associated decrease in another beta-adrenergically mediated response.

Aging↗
PubMed1988

Structural three-dimensional constitutive law for the passive myocardium.

A three-dimensional constitutive law is proposed for the myocardium. Its formulation is based on a structural approach in which the total strain energy of the tissue is the sum of the strain energies of its constituents: the muscle fibers, the collagen fibers and the fluid matrix which embeds them. The ensuing material law expresses the specific structural and mechanical properties of the tissue, namely, the spatial orientation of the comprising fibers, their waviness in the unstressed state and their stress-strain behavior when stretched. Having assumed specific functional forms for the distribution of the fibers spatial orientation and waviness, the results of biaxial mechanical tests serve for the estimation of the material constants appearing in the constitutive equations. A very good fit is obtained between the measured and the calculated stresses, indicating the suitability of the proposed model for describing the mechanical behavior of the passive myocardium. Moreover, the results provide general conclusions concerning the structural basis for the tissue overall mechanical properties, the main of which is that the collagen matrix, though comprising a relatively small fraction of the whole tissue volume, is the dominant component accounting for its stiffness.

Animals↗
PubMed1988

Effects of left ventricular loading by negative intrathoracic pressure in dogs.

There are many factors, both intrinsic and extrinsic to the left ventricle, that can affect its function when negative intrathoracic pressure is imposed. In this study, we examined whether the left ventricular response to the afterload imposed by negative intrathoracic pressure was similar to that imposed by partial aortic constriction. We used steady-state right heart bypass to control pulmonary venous return to the left ventricle and reflex blockade to maintain constant heart rate and contractility. To impose negative intrathoracic pressure we used a pressure chamber fitted over a midsternal thoracotomy, which allowed steady negative pressure to be applied to all intrathoracic surfaces. Left ventricular volumes were measured from biplane cineradiograms of multiple 1-mm markers implanted in the left ventricular midwall. With cardiac output and heart rate constant, we compared the left ventricular response to two different levels of negative intrathoracic pressure and to increasing aortic pressure by partial aortic constriction. In each case, negative intrathoracic pressure produced a rise in the left ventricular end-systolic and end-diastolic volumes as well as transmural pressures similar to the effects of partial aortic occlusion. Thus, when cardiac output, heart rate, and contractility are maintained constant and all external restraints on the left ventricle are removed, the left ventricle responds in a similar manner to an increase in hydraulic load whether produced by a decrease in intrathoracic pressure or by partial aortic occlusion.

Animals↗
PubMed1987

Augmentation of pressure in a vessel indenting the surface of the lung.

The lungs and intrathoracic cardiovascular structures compete for space within the thorax, interacting through their adjacent surfaces via the pleural space. Theoretical analysis and in vitro model studies (detailed here) established that when a vessel indents the lung surface, the increase in intravascular pressure with positive pressure lung inflation can be greater than the change in the pleural surface pressure measured outside of the interaction area. We define this phenomenon as intravessel pressure augmentation. We determined the average intravessel pressure gain as the slope of the linear regression of the pressure in a vascular structure or balloon indenting the lung on the pleural surface pressure measured by a flat disk-shaped device (disk). The analysis showed that the disk pressure overestimates the pleural pressure. Therefore, the derived pressure gain underestimated the pressure augmentation. In five dogs, the disk and a 2-ml balloon were placed in the lateral pleural space, and a segment of IVC was ligated at both ends and filled with saline. The dogs were ventilated with fixed tidal volumes, while the positive end-expiratory pressure was changed. The pressures were compared at the end of expiration. For the IVC segment, the pressure gains under four different tidal volumes were significantly greater than one [95% confidence interval of mean value (CIM) = 1.57 +/- 0.16, P less than 10(-4)], and for the small balloon, this was the case for three of four tidal volumes (95% CIM for all four volumes 1.13 +/- 0.04, P less than 10(-4)). We conclude that the surface interaction of the lungs with adjacent cardiovascular structures causes appreciable pressure augmentation in those structures during ventilation with the positive end-expiratory pressure.

Animals↗
PubMed1987

Use of negative intrathoracic pressure to obtain end-systolic pressure volume relations in dogs.

Left ventricular contractility can be assessed from the end-systolic pressure-volume relationship (ESPVR). In this study we test the hypothesis that the same ESPVR can be obtained by varying LV loading with different levels of negative intrathoracic pressure as by varying LV filling. In six dogs mean aortic transmural pressure was used to approximate LV end-systolic pressure and LV volume was determined from data gathered from biplane cineradiograms of multiple markers placed in the LV midwall. In each preparation right heart bypass allowed control of cardiac output while the thoracic pressure was varied with a box surrounding a midsternal thoracotomy. Reflex effects were minimized by ganglionic blockade and bilateral vagotomy. ESPVRs were obtained by varying the cardiac output at constant thoracic pressure or by changing intrathoracic pressure at constant cardiac output. The slopes of the ESPVRs were not significantly different. This result implies that LV loading by negative intrathoracic pressure, in this highly controlled preparation, can be used to generate a systolic LV elastance similar to that obtained by varying LV filling.

Animals↗
PubMed1987

Quantification of the mechanical properties of noncontracting canine myocardium under simultaneous biaxial loading.

Detailed understanding of cardiac mechanics depends upon accurate and complete characterization of the three-dimensional properties of both normal and diseased myocardial tissue. This, however, can only be obtained by performing multiaxial tests on cardiac tissue. In this study we subjected thin sheets of passive canine left ventricular myocardium to various combinations of simultaneous biaxial stretching. During each stretch the ratio of the orthogonal strains was kept constant and the corresponding stresses remained proportional. We fitted the biaxial stress-strain data both with exponential strain-energy functions with quadratic powers of strains as well as with an alternative function with nonintegral powers of strains. We used our recently developed nonparametric method to assess the reliability of the coefficients for each of these functions. The quadratic strain-energy functions resulted in wide intra- and interspecimen variability in the coefficients. Moreover, both their absolute and relative values demonstrated marked load history dependence such that interpretation of the direction of anisotropy was difficult. Fitting the data with the alternative nonintegral strain-energy function seemed to alleviate these problems. This alternative strain-energy function may provide more self-consistent results than the more commonly used quadratic strain-energy functions.

Animals↗
PubMed1987

A new constitutive formulation for characterizing the mechanical behavior of soft tissues.

We present a new constitutive formulation that combines certain desirable features of two previously used approaches (phenomenological and microstructural). Specifically, we assume that certain soft tissues can be idealized as composed of various families of noninteracting fibers and a homogeneous matrix. Both the fibers and the matrix are assumed to follow the gross deformation. Within the usual framework of pseudoelasticity, incompressibility, homogeneity, and the continuum hypothesis, a pseudostrain-energy function (W) is proposed wherein W is expressed in terms of matrix and fibrous contributions. Unlike phenomenological approaches where a W is usually chosen in an ad hoc manner, the present approach can be used to postulate reasonable forms of W based on limited structural information and multiaxial stress-strain data. Illustrative applications of the theory are discussed for visceral pleura and myocardium. Concise structurally motivated constitutive relations result, wherein load-dependent anisotropy, nonlinear material behavior, finite deformations, and incompressibility are accounted for.

Animals↗
PubMed1987

On constitutive relations and finite deformations of passive cardiac tissue: I. A pseudostrain-energy function.

A three-dimensional constitutive relation for passive cardiac tissue is formulated in terms of a structurally motivated pseudostrain-energy function, W, while the mathematical simplicity of phenomenological approaches is preserved. A specific functional form of W is proposed on the basis of limited structural information and multiaxial experimental data. The material parameters are determined in a least-squared sense from both uniaxial and biaxial data. Our results suggest that (1) multiaxially-loaded cardiac tissue is nearly transversely-isotropic with respect to local muscle fiber directions, at least for a limited range of strain histories, (2) material parameters determined from uniaxial papillary muscle data result in gross underestimates of the stresses in multiaxially-loaded specimens, and (3) material parameters determined from equibiaxial tests predict the behavior of the tissue under various nonequibiaxial stretching protocols reasonably well.

Animals↗
PubMed1987

Interaction between cardiac chambers and thoracic pressure in intact circulation.

A comprehensive model that describes the interaction between the cardiovascular system (CVS) and the intrathoracic pressure (ITP) based on a lumped parameter vascular representation and a time-varying elastance concept for the four cardiac chambers is presented. Special attention is given to two possible mechanisms of interventricular interaction; the constraining effects of the pericardium and direct interventricular interaction that results from the fact that the two ventricles share a common interventricular septum. The response of the CVS to positive and negative perturbations in the ITP and to injection of fluid into the pericardium was simulated and compared with experimental literature data. The results show that 1) the total heart volume is relatively constant throughout the cycle both for ITP of 0 and +15 mmHg, which is consistent with experimental data in dogs, thus suggesting that intrinsic properties of the cardiac chambers rather than a restricting pericardium is the mechanism for that observation. 2) The pericardium has a major role in modifying the transient and steady-state response to a step decrease in the ITP with a transient decrease in left ventricle (LV) end-diastolic volume followed by gradual increase afterwards. 3) The response to sudden injection of fluid into the pericardial space is a larger transient decrease in right ventricle than LV volume, which is consistent with experimental data. 4) Transmission across the septum has a relatively minor role in modifying the response of the CVS to negative pressure. Thus the model reasonably predicts the effects of intrathoracic and pericardial pressures on the circulation in a reflex-blocked animal and provides a means for placing multiple potential mechanisms in proper hierarchial order with regard to contributions to LV and overall CVS function.

Cardiovascular Physiological Phenomena↗
PubMed1987

Normalization of hemodynamic parameters: application to vascular resistance and impedance.

Comparison of hemodynamic parameters among different species is complicated by the wide variety of body sizes and shapes. Although some parameters such as blood pressure and flow velocity seem to have similar absolute values among species, most other parameters such as cardiac output, pulse wave velocity, and heart rate vary greatly. Comparisons would be facilitated if one uses indices that are independent of body size and/or shape. An ideal index will bring values for individuals or species of different sizes into the same range and yet exhibit large relative changes to responses to small increments of an intervention in an individual. In this paper we demonstrate how dimensional analysis together with Buckingham's pi theorem and the assumption of biological similarity can be used to derive dimensionless hemodynamic normalization indices. Using data from the literature and from our own laboratory, we illustrate the approach by examining resistance and impedance. We compare our results with previously proposed methods of accounting for different body sizes, namely, dividing impedance by resistance or vice versa, multiplying by body weight, and using flow velocity rather than volume flow in the calculations. Both of our proposed indices bring the values for arterial and pulmonary resistance and impedance for rats, rabbits, dogs, and humans into the same range, although the variability among species using our indices is twice that of those previously proposed. On the other hand, the relative changes of our two indices induced by vasoactive agents or exercise in a dog are from two to five times greater than either the absolute values themselves or the previously used indices.

Animals↗
PubMed1987

Transverse stiffness: a method for estimation of myocardial wall stress.

Determination of regional ventricular wall stress would allow quantification of both regional contractile state and its interplay with global function. Current methods for quantifying regional stress include mathematical modelling and measurements with strain gauges. Both methods are difficult to validate. We hypothesized that transverse stiffness (i.e., the ratio of indentation stress to strain as the ventricular wall is indented in the direction perpendicular to the wall) would be proportional to the stresses in the plane of the wall and could be used to estimate the latter. To test this hypothesis, 6 arterially perfused canine ventricular septa were mounted in an apparatus that could exert biaxial load in the plane of the wall. A servo system maintained the central third of the septa isometric during active contractions while the septa were paced at 30-60 pulses/min. In the center of the isometric region, a probe of 7 mm diameter indented the septa while the transverse indentation stress and strain were measured. For values of peak systolic in-plane stress from 0.56 to 2.6 g/mm2, the transverse stiffness varied from 1.2 to 11.7 g/mm2 and was linearly related to the in-plane wall stress in each septum (p less than 0.001, ANOVA). After cardioplegia, the transverse stiffness also correlated with passively applied wall stress for each dog (p less than 0.001). The slopes of the individual relations between transverse stiffness and wall stress from active contractions were similar to those from passively applied stress (mean +/- SEM; 1.82 +/- 0.36 versus 1.45 +/- 0.31, NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗