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F C Yin

Publications and source records attributed to F C Yin.

At least 91 records · Page 5Linked to original sources
PubMed1986

An approach to quantification of biaxial tissue stress-strain data.

Delineation of the mechanical properties of biologic tissues is one of the cornerstones of biomechanics. Abundant data from uniaxial tests exist but these cannot be extrapolated to describe three-dimensional properties of tissue. Biaxial stress-strain studies have been performed using skin, blood vessels and pericardium. Quantitative description of tissue properties in these studies has employed either polynomial or exponential strain-energy functions. Interpretation of these data, however, is difficult because of wide variability of the estimated coefficients of these functions. This variability has been attributed to experimental noise, numerical instabilities in the algorithms, or to strain-history dependence. No systematic method has been proposed to evaluate the variability. This paper describes a statistically based approach to assessing the sources of and accounting for variability of coefficients in describing biaxial stress-strain data. Our data are from canine pericardium subjected to various combinations of simultaneous biaxial stretching. We first determine a suitable strain-energy function with the least number of free parameters that will fit the data reasonably. We then perform residual analysis to see if standard statistical methods can be used to assess the variability. If not, we use a nonparametric method called bootstrapping that is suitable for assessing the uncertainty in the coefficients. Using a five-parameter exponential strain-energy function, pericardial tissue is found to be strain-history dependent and anisotropic. These findings cannot be attributed to either experimental noise or instability in the numerical algorithms.

Animals↗
PubMed1986

Biaxial stress-strain properties of canine pericardium.

Interpretation of the material properties of biological tissues under biaxial loading conditions has been hampered by wide variability in the coefficients. This variability has been attributed to experimental noise, numerical instability of the fitting algorithms, or to history dependence of the tissue. We have recently described quantitative methods for addressing some of these difficulties in interpretation. This study describes the application of these methods to anterior sheets of mongrel canine pericardium studied in vitro. Each specimen was stretched biaxially with three different combinations of strains and the resulting stress-strain relationships were fitted with a 5-parameter pseudo strain-energy function. Results show that each specimen is highly nonlinear, anisotropic, and history dependent. In addition there was wide inter-specimen variability in the material coefficients. In some cases the direction of the anisotropy was also history dependent. Possible explanations for this unexpected behaviour are discussed.

Animals↗
PubMed1986

Estimation of total arterial compliance: an improved method and evaluation of current methods.

Determination of arterial compliance in vivo has long interested physiologists. Most current methods for estimating this parameter assume that compliance is constant, i.e., that arterial pressure-volume (P-V) relations are linear, and they also assume that diastolic aortic pressure decay is an exponential function of time. Both of these assumptions, however, are questionable. This study proposes improved methods of estimating compliance based on a Windkessel model of the arterial system but which utilize the area under the pressure tracing rather than the waveform itself. Formulations accounting for both linear and three hypothetical nonlinear arterial P-V relations (exponential, logarithmic, and parabolic) are presented. Data from patients with congestive heart failure and hypertension are used for illustration. Compliances assuming linear P-V relations are reasonably close to those assuming nonlinear P-V relations only at mean aortic pressure. At end-diastolic pressure the linear assumption underestimates and at peak systolic it overestimates the compliances obtained assuming nonlinear P-V relations. The simpler linear assumption still allows a first approximation to compliance, but we show that existing methods for obtaining compliance under this assumption have severe theoretical as well as practical shortcomings. Our proposed method avoids these shortcomings primarily because deviations from an exact exponential form of the pressure wave have less influence on these compliance estimates than currently used methods.

Arteries↗
PubMed1986

Septal geometry in the unloaded living human heart.

Right ventricular loading leads to diastolic septal flattening in man without necessarily requiring right ventricular pressure to exceed left ventricular pressure. This observation suggested that the unstressed septal configuration is flat and that its normal concave shape is due to the left-to-right transseptal pressure gradient. To examine this hypothesis, we studied septal configuration by two-dimensional echocardiography in nine patients with normal global and regional left ventricular function during surgery for coronary artery disease. The transseptal pressure gradient was obtained from pulmonary capillary wedge pressure minus right atrial pressure. Measurements were obtained at control (open chest, intact pericardium [C]), with the pericardium open (OP), on cardiopulmonary bypass (CPB), and after cardiac arrest (CA). There were no changes in any measurements between C and OP or between CPB and CA. Left ventricular end-diastolic cavity area decreased from 16.5 +/- 2.1 cm2 at C to 11.1 +/- 4.5 cm2 after CPB, and further decreased to 8.9 +/- 3.5 cm2 after CA (p less than .001), yet the septum flattened, as shown by an increase in its radius of curvature from 1.7 +/- 0.5 cm during C to 2.5 +/- 0.7 cm after CPB, and to 2.9 +/- 1.0 cm after CA (p less than .001), or from 0.4 +/- 0.1 to 0.8 +/- 0.4 to 1.1 +/- 0.5 U (p less than .001) when normalized for cavity area. Diastolic transseptal pressure gradient was reduced from 4.1 +/- 2.3 mm Hg during C to 1.1 +/- 1.8 mm Hg after CPB, and to 0.5 +/- 1.4 mm Hg after CA (p less than .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Blood Pressure↗
PubMed1986

Arterial hemodynamics in human hypertension.

Differences in aortic impedance between normotensives and hypertensives are not well characterized. We examined impedance in 8 normotensive and 11 hypertensive (mean 96.7 vs. 122.2 mmHg) age-matched, Chinese patients undergoing cardiac catheterization at rest, during nitroprusside, and handgrip exercise before and after beta blockade (propranolol). Hypertensives had higher resistance (2,295 vs. 1713 dyn-s/cm5), characteristic impedance (145.7 vs. 93.9 dyn-s/cm5), total external power (1,579 vs. 1174 mW), peripheral reflections (ratio of backward to forward wave components of 0.54 vs. 0.44), and first zero crossing of impedance phase angle (4.15 vs. 2.97 Hz). These abnormalities were eliminated with vasodilatation. Differences between groups were not further exacerbated when pressure was increased during handgrip exercise. Beta blockade further increased resistance and reflections. Thus, hemodynamic abnormalities of essential hypertension (increased resistance, reflections, and pulse wave velocity, and decreased compliance) are compatible with an increased vasomotor tone that is further unmasked during generalized beta blockade.

Adult↗
PubMed1985

Applications of the finite-element method to ventricular mechanics.

The finite-element method of structural analysis is ideally suited for the analysis of complex structures. Applications of this method to analysis of the heart will be discussed in light of the insights that have been obtained as well as the limitations of each analysis. In analyzing the heart, one of the aspects that is particularly difficult to handle is the presumed anisotropy as well as the nonlinearity of the material properties of the wall. Currently, use of finite-element methods for stress analysis requires that one first assumes a reasonable constitutive relation and then use incremental methods to predict the stresses. Studies using this approach will be reviewed. In theory, one could invert this approach by measuring the loading and resulting deformation in an intact heart and using the finite-element method to predict the constitutive relations. In order for this to succeed requires accurate measurement of the loading and the deformations. As we gain increasing expertise in these measurement areas, the feasibility of achieving this seems more realistic. While this alternative approach should be useful, there has been only minimal attention paid to this area. I will discuss this approach from a theoretical viewpoint and review the state-of-the-art.

Biomechanical Phenomena↗
PubMed1984

Effect of nitroprusside on wave reflections in patients with heart failure.

The relationship between wave reflections and ventricular-vascular coupling has been the subject of considerable speculation. Since we have previously shown that low-dose nitroprusside infusion improved ventricular-vascular coupling (as evidenced by increases in cardiac output and in aortic and pulmonary arterial total external power) in patients with severe left ventricular failure and secondary pulmonary hypertension, we chose to examine the changes in their aortic and pulmonary arterial wave reflections in this study. Wave reflection indexes examined included calculated backward and forward pressure waves and the ratio of their magnitudes (reflection factor), the reflection coefficient spectrum obtained by taking the ratios of the corresponding Fourier harmonics of the backward and forward waves, two terminal reflection coefficients calculated as gamma t = (R - Zc)/(R + Zc), where Z is characteristic impedance and R is either total resistance or vascular resistance, and the difference between the maximum and minimum impedance moduli for frequencies of 4 to 15 Hz. In the systemic vasculature, nitroprusside produced large reductions in the elevated vascular resistances and decreased aortic reflections as indexed by the reflection factor and by both terminal reflection coefficients. In contrast, however, no significant changes were found in the pulmonary artery wave reflection indexes despite large reductions in the pulmonary resistances.

Aorta↗
PubMed1983

Regional coronary blood flow during relief of pacing-induced angina by nitroglycerin. Implications for mechanism of action.

The mechanism for the therapeutic effect of nitroglycerin in stress-induced angina remains controversial; it has been attributed to both increased blood supply to the ischemic myocardium and decreased myocardial oxygen demand. To investigate the contribution of each of these mechanisms, systemic pressures and great cardiac vein flow were measured in 14 patients with single-vessel disease involving the left anterior descending (LAD) coronary artery during the development of pacing-induced angina and after the administration of nitroglycerin while continuing pacing at the angina-provoking rate. Great cardiac vein flow, measured by thermodilution, represents the venous efflux from the LAD territory and therefore provided an index of flow to the poststenotic myocardium. In 11 patients, nitroglycerin was administered systemically (400 to 800 micrograms sublingually or 200 micrograms intravenously); angina was relieved in 10, concomitant with a decrease in both great cardiac vein flow (from 123 +/- 29 to 98 +/- 29 ml/min, p less than 0.001) and mean aortic pressure (from 118 +/- 22 to 104 +/- 22 mm Hg, p less than 0.001). In contrast, when 75 micrograms of nitroglycerin was administered directly into the left main coronary artery of 7 patients, it produced a small increase in great cardiac vein flow (from 108 +/- 32 to 125 +/- 31 ml/min, p = 0.059), no change in aortic pressure, and no relief of angina. This study suggests that nitroglycerin's major beneficial action in pacing-induced angina is unrelated to direct effects on the coronary circulation and is likely related to its cardiac unloading effect.

Administration, Oral↗
PubMed1983

Passive biaxial mechanical properties of isolated canine myocardium.

Excised sheets of canine myocardium were subjected to cyclic loading and unloading in the predominant fibre and cross-fibre directions to determine passive mechanical properties. Myocardium under biaxial loading exhibits both non-linear elasticity and viscoelasticity with some strain-rate dependence in the position of the stress-strain relations, but very little rate dependence in the area enclosed by the loading and unloading portions of the stress-strain loops. Fibre and cross-fibre directions demonstrate anisotropic behaviour, with both the degree and direction of the anisotropy being dependent upon the region of the heart from which specimens are obtained. In the same specimen biaxial as compared with uniaxial loading yields different interpretations as to the material properties.

Animals↗
PubMed1983

Effect of nitroprusside on hydraulic vascular loads on the right and left ventricle of patients with heart failure.

We studied the effect of nitroprusside on the hydraulic vascular load of the right and left ventricle in seven patients with severe left ventricular failure. At doses of 0.25-0.75 micrograms/kg/min, stroke volume increased progressively from 40.1 to 48.6 ml and left ventricular end-diastolic pressure decreased from 24.5 to 11.2 mm Hg. Accompanying this improvement in left ventricular performance were dose-related decreases in mean ventricular pressures, pulmonic and systemic resistances and the lower-frequency components of input impedance moduli. Characteristic impedance and both total and oscillatory external power were decreased in the pulmonic, but not the aortic, vasculature. In this class of patients, right ventricular unloading is a striking and direct effect of nitroprusside and may account, in part, for improved left ventricular performance through ventricular interdependence.

Adult↗
PubMed1983

Augmentation of regional coronary blood flow by intra-aortic balloon counterpulsation in patients with unstable angina.

Intra-aortic balloon counterpulsation is capable of reducing afterload in patients with unstable angina. Whether it is also capable of augmenting coronary blood flow to poststenotic myocardium is controversial. We studied seven patients receiving maximal drug therapy and requiring balloon pumping for unstable angina as balloon volume and assist ratio were altered. All patients had greater than 90% stenosis of the proximal left anterior descending coronary artery. With maximal augmentation (40 cc balloon volume, 1:1 assist ratio) great cardiac vein flow, representing the efflux from the left anterior descending coronary artery bed, rose from a baseline of 52 +/- 20 to 67 +/- 25 ml/min (mean +/- SD, p = .004) and mean aortic diastolic pressure increased from 77 +/- 13 to 99 +/- 33 mm Hg (p = .004). Increased great cardiac vein flow correlated with increased mean aortic diastolic pressure across changes in balloon volumes (off, 20 cc, 30 cc, and 40 cc) and changes in assist ratio (off, 1:4, 1:2, and 1:1) (p = .02). However, the intermediate balloon volumes produced great cardiac vein flows at an intermediate level between full assist and no assist (p less than .05), whereas the intermediate assist ratios did not augment flow. Thus balloon pumping increased flow to a bed fed by collateral vessels or critical stenoses; this increased flow correlated with increased aortic diastolic pressure, indicating probable loss of autoregulatory ability.

Aged↗
PubMed1983

Age-associated alterations in viscoelastic properties of canine aortic strips.

Many studies have delineated the changes in the elastic properties of arterial tissue as a function of age. Despite the fact that viscoelasticity is a prominent feature of these tissues, there is little information or characterization of age-associated changes in viscoelastic properties, over a wide range of smooth muscle activation, particularly in nonhuman tissue where atherosclerosis is not a confounding factor. In the present study, using small sinusoidal length perturbations, we determined the dynamic stiffness properties across a wide range of lengths (stretch ratios from 100 to 135%) and frequencies (from 0.25 to 35 Hz) in strips excised from ascending and descending aortas from six young (2 to 4-year-old) and 12 senescent (10- to 13-year-old) beagles. Studies were performed with the smooth muscle fully activated with calcium and norepinephrine, as well as fully inactivated with cyanide, iodoacetate, and dinitrophenol. There was a cubic nonlinear dependence of stiffness modulus on length only in senescent tissue and, surprisingly, little frequency dependence in tissue of either age. Compared to the young aortas, the three-dimensional surface representing the dependence of stiffness modulus on length and frequency from both the ascending and descending regions of aged aortas was displaced higher on the stiffness axis both with the muscle fully activated and inactivated. This age difference was accentuated at longer lengths. The phase lag between force and length was greater in the young vs. the old strips only in the activated, ascending aortic tissue. We found no age differences in the content of elastin, collagen, or in the collagen/elastin ratio, to account for these mechanical property differences.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗
PubMed1982

Limitations of pulmonary wedge V waves in diagnosing mitral regurgitation.

To study the usefulness of large V waves in pulmonary capillary wedge tracings in establishing the diagnosis of mitral regurgitation, data on 1,021 consecutive cardiac catheterizations were reviewed. Wedge tracings were obtained by Swan-Ganz catheterization in 208 patients, usually because of suspected valve disease. One hundred two patients had no trace of mitral regurgitation angiographically, 69 had mild to moderate and 37 had severe regurgitation. V waves were graded as trivial (less than 5), intermediate (5 to 10) or large (10 or more mm Hg above mean wedge pressure). Of 50 patients with large V waves, 18 (36 percent) had no or trace mitral regurgitation; these included 5 with mitral stenosis, 3 with a mitral valve prosthesis, 4 with coronary disease and congestive failure, 2 with aortic valve disease and congestive failure and 2 with a ventricular septal defect. Of 37 patients with severe mitral regurgitation, 16 (43 percent) had large and 12 (32 percent) had trivial V waves. Thus, mitral regurgitation is the most common cause of large V waves; however, large V waves are neither highly sensitive nor specific for severe regurgitation. Increased left atrial compliance may be associated with trivial V waves in the presence of severe regurgitation. Mitral obstruction, congestive heart failure and ventricular septal defect may all be associated with large V waves in the absence of significant mitral regurgitation.

Cardiac Catheterization↗
PubMed1982

Myocardial aging: functional alterations and related cellular mechanisms.

Studies that have examined the effect of age on the myocardium do not support the notion that a generalized decline in myocardial function occurs with adult aging but, rather, that certain specific changes in cardiac biochemistry and function occur. Although in many instances the molecular bases for these changes have not been precisely defined, sufficient clues regarding their nature are at hand to provide at least the first step toward an understanding of the dramatic effects of aging or time on the myocardium. A fundamental understanding of the effects of aging will provide an additional dimension to studies of myocardial biochemistry and function and may be useful in elucidating mechanisms of excitation-contraction coupling in the heart.

Adenosine Triphosphate↗
PubMed1982

Role of carotid artery resistance to collapse during high-intrathoracic-pressure CPR.

The driving force for carotid artery flow during high-intrathoracic-pressure cardiopulmonary resuscitation is a peripheral arteriovenous pressure gradient resulting from differential transmission of the high intrathoracic pressure to the carotid artery but not to the jugular vein. To study the role of carotid artery resistance to collapse in establishing this differential pressure transmission, we manipulated the upstream, downstream, and surrounding pressures and measured the resultant carotid artery flow in both intact dogs and in excised arteries. Stepwise reductions in downstream pressure produced a narrowing near the outlet from the high-pressure chamber (the thorax in vivo), but increments in flow continued despite the presence of a positive (outside--inside) transmural pressure gradient. Flow limitation occurred only when downstream pressure was further decreased. Resistance to collapse was indexed by the transmural pressure at the onset of flow limitation (Pcrit), which was 7.2 +/- 1.6 mmHg in eight intact dogs. After administration of norepinephrine Pcrit increased by 2.6 +/- 0.7 mmHg, P less than 0.001). Seven excised carotid arteries also demonstrated resistance to collapse which was enhanced somewhat with norepinephrine. Thus resistance of the carotid artery to collapse is a critical factor in maintaining forward flow during high intrathoracic pressure. This resistance to collapse is also seen in vitro and can be enhanced by vasoconstricting agents.

Animals↗
PubMed1982

Use of tibial length to quantify cardiac hypertrophy: application in the aging rat.

Fluctuations in body weight as occur with aging make body weight an unreliable reference for normalizing heart weight. We compared heart weight normalized by tibial length, which remains constant after maturity, with that normalized by body weight in 5- to 28-mo-old male Wistar rats. When normalized by tibial length or body weight, relative to the 5-mo heart, the senescent left ventricle undergoes 17 vs. 38% hypertrophy, respectively, and the right ventricle undergoes 0 vs. 28% hypertrophy, respectively. Histological measurements in the 25- compared with the 5-mo-old left ventricles reveal 6% larger myocyte diameters and 12% larger cellular cross-sectional areas, indicating about 15% hypertrophy; this value agrees more closely with the estimates based on tibial length than with those based on body weight. To allow prediction of left ventricular weight in a living rat, a regression equation using body weight, age, and tibial length was derived. This enabled us to perform a longitudinal aging study that verified that the above results were not biased by selective survival. Thus, in conditions in which body weight changes, cardiac hypertrophy can be more accurately quantified by relating heart weight to tibial length than to body weight. This approach may have applicability for assessing relative sizes of other organs as well.

Aging↗
PubMed1982

Electrocardiographic localization of coronary artery narrowings: studies during myocardial ischemia and infarction in patients with one-vessel disease.

To investigate the accuracy of the 12-lead ECG in localizing the site of coronary artery narrowings, we reviewed abnormal ECGs obtained during myocardial infarction, spontaneous angina or exercise stress testing in 134 patients with angiographically documented one-vessel disease. The presence of Q waves, ST-segment elevation and T-wave inversion in leads I, aVL and V1--V4 were all highly correlated with the presence of left anterior descending coronary artery disease (p less than 0.001), and the same ECG findings in leads II, III and aVF were associated with right (RCA) or circumflex coronary artery (LCx) narrowings (p less than 0.001). In contrast, ST depression alone was not useful in predicting the site of coronary artery narrowing. Q waves correctly identified the location of the coronary disease in 98% of cases, ST elevation in 91%, T-wave inversion in 84%, and ST depression in 60%. No electrocardiographic criteria distinguished RCA from LCx disease, even in patients with a right-dominant circulation. These findings should lead to a better understanding of the value and limitations of the 12-lead ECG in localizing coronary artery disease.

Adult↗