Biomedical subjects

F C Yin

Publications and source records attributed to F C Yin.

At least 19 recordsLinked to original sources

PubMed1992

Feasibility of 19F imaging of perfluorochemical emulsions to measure myocardial vascular volume.

19F magnetic resonance images were obtained of the ventricular walls of isolated rabbit hearts perfused with a perfluorochemical (PFC) emulsion. Since the PFC is known to stay within the vascular space in normal myocardial tissue, the 19F signal should reflect myocardial vascular volume. 19F MRI of PFC emulsions represents a new investigational tool for the study of coronary vascular volume.

Animals↗

PubMed1992

Modeling the circulation with three-terminal electrical networks containing special nonlinear capacitors.

Development, first of analog and later of digital computers, as well as algorithms for analysis of electrical circuits, stimulated the use of electrical circuits for modeling the circulation. The networks used as building blocks for electrical models can provide accurate representation of the hydrodynamic equations relating the inflow and outflow of individual segments of the circulation. These networks, however, can contain connections in which voltages and currents have no analogues in the circulation. Problems arise because (a) electrical current must flow in closed loops, whereas no such constraints exist for hydraulic models; and (b) electrical capacitors have a number of characteristics that are not analogous to those of hydraulic compliant chambers. Disregarding these differences can lead to erroneous results and misinterpretation of phenomena. To ensure against these errors, we introduce an imaginary electrical element, the nonlinear residual-charge capacitor (NRCC), with characteristics equivalent to those of a compliant chamber. If one uses appropriate circuit connections and incorporates the residual-charge capacitor, then all voltages and currents in the model are proper analogues of pressures and flows in the circulation. It is shown that the capacitive current represents the rate of change of volume of blood inside the vessel, as well as the rate of the corresponding displacement of volume of the surrounding tissue.

Blood Circulation↗

PubMed1992

A constitutive theory for biomembranes: application to epicardial mechanics.

We present a new theoretically motivated experimental approach for identifying the functional form of a constitutive relation for any nonlinear, anisotropic pseudoelastic biological membrane. The utility of this approach is illustrated by identifying, from biaxial data, a new constitutive relation for excised ventricular epicardium. Values of the associated material parameters are calculated and compared for right and left ventricular specimens. Based on our findings, we suggest that there are no significant differences in the biomechanical behavior of epicardium excised from the right and left ventricular free walls of canine hearts.

Animals↗

PubMed1992

In-plane myocardial wall stress is not the primary determinant of coronary systolic flow impediment. A study in the isolated, perfused dog septum.

The hypothesis that ventricular in-plane tensile wall stresses are the major determinant of systolic coronary flow was investigated in this study. We measured coronary artery inflow in the maximally vasodilated bed of the isolated beating septum (n = 10) during two modes of contraction characterized by markedly different levels of developed in-plane stress. An increase in contractility was induced by changing from the control steady-state pacing state to a postextrasystolic potentiated state induced by a modified rapid pacing protocol. Over a range of increments of passive stretch, the systolic flow impediment versus the diastolic wall strain was described by an inverse linear relation. Despite the differences in developed in-plane wall stresses between the two modes of contraction (p less than 0.001), the slope and intercept of these relations in both the control and potentiated states were not different for the low versus high developed stress modes. The systolic flow impediment versus diastolic wall strain relation for the potentiated beats, compared with the control beats, was characterized by an increase in the intercept in both the low developed stress beats (p less than 0.05) and the high developed stress beats (p less than 0.05). These data indicate that the impediment to coronary flow during systole is not primarily determined by systolic myocardial in-plane tensile wall stresses but rather by the contractile state of the muscle.

Animals↗

PubMed1992

Compliance changes in physiological and pathological states.

AIM: Although arterial compliance has been estimated by a variety of methods none of them can be directly validated because it is difficult to measure arterial volume. Moreover, because arterial pressure-volume relationships are non-linear, compliance is pressure-dependent. We have developed a method of estimating arterial compliance based on the Windkessel model of the arterial system that can account for the pressure-dependence of compliance. RESULTS: Compared to normotensive humans, compliance was decreased in hypertension and normalized with alpha-blockade, angiotensin converting enzyme inhibition and vasodilators, but not with beta-blockade. Compliance changes with aging, exercise and some diseases were determined. CONCLUSION: The decreased compliance seen in hypertension was due to an intrinsic change in the arterial wall, most likely due to increased smooth muscle tone.

Adrenergic alpha-Antagonists↗

PubMed1991

Servo-controlled indenter for determining the transverse stiffness of ventricular muscle.

Regional ventricular wall stress is a critical determinant of cardiac function. There are, however, no validated methods for accurately estimating this stress. We have shown in the isolated ventricular septum that, during steady-state indentations, the transverse stiffness (the ratio of indentation stress [pressure acting on indenter face] to indentation strain [amount of indentation/nonindented thickness]) can be used as an estimate of the in-plane wall stress. Because of the long acquisition time for those transverse stiffness determinations, it was not possible to follow changes in wall stress over a single contraction. We recently developed a dynamic indentation system that can determine transverse stiffness in as little as 10 ms, allowing estimation of wall stress over a single contraction cycle. The apparatus consists of an indentation probe coupled to a linear motor. This indentation system was tested on two beating canine ventricular septa that were mounted in a biaxial system the could apply strains in the plane of the septa and measure the resulting in-plane stresses. The probe indented the septa with peak displacements of 0.1-0.5 mm at frequencies of 20 and 50 Hz. The transverse stiffness was calculated as the slope of the relation between the indentation stress and indentation strain during each high-frequency indentation. Consistent with earlier studies, the transverse stiffness was related to the inplane stress. In contrast to earlier studies, however, these dynamic transverse stiffness determinations could be made during a single contraction. Thus, dynamic transverse stiffness determinations allow estimation of wall stress in the isolated septa by minimal surface contact, and may lead to methods for estimating wall stress in the intact heart.

Animals↗

PubMed1991

Arterial hemodynamics in human hypertension. Effects of adrenergic blockade.

BACKGROUND: Resistance, pulse wave velocity, and wave reflections have been shown to be increased in patients with essential hypertension compared with normotensive controls. These alterations are completely normalized by nitroprusside infusion but exacerbated during beta-adrenergic blockade, suggesting an enhanced smooth muscle tone that is in part modulated by adrenergically mediated vasodilation. The present study was performed to examine the extent to which this apparently enhanced smooth muscle tone is a result of alpha-adrenergically mediated vasoconstriction. METHODS AND RESULTS: Age-matched normotensive and hypertensive Chinese subjects were instrumented with catheter-tipped micromanometers and an electromagnetic flow velocity sensor positioned in the ascending aorta. Aortic impedance and wave reflection properties were obtained from Fourier analysis of the pressure and flow signals during baseline conditions, after beta-blockade with propranolol (0.15 mg/kg i.v.), and after alpha-blockade with intravenous phentolamine (range, 15-80 mg) that was sufficient to either normalize blood pressure or produce a pressure that could not be further lowered. Compared with normotensives, in the baseline state, hypertensives had elevated resistance (1,962 versus 1,268 dyne.sec/cm5, p less than 0.001), total power (1,893 versus 1,568 mW, p less than 0.08), reflected pressure wave component (25.6 versus 13.5 mm Hg, p less than 0.001), ratio of reflected to forward wave (0.65 versus 0.42, p less than 0.001), and pulse wave velocity as determined from the frequency of the first zero-crossing of impedance phase angle (4.6 versus 3.5 Hz, p less than 0.03). During combined alpha- and beta-adrenergic blockade, blood pressure decreased into the normal range (from 162/103 to 131/87 mm Hg) but was still somewhat higher than that in the normotensive subjects. Resistance (1,914 dyne.sec/cm5, p less than 0.03), reflected wave (19.5 mm Hg, p less than 0.01), and ratio of reflected to forward wave (0.61, p less than 0.001) were, however, persistently elevated above normal values. CONCLUSIONS: alpha-Adrenergically mediated vasoconstriction cannot account for all of the hemodynamic alterations seen in essential hypertension.

Adult↗

PubMed1990

Regional pulse wave velocities in hypertensive and normotensive humans.

STUDY OBJECTIVE: The aim was to compare regional wave transmission and reflection properties along the aorta in age matched normotensive and hypertensive subjects. DESIGN: Simultaneous upstream and downstream micromanometer pressures were recorded at five regions from the ascending aorta to the iliac artery. Recordings were made in the baseline condition in both groups and during sustained isometric handgrip exercise in the normotensives to see if increasing the blood pressure to hypertensive levels would eliminate baseline differences between the two groups. SUBJECTS: Only subjects who had no coronary or valvular heart disease were studied. Normotensive subjects (n = 8) were selected from those undergoing electrophysiological testing whose blood pressures were consistently in the normal range (systolic less than 140, diastolic less than 90 mm Hg). Age matched hypertensive subjects (n = 17) were those in whom secondary causes of hypertension were excluded and who had repeated measurements of raised blood pressure. MEASUREMENTS AND RESULTS: In each region pulse wave velocity was estimated from the simultaneous upstream and downstream pressure records using the foot to foot method, and apparent phase velocity was obtained from Fourier analysis of the two pressures. The intensity of wave reflections was judged by the magnitude of fluctuations of the harmonics of apparent phase velocity about the mean of the higher frequency harmonics. In both groups in the baseline state there were regional variations in the pulse wave velocity with the lowest values occurring just proximal to the renal arteries and highest values occurring in the iliac artery. Likewise, in both groups the extent of wave reflections varied regionally--pronounced reflections were seen in the ascending aorta and from just proximal to the renal arteries to the aortic bifurcation but not in the mid-thoracic or iliac regions. The hypertensives had higher pulse wave velocity than normotensives only in the iliac artery (780 v 567 cm.s-1) and had more reflections in the three regions in which reflections were evident. Although handgrip in the normotensive group eliminated regional differences in pulse wave velocity between the groups, reflections were still greater in the hypertensives. CONCLUSIONS: Essential hypertension affects the regional properties of the aortic wall. These alterations are manifested by increased peripheral wave-speed and increased wave reflections along the aorta. The differences in wavespeed but not reflection properties are eliminated when the pressures are matched by handgrip, suggesting that factors other than the level of blood pressure per se are responsible for the alterations in reflection properties.

Adult↗

PubMed1990

Determination of a constitutive relation for passive myocardium: I. A new functional form.

The specific aim of this study is to determine a constitutive relation for non-contracting myocardium in terms of a pseudostrain-energy function W whose form is guided by both theory and experiment. We assume that the material symmetry of myocardium is initially and locally transversely-isotropic, and seek a W which depends upon only two coordinate invariant measures of the finite deformation. The specific functional form of such a W is inferred directly from experimental protocols in which one invariant is held constant while the other is varied, and vice versa. On the basis of data from families of these "constant invariant" tests on thin slabs of myocardium taken from the mid-walls of six canine left ventricles, we propose a new polynomial form of W containing only five material parameters.

Animals↗

PubMed1990

Determination of a constitutive relation for passive myocardium: II. Parameter estimation.

In the first paper of this series, we proposed a new transversely isotropic pseudostrain-energy function W for describing the biomechanical behavior of excised noncontracting myocardium. The specific functional form of W was inferred directly from biaxial data to be a polynomial function of two coordinate invariant measures of the finite deformation and five material parameters. In this paper, best-fit values of the material parameters are determined from biaxial data using a nonlinear least-squares regression. These values of the parameters are shown to be well-determined, and the final constitutive relation is shown to have good predictive capabilities. Since the proposed constitutive relation describes much broader classes of in-vitro biaxial data than previously proposed relations, it may be better applicable to analyses of stress in the passive heart.

Animals↗

PubMed1990

Biaxial mechanical behavior of excised ventricular epicardium.

We present results from in vitro biaxial stress-strain experiments on epicardium excised from the right and left ventricular free walls of canine hearts. These data reveal that the biomechanical behavior of ventricular epicardium is qualitatively similar to atrial epicardium and parietal pericardium but different from noncontracting myocardium. In particular, ventricular epicardium exhibits a highly nonlinear stress-stretch behavior, being initially compliant but then very stiff near the limits of its extensibility. In addition, the epicardium appears to be initially isotropic but becomes markedly anisotropic upon rapid stiffening. Finally, specimens taken from the right and left ventricular free walls behaved similarly. We submit that excised ventricular epicardium is capable of carrying significant in-plane loads and that there is a need to investigate further its role in local and global cardiac mechanics and physiology.

Animals↗

PubMed1990

Effect of wall stretch on coronary hemodynamics in isolated canine interventricular septum.

The effects of stretch on coronary pressure-flow relations are not well understood. To examine the role of wall stretch per se on coronary hemodynamics, we studied arterially perfused isolated canine interventricular septa in a noncontracting state with vasodilated vessels. We compared the hemodynamic parameters of zero-flow pressure and resistance during passive stretching in the circumferential and the base-to-apex directions alone as well as during simultaneous biaxial stretching in both directions. Even in the unloaded state the zero-flow pressure was positive. Any type of stretching significantly increased the zero-flow pressure and the resistance from their unloaded values. The pressure-flow responses also showed directional dependence. When stretches with matched strains or stresses in each direction were applied sequentially, the resistance increases corresponded to the direction of higher stress. Conversely, the zero-flow pressure response increase corresponded to the direction of greater strain. However, neither response correlated with a measure of global tissue stiffness. Thus there is a complex and tight mechanical interaction between the vessels and the surrounding tissue. These interactions, but not the tissue stiffness, are important determinants of coronary pressure-flow responses during stretch.

Animals↗

PubMed1989

Biomechanical experiments on excised myocardium: theoretical considerations.

Both the specific functional form of a constitutive relation and the particular material parameters must be determined from experiments. Constitutive formulations based on experimental data are valid, however, only when the experimentally measured stresses closely represent the actual distribution of stress in a specimen. In this paper, we present theoretical considerations to help experimentalists identify conditions under which biaxial data obtained from thin slabs of non-contracting myocardium satisfy the above criterion. In particular, we present both a general framework to investigate, and specific numerical examples to illustrate, the influence of certain distributions of muscle fibers, in-plane shearing strains and particular stretching protocols on the distribution of stress within biaxially tested myocardial specimens.

Heart↗

PubMed1989

Impedance of arterial system simulated by viscoelastic t tubes terminated in windkessels.

An improved asymmetric t-tube model of the arterial system is proposed. The model consists of two viscoelastic tubes of differing lengths, each terminated in a modified windkessel with inductance as well as resistance and compliance. Equations for calculating the input impedance of this model are presented. Using typical data from the literature, the model predicts a more realistic impedance modulus and phase than previous models of the circulation. Parametric analysis shows that when peripheral compliances are altered, sharp peaks in the very low frequency portions of the impedance spectra are produced, whereas alterations of either the characteristic impedances or inductances of the terminations have little effect on input impedance. Alteration of the elasticity or relative lengths of the tubes results in shifts in the positions of the maxima and minima akin to those observed experimentally. Change in the viscosity of the walls or of the blood only affects the fluctuations of the impedance spectra without affecting the positions of the maxima and minima. Thus, with this still simple model, very realistic impedance spectra are obtainable. The model provides more insight than previously proposed models into the individual influence of various parameters of the proximal and peripheral vasculature on central hemodynamics.

Arteries↗

PubMed1989

Estimating arterial resistance and compliance during transient conditions in humans.

Almost all existing methods for estimating hemodynamic parameters are valid only during steady-state conditions. There is often a need, however, for estimating peripheral resistance and total arterial compliance during beat-to-beat transients such as during atrial fibrillation. During such transients the pressure at the onset and end of a cardiac cycle usually differ. This pressure difference necessitates a modification of usual methods used for estimating these hemodynamic parameters. In this paper we formulate a method for estimating resistance and total arterial compliance during such beat-to-beat transients. For simplicity the expressions are derived for a two-element windkessel model of the circulation. The method is a generalization of one we previously proposed. Rather than using parameter estimation techniques or having to assume a monoexponential pressure decay during diastole, our method uses the areas under the systolic and diastolic portions of the aortic pressure versus time tracing to obtain explicit expressions for compliance; both for the case where it is constant and when it is assumed to be nonlinear (exponential) function of pressure. Aortic pressure and flow data from patients undergoing cardiac catheterization are employed to illustrate the method. Results illustrate the quantitative difference between uncorrected and corrected estimates of both resistance and compliance as a function of the pressure difference between the onset and end of each beat. The uncorrected parameters were found to be linearly and highly correlated with these pressure differences. Regressions of pressure difference against normalized values revealed that the pooled data for all patients defined a single relationship.(ABSTRACT TRUNCATED AT 250 WORDS)

Arteries↗

PubMed1989

Aortic impedance and compliance in hypertensive rats.

We compared aortic impedance and compliance in normotensive control and hypertensive Wistar rats. Hypertension was induced by unilateral nephrectomy plus steroid and salt water administration. After at least 8 wk of sustained hypertension (tail-cuff systolic pressures greater than 172 mmHg), open-chest ascending aortic micromanometric pressures and electromagnetic flows were measured. We used a frequency-modulated pacing method to enhance the energy content of the pressure and flow signals at specific low frequencies and their multiples. Impedance spectra were calculated using both Fourier series and spectral analysis methods. Compliance was calculated from the low-frequency impedance moduli, assuming a windkessel model for the arterial system. During pentobarbital sodium anesthesia under baseline conditions, the hypertensive rats had higher total resistance (219,000 vs. 126,000 dyn.s.cm-5), higher characteristic impedance (7,334 vs. 5,377 dyn.s.cm-5), larger first zero crossing of impedance phase angle (13.9 vs. 10 Hz), larger ratio of backward to forward pressure waves (0.67 vs. 0.48), and lower compliance (0.00498 vs. 0.00720 ml/mmHg) than controls. The differences between the groups were eliminated when the blood pressures of the hypertensive rats were normalized by vasodilation with nitroprusside or when the control rats were made hypertensive by vasoconstriction with phenylephrine. Thus the hemodynamic alterations appear to be secondary to the increased blood pressure. These base-line differences and the responses to vasoactive drugs are similar to findings in humans, suggesting that this rat model is a good hemodynamic model of human hypertension.

Animals↗

PubMed1989

Arterial hemodynamics in a rabbit model of atherosclerosis.

Although atherosclerosis significantly alters the structural characteristics of the arterial tree, its effect on arterial impedance, which is a means of quantifying the functional characteristics of the arterial system, has not been characterized. To assess how one type of atherosclerosis affects impedance, we studied arterial impedance in New Zealand White rabbits after 11 wk on a 2% cholesterol diet. From open-chest aortic pressures and flows, impedance data were obtained from spectral analysis of randomly paced and Fourier analysis of nonpaced beats. Compliance was calculated from the low-frequency impedance moduli by assuming a windkessel model for the arterial system. Under base-line conditions, the atherosclerotic impedance phase spectrum in the low-frequency range remained negative for higher values of frequency than in controls. There was no difference between the groups in mean arterial blood pressure, impedance modulus spectrum, characteristic impedance, compliance, or total peripheral resistance. Wave reflections were, however, increased in the atherosclerotic animals. The differences between the two groups in phase and wave reflection were completely abolished after phenylephrine (3 micrograms.kg-1.min-1). Thus this study demonstrates that under base-line conditions atherosclerosis increases wave reflection at the input to the arterial system in the absence of an alteration in global arterial compliance, total peripheral resistance, or mean blood pressure. This increase is presumably secondary to atherosclerotic changes at arterial sites, which produce local impedance mismatching.

Animals↗

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↗