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Biomedical subjects

F C Yin

Publications and source records attributed to F C Yin.

12 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

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↗
PubMed1978

Evidence of incomplete left ventricular relaxation in the dog: prediction from the time constant for isovolumic pressure fall.

Although it has been proposed that incomplete relaxation explains certain increases in left ventricular end diastolic pressure relative to volume, there has been no clear demonstration that incomplete relaxation occurs in the intact working ventricle. To identify incomplete relaxation, left ventricular pressure-dimension relationships were studied in 10 canine right heart bypass preparations during ventricular pacing. The fully relaxed, exponential diastolic pressure-dimension line for each ventricle was first determined from pressure and dimension values at the end of prolonged diastoles after interruption of pacing. For 167 beats during pacing under widely varying hemodynamic conditions, diastolic pressure-dimension values encountered this line defining the fully relaxed state during the filling period indicating that relaxation was complete before end diastole. The time constant for isovolumic exponential pressure fall (T) was determined for all beats. For this exponential function, if no diastolic filling occurred, 97% of pressure fall would be complete by 3.5 T after maximal negative dP/dt. For the 167 beats the fully relaxed pressure-dimension line was always encountered before 3.5 T. With very rapid pacing rates (170-200 beats/min) and(or) with pharmacologic prolongation of relaxation, incomplete relaxation occurred as evidenced by the fact that the line defining the fully relaxed state was never reached during diastole (n = 15). This evidence of incomplete relaxation occurred only when the subsequent beat began before 3.5 T but did not always occur under these conditions. Thus, an increase in end diastolic pressure relative to diastolic volume may result from incomplete relaxation under conditions of sufficiently rapid heart rate or sufficiently prolonged ventricular relaxation. Incomplete relaxation does not occur when the next beat begins more than 3.5 T after maximum negative dP/dt.

Animals↗
PubMed1977

Characterization of left ventricular external wall motion in man by video dimension analyzer (Vidian).

Several investigators have described close relationships between left ventricular wall motion and physiologic cardiac events. Using an improved wall motion tracking devide (Vidian) in studies of 30 patients, we have compared the dynamics of left ventricular wall motion, recorded noninvasively, with high fidelity left ventricular and aortic pressures, intracardiac phonocardiograms, apexcardiograms, and cyclic left ventricular volume curves obtained during cardiac catheterization. Wall motion tracking signals comprised: pre-ejection outward deflection commencing with the first component of the first heart sound and coincident with the pre-ejection phase of the left ventricular pressure and apexcardiogram; a sharp descent during ejection, commencing with the "E" point of the apexcardiogram and with the onset of the upstroke of the aortic pressure; end ejection nadir, synchronous with the dicrotic notch of the aortic pressure; a nadir representing cessation of inward displacement, presumably reflecting slight inertial motion of the wall; a brief period of isovolumic relaxation which terminated synchronously with the "O" point of the apexcardiogram; rapid, then slow filling waves, coincident with those of the apexcardiogram, and demarcated by a transitional angulation synchronous with the third heart sound; and "a" wave, occurring simultaneously with that of the apexcardiogram. Ventricular wall motion tracking signals also corresponded to curves representing cyclic changes in left ventricular minor radius, and chamber volume derived from cineventriculograms. In 10 patients with abnormal contraction patterns detected by biplane cineventriculography, anomalous deflections were also recorded during ejection by the Vidian. Left ventricular wall motion tracking with the Vidian: 1) provides a sensitive index for timing of intracardiac events, 2) reflects cyclic changes in ventricular volumes and minor dimensions, 3) provides a convenient noninvasive technique for detection of regional asynergy involving the lateral left ventricular wall, and 4) by correlation with simultaneous ventricular pressure measurements, may provide useful information regarding left ventricular pressure/segment dimension relations.

Heart↗
PubMed1977

Increased dynamic stiffness of trabeculae carneae from senescent rats.

Dynamic stiffness was measured in both adult and senescent Wistar rats using sinusoidal length perturbations from 17 to 40 Hz, superimposed on isometric contractions at low and below Lmax. The slope of the stiffness-tension relationship was found to be increased in senescent muscles of both sexes and at two different temperatures. The age dependence of the slope was independent of resting muscle length. The slope also exhibited frequency dependence and hysteresis that indicates the presence of viscoelasticity during activation. No age dependence was seen in peak active tension, peak dT/dt, or resting stiffness. The findings of this study, taken together with other studies that have shown a decreased shortening ability in the senescent muscle, explain in part the maintenance of active tension development in the face of decreased shortening ability associated with the senescent muscle.

Aging↗
PubMed1977

Echocardiographic assessment of a normal adult aging population.

Echocardiograms were performed on 105 male participants in the National Institutes on Aging's volunteer Longitudinal Study Program. All subjects (25--84 years of age) were physically active and had no evidence of hypertension or cardiovascular disease. Measurements were made of the initial diastolic (E-F) slope of the anterior mitral valve leaflet, the aortic and left ventricular cavity dimensions, and the thickness of the posterior left ventricular wall. Fractional shortening of the minor semi-axis of the left ventricle and the velocity of circumferential fiber shortening were also determined. It was found that increasing age correlated with a decrease mitral valve E-F slope and increased aortic root diameter and left ventricular wall thickness. Aging did not affect left ventricular cavity dimension, fractional shortening of the minor semi-axis, and velocity of circumferential fiber shortening. These findings suggest that aging in the normal male is associated with altered left ventricular diastolic filling, increased aortic root diameter and left ventricle hypertrophy but little change in contractile ability in the resting state.

Adult↗