Diastolic viscous properties of the intact canine left ventricle

Strain-Dependent Stress Relaxation Behavior of Healthy Right Ventricular Free Wall

The increasing evidence of stress-strain hysteresis in large animal or human myocardium calls for extensive characterizations of the passive viscoelastic behavior of the myocardium. Several recent studies have investigated and modeled the viscoelasticity of the left ventricle while the right ventricle (RV) viscoelasticity remains poorly understood. Our goal was to characterize the biaxial viscoelastic behavior of RV free wall (RVFW) using two modeling approaches. We applied both quasi-linear viscoelastic (QLV) and nonlinear viscoelastic (NLV) theories to experimental stress relaxation data from healthy adult ovine. A three-term Prony series relaxation function combined with an Ogden strain energy density function were used in the QLV modeling, while a power-law formulation was adopted in the NLV approach. The ovine RVFW exhibited an anisotropic and strain-dependent viscoelastic behavior relative to anatomical coordinates, and the NLV model showed a higher capacity in predicting strain-dependent stress relaxation than the QLV model. From the QLV fitting, the relaxation term associated with the largest time constant played the dominant role in the overall relaxation behavior at all strains from early to late diastole, whereas the term associated with the smallest time constant was pronounced only at low strains at early diastole. From the NLV fitting, the parameters showed a nonlinear dependence on the strain. Overall, our study characterized the anisotropic, nonlinear viscoelasticity to capture the elastic and viscous resistances of the RVFW during diastole. These findings deepen our understanding of RV myocardium dynamic mechanical properties. STATEMENT OF SIGNIFICANCE: Although significant progress has been made to understand the passive elastic behavior of the right ventricle free wall (RVFW), its viscoelastic behavior remains poorly understood. In this study, we originally applied both quasi-linear viscoelastic (QLV) and nonlinear viscoelastic (NLV) models to published experimental data from healthy ovine RVFW. Our results revealed an anisotropic and strain-dependent viscoelastic behavior of the RVFW. The parameters from the NLV fitting showed nonlinear relationships with the strain, and the NLV model showed a higher capacity in predicting strain-dependent stress relaxation than the QLV model. These findings characterize the anisotropic, nonlinear viscoelasticity of RVFW to fully capture the total (elastic and viscous) resistance that is critical to diastolic function.

Biomechanics of infarcted left Ventricle-A review of experiments

Myocardial infarction (MI) is one of leading diseases to contribute to annual death rate of 5% in the world. In the past decades, significant work has been devoted to this subject. Biomechanics of infarcted left ventricle (LV) is associated with MI diagnosis, understanding of remodelling, MI micro-structure and biomechanical property characterizations as well as MI therapy design and optimization, but the subject has not been reviewed presently. In the article, biomechanics of infarcted LV was reviewed in terms of experiments achieved in the subject so far. The concerned content includes experimental remodelling, kinematics and kinetics of infarcted LVs. A few important issues were discussed and several essential topics that need to be investigated further were summarized. Microstructure of MI tissue should be observed even carefully and compared between different methods for producing MI scar in the same animal model, and eventually correlated to passive biomechanical property by establishing innovative constitutive laws. More uniaxial or biaxial tensile tests are desirable on MI, border and remote tissues, and viscoelastic property identification should be performed in various time scales. Active contraction experiments on LV wall with MI should be conducted to clarify impaired LV pumping function and supply necessary data to the function modelling. Pressure-volume curves of LV with MI during diastole and systole for the human are also desirable to propose and validate constitutive laws for LV walls with MI.

Current Understanding of the Biomechanics of Ventricular Tissues in Heart Failure

Heart failure is the leading cause of death worldwide, and the most common cause of heart failure is ventricular dysfunction. It is well known that the ventricles are anisotropic and viscoelastic tissues and their mechanical properties change in diseased states. The tissue mechanical behavior is an important determinant of the function of ventricles. The aim of this paper is to review the current understanding of the biomechanics of ventricular tissues as well as the clinical significance. We present the common methods of the mechanical measurement of ventricles, the known ventricular mechanical properties including the viscoelasticity of the tissue, the existing computational models, and the clinical relevance of the ventricular mechanical properties. Lastly, we suggest some future research directions to elucidate the roles of the ventricular biomechanics in the ventricular dysfunction to inspire new therapies for heart failure patients.

Contribution of laminar myofiber architecture to load-dependent changes in mechanics of LV myocardium

The ventricular myocardium consists of a syncytium of myocytes organized into branching, transmurally oriented laminar sheets approximately four cells thick. When systolic deformation is expressed in an axis system determined by the anatomy of the laminar architecture, laminar sheets of myocytes shear and laterally extend in an approximately radial direction. These deformations account for ~90% of normal systolic wall thickening in the left ventricular free wall. In the present study, we investigated whether the changes in systolic and diastolic function of the sheets were sensitive to alterations in systolic and diastolic load. Our results indicate that there is substantial reorientation of the laminar architecture during systole and diastole. Moreover, this reorientation is both site and load dependent. Thus as end-diastolic pressure is increased and the left ventricular wall thins, sheets shorten and rotate away from the radial direction due to transverse shearing, opposite of what occurs in systole. Both mechanisms of thickening contribute substantially to normal left ventricular wall function. Whereas the relative contributions of shear and extension are comparable at the base, sheet shear is the predominant factor at the apex. The magnitude of shortening/extension and shear increases with preload and decreases with afterload. These findings underscore the essential contribution of the laminar myocardial architecture for normal ventricular function throughout the cardiac cycle.

Load as an acute determinant of end-diastolic pressure-volume relation

Afterload-induced changes in myocardial relaxation are a mechanism for diastolic dysfunction when afterload is elevated beyond certain limits. The present study investigated the effects of acute afterload and preload changes on the position of the end-diastolic (ED) pressure-volume (P-V) relation. Beat-to-beat afterload elevations were induced in seven open-chest rabbits by gradually occluding the ascending aorta to increase peak left ventricular pressure (LVP) from baseline to isovolumetric level. Afterload elevations were performed at three ED LVP: 2.0 +/- 0.2 (low), 5.7 +/- 0.2 (mid), and 9.6 +/- 0.6 (high) mmHg. Preload was altered with caval occlusions and/or intravenous dextran. Afterload elevations induced an upward shift of the diastolic P-V relation, which became more important as afterload and/or preload increased. For instance, maximal afterload elevations shifted this relation upward 2.2 +/- 0. 5, 5.1 +/- 0.8, and 12.1 +/- 1.7 mmHg at low, mid, and high preload, respectively. These effects were partially due to changes in relaxation rate and time available to relax. In conclusion, load is an acute determinant of the ED P-V relation, which, therefore, does not provide a load-independent assessment of diastolic function.

Load dependent diastolic dysfunction in heart failure

Congestive heart failure may result from cardiovascular overload, from systolic or from diastolic dysfunction. Diastolic left ventricular dysfunction may result from structural resistance to filling such as induced by pericardial constraint, right ventricular compression, increased chamber stiffness (hypertrophy) and increased myocardial stiffness (fibrosis). A distinct and functional etiology of diastolic dysfunction is slow and incomplete myocardial relaxation. Relaxation may be slowed by pathological processes such as hypertrophy, ischemia and by asynchronous left ventricular function. The present contribution analyses the occurrence of slow and incomplete myocardial relaxation in response to changes in systolic pressure and in response to changes in venous return. The regulation of myocardial relaxation by load is critically dependent on the transition from myocardial contraction to relaxation, which occurs in dogs when 82% of peak isovolumetric pressure has developed or at a relative load of 0.82. This corresponds to early ejection in normal hearts, but is situated even before aortic valve opening in severely diseased hearts. When load is developed beyond this transition, relaxation becomes slow and even incomplete. This is load dependent diastolic dysfunction. Load dependent diastolic dysfunction occurs in normal hearts facing heavy afterload and in severely diseased hearts even with normal hemodynamic parameters. This dysfunction should contribute to elevating filling pressures in most patients with severe congestive heart failure. This dysfunction can be reverted by decreasing systolic pressures or by decreasing venous return. Load dependent diastolic dysfunction gives us an additional reason to aggressively treat CHF patients with diuretics and vasodilators.

A new method to measure regional myocardial time-varying elastance using minute vibration

We developed a new technique to evaluate regional myocardial elastance using minute vibration. In 13 isolated cross-circulated canine hearts, we applied small sinusoidal vibrations of displacement to the left ventricular surface at various frequencies (50-100 Hz). Using the measured displacement and force between the vibrator head and myocardium, we derived myocardial elastance on the basis of the equation of motion for a given moment of the cardiac cycle. Simultaneous solution of the equations of motion at different frequencies yielded a unique value of elastance. Time-varying myocardial elastance increased from diastole (0.028 +/- 0.211 x 10(6) dyn/cm) to systole (0.833 +/- 0.391 x 10(6) dyn/cm). The end-systolic elastance (ees) linearly correlated with end-systolic left ventricular elastance (r = 0.717, P < 0.001) and also with the end-systolic Young's modulus (r = 0.874, P < 0.0001). We also measured ees at both ischemic and nonischemic regions during coronary occlusion. Young's modulus, estimated by normalizing ees by the wall thickness and by the estimated mass, did not change significantly at the nonischemic regions, whereas it decreased significantly from 2.303 +/- 0.556 to 1.173 +/- 0.370 x 10(6) dyn/cm2 at the ischemic region after coronary occlusion (P < 0.005). We conclude that this technique is useful for the quantitative assessment of regional myocardial elastance.

Myocardial ischemia and reperfusion are associated with an increased stiffness of remote nonischemic myocardium

During and after an ischemic injury, maintenance and recovery of cardiac function may critically depend on remote nonischemic myocardium. Graded myocardial ischemia is associated with an approximately 50% increase in stiffness of nonischemic myocardium. We determined whether this increase in stiffness is unique to the ischemic period or persists during reperfusion. Ten anesthetized (isoflurane 1.0% vol/vol) open-chest dogs were instrumented to measure left ventricular pressure and dimensions (sonomicrometry) in ischemic and nonischemic myocardium. Regional chamber stiffness and myocardial stiffness were assessed using the end-diastolic pressure-length relationship which was modified by stepwise infusion and withdrawal of 200 mL of the animals' own blood during baseline, 45 min low flow ischemia (systolic bulge), and 60 min after the onset of reperfusion. In remote nonischemic myocardium, regional myocardial ischemia was associated with a significant (P < 0.05) increase in chamber stiffness (+44%) and myocardial stiffness (+48%). Sixty minutes after the onset of reperfusion, chamber stiffness (+54%, P < 0.05 versus baseline) and myocardial stiffness (+55%, P < 0.05 versus baseline) remained increased. Thus, the ischemia-induced increase in stiffness of remote nonischemic myocardium persists for at least 60 min after reperfusion.

Post-ischemic diastolic dysfunction

Though a sustained post-ischemic decrease in contractile function has been clearly established, post-ischemic diastolic function has not been thoroughly investigated. Accordingly, 11 anesthetized (isoflurane 1%) open-chest beagles were instrumented to measure left ventricular pressure and dimensions (circumferential length and wall thickness) in an apicoanterior area supplied by the left anterior descending coronary artery (LAD). Pressure-dimension relations were modified by stepwise infusion and withdrawal of 200 mL of the animals' own blood during baseline, 45 minutes partial occlusion of the LAD (systolic bulging), and 60 minutes after the onset of reperfusion. Stiffness constants were derived from the end-diastolic pressure-length and stress-strain relations, respectively. Myocardial ischemia was associated with significant (P < 0.05) alterations of the following parameters of diastolic function: (1) 47% increase in end-diastolic pressure; (2) 22% decrease in peak negative dP/dt; (3) 9% increase in the time constant of isovolumic relaxation (tau); (4) postcystolic contraction; (5) 6% increase in end-diastolic length and 10% decrease in end-diastolic thickness; (6) 12% increase in unstressed length (creep) and 13% decrease in unstressed thickness; (7) 51% increase in chamber stiffness and a 63% increase in myocardial stiffness; and (8) 40% decrease in the peak lengthening rate. After 60 minutes of reperfusion, only end-diastolic pressure and tau had returned to baseline values whereas systolic shortening fraction, postsystolic contraction, and end-diastolic and unstressed dimensions had only partially recovered. No recovery occurred in peak negative dP/dt, chamber stiffness, myocardial stiffness, and peak lengthening rate. Thus, both myocardial ischemia and reperfusion are associated with complex changes in global and regional left ventricular diastolic function.

Graded myocardial ischemia is associated with a decrease in diastolic distensibility of the remote nonischemic myocardium in the anesthetized dog

OBJECTIVES

This study was designed to investigate the changes in regional distensibility of the ischemic segment and of a remote nonischemic segment brought about by graded myocardial ischemia.

BACKGROUND

Ventricular distensibility is a major determinant of left ventricular end-diastolic pressure. The effects of graded myocardial ischemia on the regional distensibility of the ischemic area have not been studied. Moreover, there are few data on the effects of myocardial ischemia on the regional distensibility of the nonischemic myocardium.

METHODS

Nine anesthetized open chest mongrel dogs were fitted with instruments to measure left ventricular pressure and circumferential length (sonomicrometry) in the ischemic segment and in a nonischemic segment. The pressure-length relation was modified by stepwise infusion and withdrawal of 200 ml of each dog's own blood over 30 min in five consecutive stages of regional ischemia. Unstressed dimensions were obtained by repeated inferior vena cava occlusions. In both segments, regional distensibility was assessed at end-diastole by means of the constants of the pressure-length (chamber stiffness), the pressure-strain and the force-strain (myocardial stiffness) relations.

RESULTS

In the ischemic segment, partial and complete coronary occlusions were associated with a twofold increase in the chamber stiffness constant, the pressure-strain constant and the myocardial stiffness constant, whereas in the nonischemic segment the chamber stiffness constant, the pressure-strain constant and the myocardial stiffness constant increased by 50%.

CONCLUSIONS

Regional myocardial ischemia is associated with a decrease in distensibility of both the ischemic and the remote nonischemic myocardium.

Clinical evaluation of left ventricular diastolic performance

Diastole can be divided into four phases: isovolumic relaxation, early filling, diastasis, and atrial systole. The amount of LV filling that occurs during each of these phases depends on myocardial relaxation, the passive characteristics of the LV, the characteristics of the left atrium, pulmonary veins, and mitral valve, and the heart rate. When diastolic function is normal, the net effect of these factors results in an LV filling sufficient to produce an adequate cardiac output, while mean pulmonary venous pressure is maintained below 12 mm Hg. In the absence of systolic dysfunction, abnormal diastolic performance is usually due to abnormal relaxation and/or changes in the passive LV characteristics. Invasive studies can quantitate the rate of myocardial relaxation and the LV diastolic pressure-volume relation. More recently, RNA and Doppler echocardiography have been used to noninvasively evaluate diastolic performance by determining the pattern of LV diastolic filling. At rest, most LV filling occurs early in diastole. Conditions that produce diastolic dysfunction, such as LV hypertrophy and ischemia, are associated with reduced early diastolic filling and an augmented importance of atrial systole. It is important to recognize that such patterns can occur in patients who do not have clinically apparent diastolic dysfunction and in normals. Furthermore, a normal pattern can occur in patients who have severe diastolic dysfunction. A reduced early diastolic filling, in the absence of pulmonary congestion, indicates the loss of diastolic reserve, since the left atrium is being used as a booster pump. This pattern of diastolic filling in a patient who has symptoms of pulmonary congestion may suggest diastolic dysfunction, even if the systolic LV performance is normal. Since diastolic filling of the LV results from a complex interplay of factors, it is unlikely that a single, easily interpreted index of LV diastolic performance will ever be developed. However, the recent development of a noninvasive evaluation of the pattern of LV diastolic filling by RNA or Doppler echocardiography is an important advance. When interpreted with an understanding of the determinants of LV filling and the patient's clinical status, these noninvasive tests can contribute to the rational assessment of LV diastolic performance.

Functional and structural abnormalities in patients with dilated cardiomyopathy

Passive diastolic properties of the left ventricle were determined in 10 control subjects and 12 patients with dilated cardiomyopathy. Simultaneous left ventricular angiography and high fidelity pressure measurements were performed in all patients. Left ventricular chamber stiffness was calculated from left ventricular pressure-volume and myocardial stiffness from left ventricular stress-strain relations with use of a viscoelastic model. Patients with dilated cardiomyopathy were classified into two groups according to the diastolic constant of myocardial stiffness (beta). Group 1 consisted of seven patients with a normal constant of myocardial stiffness less than or equal to 9.6 (normal range 2.2 to 9.6) and group 2 of 5 patients with a beta greater than 9.6. Structural abnormalities (percent interstitial fibrosis, fibrous content) in patients with dilated cardiomyopathy were assessed by morphometry from right ventricular endomyocardial biopsies. Heart rate was similar in the three groups. Left ventricular end-diastolic pressure was significantly greater in patients with cardiomyopathy (18 mm Hg in group 1 and 22 mm Hg in group 2) than in the control patients (10 mm Hg). Left ventricular ejection fraction was significantly lower in groups 1 (37%) and 2 (36%) than in the control patients (66%). Left ventricular muscle mass index was significantly increased in both groups with cardiomyopathy. The constant of chamber stiffness (beta*) was slightly although not significantly greater in groups 1 and 2 (0.58 and 0.58, respectively) than in the control group (0.35). The constant of myocardial stiffness beta was normal in group 1 (7.0; control group 6.9, p = NS) but was significantly increased in group 2 (23.5). Interstitial fibrosis was 19% in group 1 and 43% (p less than 0.001) in group 2 (normal less than or equal to 10%). There was an exponential relation between both diastolic constant of myocardial stiffness (beta) and interstitial fibrosis (IF) (r = 0.95; p less than 0.001) and beta and fibrous content divided by end-diastolic volume index (r = 0.93; p less than 0.001). It is concluded that myocardial stiffness can be normal in patients with dilated cardiomyopathy despite severely depressed systolic function. Structural alterations of the myocardium with increased amounts of fibrous tissues are probably responsible for the observed changes in passive elastic properties of the myocardium in patients with dilated cardiomyopathy. The constant of myocardial stiffness (beta) helps to identify patients with severe structural alterations (group 2), representing possibly a more advanced stage of the disease.

Diastolic function after cardiac and heart-lung transplantation

The mechanical efficiency of left ventricular contraction and relaxation, the asynchrony of the onset of left ventricular relaxation, the time constant of left ventricular isovolumic pressure decay, and left ventricular chamber and myocardial stiffness were analysed in 32 patients after cardiac (24) and heart-lung transplantation (8). After cardiac transplantation left ventricular myocardial stiffness was increased and a mild degree of incoordinate contraction and relaxation was seen. In contrast, after heart-lung transplantation diastolic function was almost normal. Impairment of passive diastolic properties was significantly related to the ischaemic time of the donor heart and the donor's age. The index of left ventricular asynchrony was related to the ischaemic time and the recipient's age. The interval between transplantation and study did not influence the number of rejection episodes. This study confirms the presence of diastolic dysfunction after cardiac transplantation. Impairment of diastolic function seems to be related to the ischaemic time of the donor heart and to a mismatch between the size of the donor heart and the recipient's needs.

Left ventricular pressure-length relation during exercise-induced ischemia

The pressure-length relation in normal and ischemic segments was analyzed with use of left ventriculography and simultaneous micromanometry during supine exercise in 9 normal subjects and 12 patients with effort angina. Segmental analysis was done in the right anterior oblique projection using a long axis with three perpendicular, equidistant chords. The apical segment in the 12 patients with coronary artery disease represented the ischemic region. In 5 of the 12 patients with coronary artery disease, the basal segment that showed no exercise-induced deterioration in wall motion was used as an intrapatient control (nonischemic segment). In the 12 patients with coronary artery disease, left ventricular ejection fraction decreased (from 65% to 50%, p less than 0.001), end-diastolic pressure increased (from 24 to 40 mm Hg, p less than 0.001) and the lowest diastolic filling pressure increased (from 9 to 22 mm Hg, p less than 0.001) during exercise-induced ischemia. In normal subjects, ejection fraction increased (from 64% to 70%, p less than 0.01) with unchanged end-diastolic pressure, whereas the lowest diastolic filling pressure decreased during exercise (from 9 to 3 mm Hg, p less than 0.01). Global left ventricular diastolic pressure-volume curves showed an upward and rightward shift during exercise-induced ischemia. Regional pressure-length curves of both nonischemic (n = 5) and ischemic (n = 12) segments were shifted upward in early diastole, but moved to a higher portion of the rest pressure-length curve without an upward shift during mid- to end-diastole. In contrast, the apical segment in normal subjects showed a downward shift during exercise.(ABSTRACT TRUNCATED AT 250 WORDS)

Is stiffness increased during ischemia?

Possible sources of increased ventricular stiffness can be more easily appreciated when pressure and volume patterns are considered as a function of time. A discussion on sources of effective or apparent stiffness or stiffness changes includes viscoelastic properties and active behavior at the muscular level. Chamber geometry and coronary vascular pressure and flow are intrinsic ventricular components. Together with the pressure head and crosstalk as extraventricular components, all these properties are integrated to determine intact heart behavior in late relaxation and diastole.

Effects of ischemia, bypass surgery and past infarction on myocardial contraction, relaxation and compliance during exercise

Abnormalities of left ventricular function during ischemia have been described in animal models and in humans. Exercise, while a physiologic means of inducing ischemia, has a complex effect on left ventricular function by itself. In addition, patients with coronary artery disease have a diversity of chronic changes in myocardial structure and function. Therefore, with use of micromanometer left ventricular pressure measurements and ventricular volumes, calculated from biplane cineangiograms, left ventricular function at rest and during exercise was studied in 57 patients. Exercise-induced ischemia produced a decrease in ejection fraction, an increase in end-systolic volume, dramatic increases in diastolic pressures and an upward shift in the diastolic pressure-volume relation. Central to these changes was abnormal myocardial contraction and relaxation, with reduced regional shortening and impaired left ventricular pressure decay. However, nonischemic areas were capable of augmented shortening, and global pressure decay did accelerate slightly. These findings demonstrate that exercise-induced adjustments in contraction and relaxation are intertwined with ischemia-related abnormalities. Exercise studies in patients after bypass surgery and in patients with scars from distant myocardial infarction were useful in clarifying confounding factors. For example, asynchrony of contraction and relaxation, and chronic changes in passive chamber properties, also compromise systolic and diastolic function during exercise. In patients with coronary artery disease without ischemia during exercise, left ventricular end-diastolic pressure, but not early diastolic pressure, increased during exercise. The increase in pressure was appropriate for a slight increase in end-diastolic volume in a ventricle with a steep pressure-volume relation. Furthermore, end-systolic volume, while maintained during exercise, was not reduced, as occurs normally.(ABSTRACT TRUNCATED AT 250 WORDS)

Human right ventricular end-systolic pressure-volume relation defined by maximal elastance

This study was undertaken to determine 1) whether a combined radionuclide-hemodynamic technique could define the right ventricular end-systolic pressure-volume relation (RV ESPVR) in the clinical setting, 2) whether the human RV ESPVR defined by maximal elastance is linear and responsive to inotropic interventions, and 3) whether more easily measured modifications of the ESPVR are reliable substitutes as an index of RV function. Eight patients with normal RV function were studied with simultaneous micromanometer RV pressure measurements and radionuclide ventriculography to construct RV pressure-volume loops. Data were collected at baseline and after at least two alterations in loading conditions with nitroglycerin, phenylephrine, or saline. End systole was defined by maximal elastance (E(t) = P(t)/[V(t) - V0]). Data were also obtained during administration of dobutamine in four patients and after atrial pacing tachycardia in one patient. The RV ESPVR defined by maximal elastance was highly linear (r = 0.988-0.999) throughout the range of pressures and volumes tested. Furthermore, the linear correlations were significantly higher (p less than 0.005), and the linear regression standard error of the estimate (SEE) was significantly lower (p less than 0.005) for the RV ESPVR defined by maximal elastance compared with modifications of the ESPVR with the ratio of pulmonary artery-dicrotic notch pressure or RV peak pressure to end-ejection volume. Dobutamine or atrial pacing tachycardia produced a leftward shift of the entire RV pressure-volume loop, and in each patient (five of five), the point of maximal elastance fell outside the 95% confidence interval defined by the baseline ESPVR. However, because of the larger SEE, the leftward shift with modifications of the ESPVR was not statistically significant in any patient by the pulmonary artery-dicrotic notch pressure: end-ejection volume ratio and was significant in only one of five patients by the RV peak pressure: end-ejection volume ratio (p less than 0.03). Therefore, it appears that the steady-state RV ESPVR defined by maximal elastance in patients with normal RV function is responsive to alterations in inotropic state and is more sensitive to alterations in RV function than the frequently used, more easily measured modifications of the RV ESPVR.

Diastolic properties of the normal left ventricle during supine exercise

Diastolic function in response to dynamic exercise was studied by biplane left ventriculography and by measuring left ventricular pressure with a high fidelity micromanometer tipped catheter at rest and during supine bicycle exercise in nine normal subjects. During exercise there was a fall in end systolic volume, in the time constant of left ventricular isovolumic pressure decay, and in the lowest diastolic pressure. Stroke volume, peak filling rate, mean passive filling rate, and the volume at the lowest diastolic pressure increased. There was an increase in the number of time constants that had elapsed before the lowest diastolic pressure was reached and the slope of the pressure-volume curves during passive filling (delta P/delta V) increased without changes in end diastolic pressure and volume. These results show that during exercise elastic recoil is enhanced and left ventricular relaxation is faster and more complete. Both phenomena reduce the lowest diastolic filling pressure. The observed increase in chamber stiffness from rest to exercise is probably related to increased resistance of the left ventricular wall caused by higher passive filling rates. The enhanced early diastolic pressure decay during exercise allows stroke volume to increase despite an increase in diastolic viscoelastic resistance and chamber stiffness.

Impaired regional diastolic distensibility in coronary artery disease: relations with dynamic left ventricular compliance

The regional left ventricular distensibility and its relations with the dynamic left ventricular chamber compliance were studied in 11 normal subjects and in 30 patients with coronary artery disease. The regional peak filling rates were calculated from angiographic data in eight ventricular segments and used as an index of regional distensibility. A depressed global peak filling rate was observed in only 30% of the patients with angina pectoris, but regional abnormalities in peak filling rate were detected in 75% of these patients. A relation between alterations in regional peak filling rate and left ventricular compliance was evident in these patients. Despite comparable end diastolic volume and pressure (10 +/- 2 mm Hg vs. 10 +/- 3 in normal subjects; not significant), the patients with angina pectoris, whose ventricle had at least three segments with a reduced peak filling rate, had indeed significant increases in mean left ventricular filling pressure (14 +/- 4 mm Hg vs. 8 +/- 3 in normal subjects; p less than 0.01) and upward shifts of their left ventricular pressure-volume relation during rapid filling. Conversely, an increase in regional peak filling rate produced by intravenous administration of the calcium antagonist nicardipine in a subgroup of patients with poor diastolic function was accompanied by a reduction in mean left ventricular filling pressure and by a downward shift of the early diastolic left ventricular pressure-volume relation. It is concluded that even in the absence of clinical signs of ischemia and of a previous myocardial infarction, large areas with impaired distensibility are frequently present in patients with angina pectoris.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of coronary occlusion during percutaneous transluminal angioplasty in humans on left ventricular chamber stiffness and regional diastolic pressure-radius relations

The effect of repeated (3 to 10 second) and transient (15 to 75 second) abrupt coronary occlusion on the global and regional chamber stiffness was studied in nine patients undergoing angioplasty of a single proximal left anterior descending coronary artery stenosis. The left ventricular high fidelity pressure and volume relation was obtained before and after the procedure as well as during coronary occlusion, after 20 seconds (n = 9) and after 50 seconds (n = 5). During ischemia, there was an upward shift of the pressure-volume relation. The nonlinear simple elastic constant of chamber stiffness increased from 0.0273 +/- 0.017 before angioplasty (mean +/- SD) to 0.0621 +/- 0.026 after 20 seconds of occlusion (p less than 0.05) and 0.0605 +/- 0.015 after 50 seconds of occlusion (p less than 0.01). In five patients, the postangioplasty value remained higher than the control value, but at the group level the mean value (0.0529 +/- 0.037) was not statistically different. The regional stiffness was determined from the changes in the length of six segmental radii during diastole, from the lowest diastolic to the end-diastolic pressure. The regional constant of elastic stiffness was unaffected in the nonischemic zone. In the adjacent and ischemic zones, the regional stiffness was increased during occlusion (p less than 0.05). These regional abnormalities in diastolic function persisted at the time of postangioplasty measurements, 12 minutes after the end of the procedure. This suggests that recovery of normal diastolic function after repeated ischemic injuries is delayed after restoration of normal blood flow and systolic function.

Diastolic properties of the hypertrophied left ventricle in spontaneously hypertensive rats

The influence of myocardial hypertrophy on left ventricular volume compliance was studied in vitro in isolated hearts of 4 and 19 month old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). In both SHR groups diastolic volume compliance was similar to that in the controls, despite the presence of left ventricular hypertrophy. This seems to be mainly due to an altered geometric situation, since with increased wall thickness to internal radius ratio (w/ri), which was at hand, the less are outer myocardial layers stretched at a given increase in ventricular volume. This may imply that these layers will only little interfere with luminal distension (and thereby with diastolic volume compliance) in SHR. It was also observed that the progressive increase of ventricular hypertrophy from 4 to 19 months of age did not further increase w/ri in SHR, indicating an increase in overall ventricular size with age. Left ventricular end diastolic pressure (LVEDP) was also measured in conscious 5 week and 4 month old SHR compared with matched controls. LVEDP increased with the development of hypertension and was significantly elevated in 4 month old SHR. This will increase also the average diastolic pre-stretch of the SHR left ventricle and mobilize the "Starling mechanism" to maintain a normal stroke volume against the increased afterload for the heart in established hypertension. This seems particularly important since the hypertrophic w/ri increase (about 20%) is smaller than the great elevation of mean arterial pressure (40-50%) in SHR.

Influence of the velocity of changes in end-diastolic volume on the starling mechanism of isolated left ventricles

1. In this study the relationships between active developed systolic pressure, end-diastolic pressure and different diastolic volumes are studied in Tyrode perfused isolated rabbit left ventricles. Contractions were isovolumic. 2. Rapid diastolic volume changes were imposed on top of different preset basic diastolic volumes. These volume changes are shown to produce systolic and diastolic pressure values that cannot be explained by assuming a single pressure-volume relation during systole and diastole. The changes in pressure are in the same direction but higher than is expected on the basis of the increase or decrease of the ventricular end-diastolic volume alone. 3. The variation of the diastolic pressure-volume relation cannot be explained by assuming variations of the heart's passive elasticity or viscous effects within its wall. During diastole the effect is completely reversible without concomitant systolic effects. No velocity dependent effect of the quick volume change could be observed if the time duration was varied between 10 and 65 ms. The results are in keeping with the hypothesis that active force generating mechanisms may be present during the diastolic pause. 4. The effects observed during systole suggest the possibility of length dependent activation of the myocardial cells. This results in different inotropic conditions of the heart at identical volumes, depending on how these volumes were installed. These volumes may be considered to affect intrinsic properties of the muscle cells on a beat to beat basis.

Passive stress-strain relation for the right ventricle in diastole

Wall thickness and medial line radii of an in vitro canine heart are measured. These data are assumed to be characteristic of the in vivo ventricles subject to zero pressure, and in the absence of filling. The myocardium is taken to be homogeneous, isotropic, non-linearly elastic, and incompressible. The right ventricular free wall is modeled as a circular arch of constant thickness, fixed at the interventricular groove. Circumferential stress is determined from thrust, and circumferential strain from displacement, both at the crown of the midwall. Our purpose was to obtain a stress-strain relationship without inertia and ventricular filling, termed passive. The passive circumferential stress-strain relation for the right ventricle in diastole is shown to be an exponential equation with two parameters. These parameters are related to the product of material constants of in vivo heart, and functions of right ventricular geometry in terms of the ratio of wall thickness to arch radius, and the terminal value of the central angle. Using mean values of observations, right and left ventricular passive curves are plotted over the same representative strain interval in an example from lowest diastolic pressure to the start of atrial contraction.

Myocardial relaxation. III. Reoxygenation mechanics in the intact dog heart

Reoxygenation of hypoxic isolated cardiac muscle results in prolonged duration of contraction-relaxation. To determine whether similar mechanical changes occur in the intact left ventricle (LV), and especially to assess the influence of prolonged relaxation on LV diastolic stiffness, we examined LV pressure transients (micromanometer) and changes in myocardial segment length (ultrasonic transit time) during reoxygenation in 22 anesthetized dogs following 15 minutes of hypoxia (PaO2 = 21 +/- 2 mm Hg). The time constant (T) of LV isovolumic exponential pressure decline was used as an index of myocardial relaxation; LV end-diastolic stiffness was assessed from stiffness constants derived from multiple coordinates of end-diastolic pressure and segment length (volume loading). During reoxygenation, after LV systolic pressure and segment length measurements had returned to control levels, relaxation was prolonged; T increased from a control of 32 +/- 2 to 44 +/- 3 msec at 5 minutes of reoxygenation (P less than 0.01). Prolonged relaxation resulted in a consistent increase in LV early-diastolic pressures. Furthermore, calculated values for LV end-diastolic stiffness increased during reoxygenation when the next beat began less than 3.5 T after maximum negative dP/dt; this condition was present more frequently at a heart rate of 150 beats/min than at 120 beats/min. Thus, rapid correction of acute hypoxia in the dog results in prolonged LV relaxation; prolonged relaxation can influence LV end-diastolic stiffness when relaxation is sufficiently slow and/or when diastole is sufficiently short.

Influence of potassium cardioplegia versus ischemic arrest on regional left ventricular diastolic compliance in humans

To compare the effects of hypothermic ischemic arrest versus hypothermic potassium cardioplegia, regional left ventricular performance was monitored in 20 adult male patients undergoing saphenous vein bypass operation. Twelve patients received ischemic arrest (Group 1), and 8 received potassium cardioplegia (Group 2). Groups 1 and 2 did not differ in left ventricular ejection fraction (0.62 versus 0.60), number of bypassed vessels (3.7 versus 3.4), mean cross-clamp time (75 versus 63 minutes), or mean cardiopulmonary bypass time (182 versus 170 minutes). Before cardiopulmonary bypass was begun, a pair of ultrasonic crystals was secured in the left ventricular anterior myocardium to measure segment motion and a micromanometer-tipped catheter was placed in the left ventricular chamber. All patients received a saphenous vein bypass graft to a vessel supplying the anterior left ventricular wall in the region of the ultrasonic crystals. Comparison of changes in systolic measurements revealed no significant differences between Groups 1 and 2. After saphenous vein bypass grafting, the left ventricular end-diastolic pressure (11.4 to 17.0 mm HG) and modulus of left ventricular segment stiffness (0.37 to 0.67, p less than 0.02) were elevated in Group 1 but no changes were observed in Group 2 (14.0 to 15.6 mm Hg, and 0.16 to 0.24, respectively). Compared with hypothermic ischemic arrest, hypothermic potassium cardioplegia is not associated with an increased left ventricular diastolic stiffness shortly after saphenous vein bypass grafting in humans.

Interaction of left ventricular relaxation and filling during early diastole in human subjects

Seventeen patients with coronary artery disease were studied with cineangiography and simultaneous tip manometry at resting heart rate and maximal tachycardia induced by atrial pacing. During early diastole, defined as the interval from the opening of the mitral valve to the point of minimal left ventricular pressure, 20 percent of total ventricular filling took place at resting heart rate, but 62 percent occurred during tachycardia. Minimal pressure was significantly correlated with the time constant of pressure decay during the isovolumic phase (r = 0.75 at resting heart rate and r = 0.81 during tachycardia). The measured minimal pressure could be predictd by extrapolating the exponential decay of ventricular isovolumic pressure to the time of occurence of the minimal pressure, which occurred on average 2.7 time constants from the peak negative rate of change of pressure. At resting heart rate the time constant of relaxation was inversely correlated with ventricular inflow volume (r = -0.64) and inflow rate (r = -0.72). It is concluded that left ventricular relaxatin has a relevant role in early diastolic pressure-volume relations and increases during tachycardia.