Viscoelastic properties of the diastolic left ventricle in the conscious dog

Don't go breakin' my heart: cardioprotective alterations to the mechanical and structural properties of reperfused myocardium during post-infarction inflammation

Myocardial infarctions (MIs) kickstart an intense inflammatory response resulting in extracellular matrix (ECM) degradation, wall thinning, and chamber dilation that leaves the heart susceptible to rupture. Reperfusion therapy is one of the most effective strategies for limiting adverse effects of MIs, but is a challenge to administer in a timely manner. Late reperfusion therapy (LRT; 3 + hours post-MI) does not limit infarct size, but does reduce incidences of post-MI rupture and improves long-term patient outcomes. Foundational studies employing LRT in the mid-twentieth century revealed beneficial reductions in infarct expansion, aneurysm formation, and left ventricle dysfunction. The mechanism by which LRT acts, however, is undefined. Structural analyses, relying largely on one-dimensional estimates of ECM composition, have found few differences in collagen content between LRT and permanently occluded animal models when using homogeneous samples from infarct cores. Uniaxial testing, on the other hand, revealed slight reductions in stiffness early in inflammation, followed soon after by an enhanced resistance to failure for cases of LRT. The use of one-dimensional estimates of ECM organization and gross mechanical function have resulted in a poor understanding of the infarct's spatially variable mechanical and structural anisotropy. To resolve these gaps in literature, future work employing full-field mechanical, structural, and cellular analyses is needed to better define the spatiotemporal post-MI alterations occurring during the inflammatory phase of healing and how they are impacted following reperfusion therapy. In turn, these studies may reveal how LRT affects the likelihood of rupture and inspire novel approaches to guide scar formation.

Micron-scale hysteresis measurement using dynamic optical coherence elastography

We present a novel optical coherence elastography (OCE) method to characterize mechanical hysteresis of soft tissues based on transient (milliseconds), low-pressure (<20 Pa) non-contact microliter air-pulse stimulation and micrometer-scale sample displacements. The energy dissipation rate (sample hysteresis) was quantified for soft-tissue phantoms (0.8% to 2.0% agar) and beef shank samples under different loading forces and displacement amplitudes. Sample hysteresis was defined as the loss ratio (hysteresis loop area divided by the total loading energy). The loss ratio was primarily driven by the sample unloading response which decreased as loading energy increased. Samples were distinguishable based on their loss ratio responses as a function loading energy or displacement amplitude. Finite element analysis and mechanical testing methods were used to validate these observations. We further performed the OCE measurements on a beef shank tissue sample to distinguish the muscle and connective tissue components based on the displacement and hysteresis features. This novel, noninvasive OCE approach has the potential to differentiate soft tissues by quantifying their viscoelasticity using micron-scale transient tissue displacement dynamics. Focal tissue hysteresis measurements could provide additional clinically useful metrics for guiding disease diagnosis and tissue treatment responses.

Need for Speed: The Importance of Physiological Strain Rates in Determining Myocardial Stiffness

The heart is viscoelastic, meaning its compliance is inversely proportional to the speed at which it stretches. During diastolic filling, the left ventricle rapidly expands at rates where viscoelastic forces impact ventricular compliance. In heart disease, myocardial viscoelasticity is often increased and can directly impede diastolic filling to reduce cardiac output. Thus, treatments that reduce myocardial viscoelasticity may provide benefit in heart failure, particularly for patients with diastolic heart failure. Yet, many experimental techniques either cannot or do not characterize myocardial viscoelasticity, and our understanding of the molecular regulators of viscoelasticity and its impact on cardiac performance is lacking. Much of this may stem from a reliance on techniques that either do not interrogate viscoelasticity (i.e., use non-physiological rates of strain) or techniques that compromise elements that contribute to viscoelasticity (i.e., skinned or permeabilized muscle preparations that compromise cytoskeletal integrity). Clinically, cardiac viscoelastic characterization is challenging, requiring the addition of strain-rate modulation during invasive hemodynamics. Despite these challenges, data continues to emerge demonstrating a meaningful contribution of viscoelasticity to cardiac physiology and pathology, and thus innovative approaches to characterize viscoelasticity stand to illuminate fundamental properties of myocardial mechanics and facilitate the development of novel therapeutic strategies.

Identifying the parameters of viscoelastic model for a gel-type material as representative of cardiac muscle in dynamic tests

In this paper, mechanical parameters of a calf heart muscle are identified and a gel-type material as the representative of the cardiac muscle in dynamic tests is introduced. The motivation of this study is to introduce a replacement material of the heart muscle to use in experimental studies of the leadless pacemaker. A particular test setup is developed to capture the experimental data based on the stress relaxation test method where its outputs are time histories of the force and displacement. The standard linear solid model is used for mathematical modeling of the heart muscle sample and a gel-type material specimen namely α-gel. Five tests with different strain history (13.6%,17.1%,20.6%22.4%and,23.8%) are performed by regarding and disregarding the influence of the initial ramp of the loading. The mechanical parameters of the standard linear solid model were identified with precise curve fitting. Consideration of the initial ramp significantly influences the consequences and they are so close to their experimental counterparts. The identified parameters of the standard linear solid model by regarding the influence of the initial ramp for the gel-type material are within an acceptable range for the viscoelastic properties of the calf heart tissue. These results show that the gel-type material has the potential to represent the cardiac muscle in the leadless pacemaker experimental studies. Dynamic mechanical analysis is used to characterize the dynamic viscoelastic properties for the gel by utilizing the identified parameters with taking into account the initial ramp in the frequency domain. Results show that Storage modulus, Loss modulus, and Loss tangent are strongly frequency-dependent especially at low-frequency around the heartbeat frequency range (0-2 Hz).

Diffuse Myocardial Fibrosis and Diastolic Function in Aortic Stenosis

OBJECTIVES

The aim of this study was to investigate the relationship between extracellular volume fraction (ECV), a noninvasive parameter that quantifies the degree of diffuse myocardial fibrosis on cardiac magnetic resonance (CMR), and left ventricular diastolic dysfunction (LVDD) in patients with aortic stenosis (AS).

BACKGROUND

Myocardial fibrosis on invasive myocardial biopsy is associated with LVDD. However, there is a paucity of data on the association between noninvasively quantified diffuse myocardial fibrosis and the degree of LVDD and how these are related to symptoms and long-term prognosis in patients with AS.

METHODS

Patients with moderate or severe AS (n = 191; mean age 68.4 years) and 30 control subjects without cardiovascular risk factors underwent CMR. LVDD grade was evaluated using echocardiography according to the 2016 American Society of Echocardiography/European Association of Cardiovascular Imaging guidelines. Clinical outcomes were defined as a composite of all-cause mortality or hospitalization for heart failure aggravation.

RESULTS

Patients in higher ECV quintiles had a significantly higher prevalence of LVDD. Higher ECV was particularly associated with decreased myocardial relaxation (septal e' <7 cm/s) and increased LV filling pressure (E/e' ratio ≥15). Although both impaired diastolic function and higher ECV were significantly associated with a worse degree of dyspnea, patients with higher ECV showed greater dyspnea within the same grade of LVDD. During a median follow-up period of 5.6 years, 37 clinical events occurred. Increased ECV, as well as lower septal e' and higher E/septal e' ratio, were independent predictors of clinical events, irrespective of age, AS severity, aortic valve replacement, and left ventricular (LV) ejection fraction. ECV provided incremental prognostic value on top of clinical factors and LV systolic and diastolic function.

CONCLUSIONS

Diffuse myocardial fibrosis, assessed using ECV on CMR, was associated with LVDD in patients with AS, but both ECV and LV diastolic function parameters provided a complementary explanation for dyspnea and clinical outcomes. Concomitant assessment of both LVDD and diffuse myocardial fibrosis may further identify patients with AS with greater symptoms and worse prognosis.

Left ventricular geometry during unloading and the end-systolic pressure volume relationship: Measurement with a modified real-time MRI-based method in normal sheep

The left ventricular (LV) end-systolic (ES) pressure volume relationship (ESPVR) is the cornerstone of systolic LV function analysis. We describe a 2D real-time (RT) MRI-based method (RTPVR) with separate software tools for 1) semi-automatic level set-based shape prior method (LSSPM) of the LV, 2) generation of synchronized pressure area loops and 3) calculation of the ESPVR. We used the RTPVR method to measure ventricular geometry, ES pressure area relationship (ESPAR) and ESPVR during vena cava occlusion (VCO) in normal sheep. 14 adult sheep were anesthetized and underwent measurement of LV systolic function. Ten of the 14 sheep underwent RTMRI and eight of the 14 underwent measurement with conductance catheter; 4 had both RTMRI and conductance measurements. 2D cross sectional RTMRI were performed at apex, mid-ventricle and base levels during separate VCOs. The Dice similarity coefficient was used to compare LSSPM and manual image segmentation and thus determine LSSPM accuracy. LV cross-sectional area, major and minor axis length, axis ratio, major axis orientation angle and ESPAR were measured at each LV level. ESPVR was calculated with a trapezoidal rule. The Dice similarity coefficient between LSSPM and manual segmentation by two readers was 87.31±2.51% and 88.13±3.43%. All cross sections became more elliptical during VCO. The major axis orientation shifted during VCO but remained in the septo-lateral direction. LV chamber obliteration at the apical level occurred during VCO in 7 of 10 sheep that underwent RTMRI. ESPAR was non-linear at all levels. Finally, ESPVR was non-linear because of apical collapse. ESPVR measured by conductance catheter (E_ES,Index = 2.23±0.66 mmHg/ml/m²) and RT (E_ES,Index = 2.31±0.31 mmHg/ml/m²) was not significantly different. LSSPM segmentation of 2D RT MRI images is accurate and allows calculation of LV geometry, ESPAR and ESPVR during VCO. In the future, RTPVR will facilitate determination of regional systolic material parameters underlying ESPVR.

Microtubules Increase Diastolic Stiffness in Failing Human Cardiomyocytes and Myocardium

BACKGROUND

Diastolic dysfunction is a prevalent and therapeutically intractable feature of heart failure (HF). Increasing ventricular compliance can improve diastolic performance, but the viscoelastic forces that resist diastolic filling and become elevated in human HF are poorly defined. Having recently identified posttranslationally detyrosinated microtubules as a source of viscoelasticity in cardiomyocytes, we sought to test whether microtubules contribute meaningful viscoelastic resistance to diastolic stretch in human myocardium.

METHODS

Experiments were conducted in isolated human cardiomyocytes and trabeculae. First, slow and rapid (diastolic) stretch was applied to intact cardiomyocytes from nonfailing and HF hearts and viscoelasticity was characterized after interventions targeting microtubules. Next, intact left ventricular trabeculae from HF patient hearts were incubated with colchicine or vehicle and subject to pre- and posttreatment mechanical testing, which consisted of a staircase protocol and rapid stretches from slack length to increasing strains.

RESULTS

Viscoelasticity was increased during diastolic stretch of HF cardiomyocytes compared with nonfailing counterparts. Reducing either microtubule density or detyrosination reduced myocyte stiffness, particularly at diastolic strain rates, indicating reduced viscous forces. In myocardial tissue, we found microtubule depolymerization reduced myocardial viscoelasticity, with an effect that decreased with increasing strain. Colchicine reduced viscoelasticity at strains below, but not above, 15%, with a 2-fold reduction in energy dissipation upon microtubule depolymerization. Post hoc subgroup analysis revealed that myocardium from patients with HF with reduced ejection fraction were more fibrotic and elastic than myocardium from patients with HF with preserved ejection fraction, which were relatively more viscous. Colchicine reduced viscoelasticity in both HF with preserved ejection fraction and HF with reduced ejection fraction myocardium.

CONCLUSIONS

Failing cardiomyocytes exhibit elevated viscosity and reducing microtubule density or detyrosination lowers viscoelastic resistance to diastolic stretch in human myocytes and myocardium. In failing myocardium, microtubules elevate stiffness over the typical working range of strains and strain rates, but exhibited diminishing effects with increasing length, consistent with an increasing contribution of the extracellular matrix or myofilament proteins at larger excursions. These studies indicate that a stabilized microtubule network provides a viscous impediment to diastolic stretch, particularly in HF.

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.

Captopril Attenuates Cardiovascular and Renal Disease in a Rat Model of Heart Failure With Preserved Ejection Fraction

Heart failure with preserved ejection fraction (HFpEF), a prevalent form of heart failure, is frequently accompanied by the metabolic syndrome and kidney disease. Because current treatment options of HFpEF are limited, evaluation of therapies in experimental models of HFpEF with the metabolic syndrome and kidney disease is needed. In this study, we evaluated the effects of captopril, furosemide, and their combination in aged, obese ZSF1 rats, an animal model of HFpEF with the metabolic syndrome and chronic kidney disease as comorbidities. Captopril (100 mg/kg), furosemide (50 mg/kg), or their combination was administered orally to obese ZSF1 rats aged 20 to 44 weeks. Untreated ZSF1 rats served as controls. After 24 weeks of treatment, captopril significantly lowered systemic blood pressure and attenuated HFpEF as evidenced by significantly reduced left ventricular end diastolic pressures (10.5 ± 1.4 vs. 4.9 ± 1.3 mm Hg in Control vs. Captopril, respectively) and significantly lower left ventricular relaxation time constants (28.1 ± 2.9 vs. 18.3 ± 3.1 ms in Control vs. Captopril, respectively). The captopril-induced improvement in left ventricular function was associated with reduced cardiac hypertrophy, ischemia, necrosis, and vasculitis. Captopril also increased renal blood flow and glomerular filtration rate, reduced renal vascular resistance and proteinuria, and improved renal histology (ie, reduced renal hypertrophy, glomerulosclerosis, and tubular atrophy/dilation). Furosemide alone provided little benefit; moreover, furosemide did not augment the therapeutic benefits of captopril. This study suggests that chronic administration of captopril, but not furosemide, could be beneficial in patients with HFpEF, particularly in those with comorbidities such as obesity, diabetes, and dyslipidemias.

Mitral annuloplasty ring suture forces: Impact of surgeon, ring, and use conditions

OBJECTIVE

The study objective was to quantify the effect of ring type, ring-annulus sizing, suture position, and surgeon on the forces required to tie down and constrain a mitral annuloplasty ring to a beating heart.

METHODS

Physio (Edwards Lifesciences, Irvine, Calif) or Profile 3D (Medtronic, Dublin, Ireland) annuloplasty rings were instrumented with suture force transducers and implanted in ovine subjects (N = 23). Tie-down forces and cyclic contractile forces were recorded and analyzed at 10 suture positions and at 3 levels of increasing peak left ventricular pressure.

RESULTS

Across all conditions, tie-down force was 2.7 ± 1.4 N and cyclic contractile force was 2.0 ± 1.2 N. Tie-down force was not meaningfully affected by any factor except surgeon. Significant differences in overall and individual tie-down forces were observed between the 2 primary implanting surgeons. No other factors were observed to significantly affect tie-down force. Contractile suture forces were significantly reduced by ring-annulus true sizing. This was driven almost exclusively by Physio cases and by reduction along the anterior aspect, where dehiscence is less common clinically. Contractile suture forces did not differ significantly between ring types. However, when undersizing, Profile 3D forces were significantly more uniform around the annular circumference. A suture's tie-down force did not correlate to its eventual contractile force.

CONCLUSIONS

Mitral annuloplasty suture loading is influenced by ring type, ring-annulus sizing, suture position, and surgeon, suggesting that reports of dehiscence may not be merely a series of isolated errors. When compared with forces known to cause suture dehiscence, these in vivo suture loading data aid in establishing potential targets for reducing the occurrence of ring dehiscence.

How Local Annular Force and Collagen Density Govern Mitral Annuloplasty Ring Dehiscence Risk

BACKGROUND

Annuloplasty ring dehiscence is a well described mode of mitral valve repair failure. Defining the mechanisms underlying dehiscence may facilitate its prevention.

METHODS

Factors that govern suture dehiscence were examined with an ovine model. After undersized ring annuloplasty in live animals (n = 5), cyclic force (FC) that acts on sutures during cardiac contraction was measured with custom transducers. FC was measured at ten suture positions, throughout cardiac cycles with peak left ventricular pressure (LVPmax) of 100, 125, and 150 mm Hg. Suture pullout testing was conducted on explanted mitral annuli (n = 12) to determine suture holding strength at each position. Finally, relative collagen density differences at suture sites around the annulus were assessed by two-photon excitation fluoroscopy.

RESULTS

Anterior FC exceeded posterior FC at each LVPmax (eg, 2.8 ± 1.3 N versus 1.8 ± 1.2 N at LVPmax = 125 mm Hg, p < 0.01). Anterior holding strength exceeded posterior holding strength (6.4 ± 3.6 N versus 3.9 ± 1.6 N, p < 0.0001). On the basis of FC at LVPmax of 150 mm Hg, margin of safety before suture pullout was vastly higher between the trigones (exclusive) versus elsewhere (4.8 ± 0.9 N versus 1.9 ± 0.5 N, p < 0.001). Margin of safety exhibited strong correlation to collagen density (R(2) = 0.947).

CONCLUSIONS

Despite lower cyclic loading on posterior sutures, the weaker posterior mitral annular tissue creates higher risk of dehiscence, apparently because of reduced collagen content. Sutures placed atop the trigones are less secure than predicted, because of a combination of reduced collagen and higher overall rigidity in this region. These findings highlight the inter-trigonal tissue as the superior anchor and have implications on the design and implantation techniques for next-generation mitral prostheses.

Dual A1/A2B Receptor Blockade Improves Cardiac and Renal Outcomes in a Rat Model of Heart Failure with Preserved Ejection Fraction

Heart failure with preserved ejection fraction (HFpEF) is prevalent and often accompanied by metabolic syndrome. Current treatment options are limited. Here, we test the hypothesis that combined A1/A2B adenosine receptor blockade is beneficial in obese ZSF1 rats, an animal model of HFpEF with metabolic syndrome. The combined A1/A2B receptor antagonist 3-[4-(2,6-dioxo-1,3-dipropyl-7H-purin-8-yl)-1-bicyclo[2.2.2]octanyl]propanoic acid (BG9928) was administered orally (10 mg/kg/day) to obese ZSF1 rats (n = 10) for 24 weeks (from 20 to 44 weeks of age). Untreated ZSF1 rats (n = 9) served as controls. After 24 weeks of administration, BG9928 significantly lowered plasma triglycerides (in mg/dl: control group, 4351 ± 550; BG9928 group, 2900 ± 551) without adversely affecting plasma cholesterol or activating renin release. BG9928 significantly decreased 24-hour urinary glucose excretion (in mg/kg/day: control group, 823 ± 179; BG9928 group, 196 ± 80) and improved oral glucose tolerance, polydipsia, and polyuria. BG9928 significantly augmented left ventricular diastolic function in association with a reduction in cardiac vasculitis and cardiac necrosis. BG9928 significantly reduced 24-hour urinary protein excretion (in mg/kg/day: control group, 1702 ± 263; BG9928 group, 1076 ± 238), and this was associated with a reduction in focal segmental glomerulosclerosis, tubular atrophy, tubular dilation, and deposition of proteinaceous material in the tubules. These findings show that, in a model of HFpEF with metabolic syndrome, A1/A2B receptor inhibition improves hyperlipidemia, exerts antidiabetic actions, reduces HFpEF, improves cardiac histopathology, and affords renal protection. We conclude that chronic administration of combined A1/A2B receptor antagonists could be beneficial in patients with HFpEF, in particular those with comorbidities such as obesity, diabetes, and dyslipidemias.

Viscoelastic properties of normal and infarcted myocardium measured by a multifrequency shear wave method: comparison with pressure-segment length method

Our aims were (i) to compare in vivo measurements of myocardial elasticity by shear wave dispersion ultrasound vibrometry (SDUV) with those by the conventional pressure-segment length method, and (ii) to quantify changes in myocardial viscoelasticity during systole and diastole after reperfused acute myocardial infarction. The shear elastic modulus (μ1) and viscous coefficient (μ2) of left ventricular myocardium were measured by SDUV in 10 pigs. Young's elastic modulus was independently measured by the pressure-segment length method. Measurements made with the SDUV and pressure-segment length methods were strongly correlated. At reperfusion, μ1 and μ2 in end-diastole were increased. Less consistent changes were found during systole. In all animals, μ1 increased linearly with left ventricular pressure developed during systole. Preliminary results suggest that μ1 is preload dependent. This is the first study to validate in vivo measurements of myocardial elasticity by a shear wave method. In this animal model, the alterations in myocardial viscoelasticity after a myocardial infarction were most consistently detected during diastole.

The diastatic pressure-volume relationship is not the same as the end-diastolic pressure-volume relationship

The end-diastolic pressure-volume (P-V) relationship (EDPVR) is routinely used to determine the passive left ventricular (LV) stiffness, although the diastatic P-V relationship (D-PVR) has also been measured. Based on the physiological difference between diastasis (the LV and atrium are relaxed and static) and end diastole (LV volume increased by atrial systole and the atrium is contracted), we hypothesized that, although both D-PVR and EDPVR include LV chamber stiffness information, they are two different, distinguishable P-V relations. Cardiac catheterization determined LV pressures, and conductance volumes in 31 subjects were analyzed. Physiological, beat-to-beat variation of the diastatic and end-diastolic P-V points were fit by linear and exponential functions to generate the D-PVR and EDPVR. The extrapolated exponential D-PVR underestimated LVEDP in 82% of the heart beats (P < 0.001). The extrapolated EDPVR overestimated pressure at diastasis in 84% of the heart beats (P < 0.001). If each subject's diastatic and end-diastolic P-V data were combined to form a continuous data set to be fit by one exponential relation, the goodness of fit was always worse than if the diastatic and end-diastolic data were grouped separately and fit by two distinct exponential relations. Diastatic chamber stiffness was less than EDPVR stiffness (defined by the slope of P-V relation) for all 31 subjects (0.16 +/- 0.11 vs. 0.24 +/- 0.15 mmHg/ml, P < 0.001). We conclude that the D-PVR and EDPVR are distinguishable. Because it is not coupled to a contracted atrium, the D-PVR conveys passive LV stiffness better than the EDPVR. Additional studies that fully elucidate the physiology and biology of diastasis in health and disease are in progress.

E-wave deceleration time may not provide an accurate determination of LV chamber stiffness if LV relaxation/viscoelasticity is unknown

Average left ventricular (LV) chamber stiffness (ΔPavg/ΔVavg) is an important diastolic function index. An E-wave-based determination of ΔPavg/ΔVavg (Little WC, Ohno M, Kitzman DW, Thomas JD, Cheng CP. Circulation 92: 1933–1939, 1995) predicted that deceleration time (DT) determines stiffness as follows: ΔPavg/ΔVavg = N(π/DT)2 (where N is constant), which implies that if the DTs of two LVs are indistinguishable, their stiffness is indistinguishable as well. We observed that LVs with indistinguishable DTs may have markedly different ΔPavg/ΔVavg values determined by simultaneous echocardiography-catheterization. To elucidate the mechanism by which LVs with indistinguishable DTs manifest distinguishable chamber stiffness, we use a validated, kinematic E-wave model (Kovács SJ, Barzilai B, Perez JE. Am J Physiol Heart Circ Physiol 252: H178–H187, 1987) with stiffness ( k) and relaxation/viscoelasticity ( c) parameters. Because the predicted linear relation between k and ΔPavg/ΔVavg has been validated, we reexpress the DT-stiffness (ΔPavg/ΔVavg) relation of Little et al. as follows: DT k ≈ [Formula: see text]. Using the kinematic model, we derive the general DT-chamber stiffness/viscoelasticity relation as follows: DT k, c = [Formula: see text](where c and k are determined directly from the E-wave), which reduces to DT k when c ≪ k. Validation involved analysis of 400 E-waves by determination of five-beat averaged k and c from 80 subjects undergoing simultaneous echocardiography-catheterization. Clinical E-wave DTs were compared with model-predicted DT k and DT k, c. Clinical DT was better predicted by stiffness and relaxation/viscoelasticity ( r2 = 0.84, DT vs. DT k, c) jointly rather than by stiffness alone ( r2 = 0.60, DT vs. DT k). Thus LVs can have indistinguishable DTs but significantly different ΔPavg/ΔVavg if chamber relaxation/viscoelasticity differs. We conclude that DT is a function of both chamber stiffness and chamber relaxation viscoelasticity. Quantitative diastolic function assessment warrants consideration of simultaneous stiffness and relaxation/viscoelastic effects.

Examining potential therapies targeting myocardial fibrosis through the inhibition of transforming growth factor-beta 1

After injury, the heart undergoes a remodeling process consisting primarily of myocyte hypertrophy, apoptosis and interstitial fibrosis. Although initially beneficial, excess fibrosis gradually results in alteration of left ventricular properties and cardiac dysfunction. Transforming growth factor-beta 1 (TGF-beta(1)) is thought to be a primary mediator of fibrosis within the heart after injury. Currently, angiotensin II blockade is used to inhibit the actions of TGF-beta(1). However, recent studies indicate that angiotensin II blockade alone may not be sufficient to prevent TGF-beta(1)-induced fibrosis. Thus far, both in vivo and in vitro models have shown that direct TGF-beta(1) inhibition, NAPDH oxidase inhibitors, growth factors and hormonal treatment regimens targeting TGF-beta(1) may significantly reduce cardiac fibrosis after injury. This study attempts to underline these alternatives to angiotensin II blockade in combating TGF-beta(1)-induced cardiac dysfunction.

Stiffness- and relaxation-based quantitation of radial left ventricular oscillations: elucidation of regional diastolic function mechanisms

Traditionally, global and longitudinal (i.e., regional) left ventricular (LV) diastolic function (DF) assessment has utilized features of transmitral Doppler E and A waves or Doppler tissue imaging (DTI)-derived mitral annular E' and A' waves, respectively. Quantitation of regional DF has included M-mode echocardiography-based approaches and strain and strain rate imaging (in selected imaging planes), while analysis of mitral annular "oscillations" has recently provided a new window into longitudinal (long-axis) function. The remaining major spatial degree of kinematic freedom during diastole, radial (short-axis) motion, has not been fully characterized, nor has it been exploited for its potential to provide radial LV stiffness (k'(rad)) and relaxation/damping (c'(rad)) indexes. Prior characterization of regional (longitudinal) DF used only annular E'- and A'-wave peak velocities or, alternatively, myocardial strain and strain rate. By kinematically modeling short-axis tissue motion as damped radial oscillation, we present a novel method of estimating k'(rad) and c'(rad) during early filling. As required by the (near) constant-volume property of the heart and tissue/blood incompressibility, in subjects (n = 10) with normal DF, we show that oscillation duration-determined longitudinal (k'(long) and c'(long)) and radial (k'(long) and c'(rad)) parameters are highly correlated (R = 0.69 and 0.92, respectively). Selected examples of diabetic and LV hypertrophic subjects yield radial (k'(long) and c'(rad)) parameters that differ substantially from controls. Results underscore the utility of the incompressibility-based causal relation between DTI-determined mitral annular long-axis (longitudinal mode) and short-axis (radial mode) oscillations in healthy subjects. Selected pathological examples provide mechanistic insight and illustrate the value and potential role of regional (longitudinal and radial) DF indexes in fully characterizing normal vs. impaired DF states.

Effects of sodium nitroprusside in aortic stenosis associated with severe heart failure: pressure-volume loop analysis using a numerical model

In the recently published clinical study [Use of Nitroprusside in Left Ventricular Dysfunction and Obstructive Aortic Valve Disease (UNLOAD)], sodium nitroprusside (SNP) improved cardiac function in patients with severe aortic stenosis (AS) and left ventricular (LV) systolic dysfunction. We explored the possible mechanisms of these findings using a series of numerical simulations. A closed-loop lumped parameters model that consists of 24 differential equations relating pressure and flow throughout the circulation was used to analyze the effects of varying hemodynamic conditions in AS. Hemodynamic data from UNLOAD study subjects were used to construct the initial simulation. Systemic vascular resistance (SVR), heart rate, and aortic valve area were directly entered into the model while end-systolic and end-diastolic pressure-volume (P-V) relationships were adjusted using previously published data to match modeled and observed end-systolic and end-diastolic pressures and volumes. Initial simulation of SNP treatment by a reduction of SVR was not adequate. To obtain realistic model hemodynamics that reliably reproduce SNP treatment effects, we performed a series of simulations while simultaneously changing end-systolic elastance ( Ees), end-systolic volume at zero pressure (V0), and diastolic P-V shift. Our data indicate that either an Ees increase or V0 decrease is necessary to obtain realistic model hemodynamics. In five patients, we corroborated our findings by using the model to duplicate individual P-V loops obtained before and during SNP treatment. In conclusion, using a numerical model, we identified ventricular function parameters that are responsible for improved hemodynamics during SNP infusion in AS with LV dysfunction.

Diastolic dysfunction in the elderly--the interstitial issue

Diastolic dysfunction is increasingly recognized as a cause of congestive heart failure. Meta-analyses of earlier studies of this disorder suggest that 40%-50% of patients with the congestive heart failure syndrome have preserved left ventricular systolic function, with current estimates ranging up to 74%. Among patients >or=65 years of age with congestive heart failure, 55% of all subjects and 67% of women had normal systolic function. Histopathologic evaluation reveals a maladaptive remodeling of the interstitium associated with aging, resulting in an increase in interstitial collagen content. The interstitium normally plays a critical role in the generation of early diastolic suction. When there is a significant enough increase in myocardial collagen volume fraction, with its increased viscoelastic burden, this normal early diastolic suction is compromised and diastolic pressures increase. Left ventricular diastolic dysfunction ensues. Neurohumoral abnormalities associated with diastolic dysfunction include activation of the renin-angiotensin-aldosterone system, including increased elaboration of myocardial aldosterone. This excess of aldosterone appears to play a major role in the development of myocardial fibrosis. Recent observations in animal models and humans have demonstrated regression of interstitial collagen volume fraction in response to inhibition of the renin-angiotensin-aldosterone system by angiotensin-converting enzyme inhibitors and aldosterone inhibition, with improvement in diastolic function. Therapeutic implications of these observations suggest targeting the maladaptive remodeling of the interstitium via inhibition of the renin-angiotensin-aldosterone system.

Mechanisms of diastolic intraventricular regional pressure differences and flow in the inflow and outflow tracts

OBJECTIVES

We sought to investigate the mechanisms of left ventricular (LV) intracavitary early diastolic flow during changes in contractility and loading.

BACKGROUND

There is limited understanding of how intracavitary flow velocities relate to intraventricular driving pressures.

METHODS

In 12 anesthetized dogs, we measured pressures in the left atrium (LA), LV at the mitral tip, apex, and subaortic region; intraventricular velocities by color M-mode Doppler echocardiography (CMD); and volume by sonomicrometry. We also investigated responses to isoprenaline, ischemic failure, and volume loading.

RESULTS

During rapid, early filling, the mitral to apical pressure gradient (LVP(mitral-apex)) correlated with the peak mitral to apical velocity (r = 0.92). The LVP(mitral-apex) increased from 1.4 +/- 0.6 (SD) to 3.2 +/- 1.8 mm Hg during isoprenaline (p < 0.05) and decreased to 0.6 +/- 0.5 during ischemic failure (p < 0.01). The pressure gradient correlated positively with the time constant of isovolumic relaxation (tau) (r = 0.82) and negatively with LV end-systolic volume (ESV) (r = -0.77). Volume loading increased LA pressure, tau, and ESV, but caused no significant change in LVP(mitral-apex). At baseline and during isoprenaline, tau was shorter (p < 0.05) at the apex than at the base. When the mitral to apical gradient approached zero, filling velocities were directed toward the LV outflow tract, and a pressure gradient was established between the apex and subaortic region.

CONCLUSIONS

Changes in LVP(mitral-apex) induced by inotropic stimuli, loading, and ischemia appeared to reflect dependency of the pressure gradient on the rate of relaxation, ESV, and LA pressure. Regional differences in the rate of relaxation may also contribute to intraventricular pressure gradients. These findings have implications for how to interpret intraventricular filling in a clinical context.

Constitutive properties of hypertrophied myocardium: cellular contribution to changes in myocardial stiffness

Recent studies have suggested that pressure overload hypertrophy (POH) alters the viscoelastic properties of individual cardiocytes when studied in isolation. However, whether these changes in cardiocyte properties contribute causally to changes in the material properties of the cardiac muscle as a whole is unknown. Accordingly, a selective, isolated, acute change in cardiocyte constitutive properties was imposed in an in vitro system capable of measuring the resultant effect on the material properties of the composite cardiac muscle. POH caused an increase in both myocardial elastic stiffness, from 20.5 +/- 1.3 to 28.4 +/- 1.8, and viscous damping, from 15.2 +/- 1.1 to 19.8 +/- 1.5 s (normal vs. POH, P < 0.05), respectively. Recent studies have shown that cardiocyte constitutive properties could be acutely altered by depolymerizing the microtubules with colchicine. Colchicine caused a significant decrease in the viscous damping in POH muscles (19.8 +/- 1.5 s at baseline vs. 14.7 +/- 1.3 s after colchicine, P < 0.05). Therefore, myocardial material properties can be altered by selectively changing the constitutive properties of one element within this muscle tissue, the cardiocyte. Changes in the constitutive properties of the cardiocytes themselves contribute to the abnormalities in myocardial stiffness and viscosity that develop during POH.

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.

Quantification of left ventricular diastolic pressure-volume relations during routine cardiac catheterization by two-dimensional digital echo quantification and left ventricular micromanometer

BACKGROUND

Currently there is no simple clinical method for quantifying the left ventricular (LV) diastolic pressure-volume relation. Echocardiographic-automated endocardial border detection, however, may be combined with LV micromanometer to construct LV pressure-volume loops. We investigated the feasibility of on-line display and sampling of LV pressure-volume loops by such an approach. For this purpose we used a new echocardiographic digital echo quantification (DEQ) method in combination with LV pressures on-line and in real-time.

METHODS

Eighteen patients were screened by conventional echocardiography and DEQ. Ten of the patients with high quality images were included in the study. Left ventricular pressures and volumes were recorded simultaneously and were displayed on-line as pressure-volume loops. Changes in LV volume were induced by intravenous saline. Left ventricular chamber compliance was estimated as change in volume divided by change in pressure from minimum diastolic pressure to end-diastolic pressure (average LV chamber compliance).

RESULTS

Left ventricular pressure-volume loops were displayed on-line during the examination. When compared with the Simpson's method, DEQ underestimated end-diastolic volume (EDV) by 35% and overestimated end-systolic volume (ESV) by 14%. Beat-to-beat variability for ESV and EDV were 7.4% +/- 0.8% and 7.2% +/- 0.7 %, respectively. Volume loading increased LV end-diastolic pressure (LVEDP) from 14.0 +/- 1.6 to 24.7 +/- 2.0 mm Hg (P <.05) and EDV from 79 +/- 10 to 85 +/- 11 mL (NS), and decreased LV chamber compliance from 4.0 +/- 0.7 to 2.0 +/- 0.3 mL/mm Hg (P <.05).

CONCLUSION

The current study demonstrates that LV pressure-volume loops can be displayed and evaluated in real-time during routine cardiac catheterization. This may represent a clinically useful method for identifying patients with reduced chamber compliance. The underestimation of the volumes by DEQ compared with the Simpson's method suggests that further refinements should be performed to improve the endocardial border detection algorithm.

Relationship of hydraulic impedance and elasticity in the pulmonary artery of maturing newborn pigs

The current study determined the dynamic stress-strain elastic moduli (E(Y)) and characteristic impedances (Z(0(2-7Hz))) of the main pulmonary artery in open-chest, anesthetized newborn pigs at 2 days, 2 weeks, and 3 months of age. E(Y) and Z(0(2-7Hz)) were compared to those values derived from the Womersley and Moens-Korteweg equations (denoted E(W-MK) and Z(0W-MK), respectively) to test the validity of these equations in describing the elasticity of the intact newborn pulmonary artery. E(Y) was defined as the ratio of stress to strain. The current study hypothesized that: (1) E(Y) and E(W-MK) are numerically similar, and therefore the Womersley and Moens-Korteweg equations accurately describe the viscoelastic properties of the main pulmonary artery of the newborn pig, and (2) that both E(Y) and Z(0) are elevated at birth and undergo a steady decline with maturation. E(Y) was not significantly different from E(W-MK), while Z(0(2-7Hz)) was nearly identical to Z(0W-MK) in all groups. The elastic modulus peaked (P < 0.001) in 2-week-old pigs compared with both younger and older animals, while Z(0(2-7Hz)) decreased with increasing age (48 h = 1237 +/- 251 [SEM] dyn s cm(-5), 2-week = 433 +/- 95 dyn s cm(-5), 3 month = 162 +/- 17 dyn s cm(-5), P < 0.001). These experiments validate the Womersley and Moens-Korteweg equations as accurately describing the elastic properties of the intact newborn pig pulmonary artery. These data also demonstrate that a diminution in Z(0) may occur even with increased wall stiffness, as observed in our 2-week-old pigs.

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.

Modeling of diastole

Modeling methods have been employed to further characterize the physical and physiologic processes of filling and diastolic function. They have led to more detailed understanding of the effect of alteration of physiologic parameters on the Doppler E-wave contour as well as pulmonary vein flow. Depending on the modeling approach, different aspects of the filling process have been considered from AV gradient and net compliance to atrial appendage function to the mechanical suction pump attribute of the heart. The models have been applied for further characterization of diastolic function and elucidation of novel basic physiologic relations. We trust that readers recognize that this article could not serve as a comprehensive and global review of the state-of-the-art in physiologic modeling, but rather as a selective overview, with emphasis on the main modeling principles and options currently in use. Modeling of systems physiology, especially as it relates to the function of the four-chamber heart, remains a fertile area of investigation. Future progress is likely to have profound influence on (noninvasive) diagnosis and quantitation of the effect of therapy and lead to continued discovery of "new" (macroscopic, cellular, and molecular biologic) physiology.

Physiology of diastolic function and transmitral pressure-flow relations

The study of diastolic function, in particular, the creative application of noninvasive modalities, such as echocardiography and MR imaging, requires an understanding and appreciation of the basic physiology of left ventricular filling dynamics. The physics and physiology of diastolic function and dysfunction is examined by relating the phasic patterns of transmitral flow to the properties of the cardiac chambers. Particular attention is paid to the equations governing the transmitral pressure-flow relations and the active and passive chamber properties that determine the flow patterns: Active relaxation, passive compliance, viscoelasticity, and elastic deformation. The physiologic role of diastolic suction is discussed within this context.

Assessment of diastolic dysfunction. Invasive modalities

In this article, the author sought to review the two primary components of diastolic function that are most directly and accurately determined using invasive methodologies. For chamber relaxation this is optimally achieved using a micromanometer catheter, whereas for chamber compliance (or its inverse stiffness) this is best achieved by combining this catheter with a measure of instantaneous volume from a conductance catheter, using data from multiple cycles. Even with the ideal data set, the analysis of both properties involves physiologic and often mathematical assumptions, and the extent to which the data do not match these assumptions, the derived indexes may be misleading. Care in the data collection, and awareness of the various factors and pitfalls involved with their analysis can undoubtedly improve the interpretations. As advances in noninvasive methods continue to evolve, reliance on invasive methodologies will continue to fade into the background. At present, however, they remain the gold standard for the two primary diastolic properties described, and have clearly played a central role in the evolution of our understanding of cardiac diastolic disease and its treatment.

Comparison of benefits on myocardial performance of cellular cardiomyoplasty with skeletal myoblasts and fibroblasts

Cellular cardiomyoplasty (CCM), or introduction of immature cells into terminally injured heart, can mediate repair of chronically injured myocardium. Several different cell types, ranging from embryonic stem cells to autologous skeletal myoblasts, have been successfully propagated within damaged heart and shown to improve myocardial performance. However, it is unclear if the functional advantages associated with CCM depend upon the use of myogenic cells or if similar results can be seen with other cell types. Thus, we compared indices of regional contractile (systolic) and diastolic myocardial performance following transplantation of either autologous skeletal myoblasts (Mb) or dermal fibroblasts (Fb) into chronically injured rabbit heart. In vivo left ventricular (LV) pressure (P) and regional segment length (SL) were determined in 15 rabbits by micromanometry and sonomicrometry 1 week following LV cryoinjury (CRYO) and again 3 weeks after autologous skeletal Mb or dermal Fb transplantation. Quantification of systolic performance was based on the linear regression of regional stroke work and end-diastolic (ED) SL. Regional diastolic properties were assessed using the curvilinear relationships between LVEDP and strain (epsilon) as well as LVEDP and EDSL. At study termination, cellular engraftment was characterized histologically in a blinded fashion. Indices of diastolic performance were improved following CCM with either Mb or Fb. However, only Mb transplantation improved systolic performance; Fb transfer actually resulted in a significant decline in systolic performance. These data suggest that both contractile and noncontractile cells can improve regional material properties or structural integrity of terminally injured heart, as reflected by improvements in diastolic performance. However, only Mb improved systolic performance in the damaged region, supporting the role of myogenic cells in augmenting contraction. Further studies are needed to define the mechanism by which these effects occur and to evaluate the long-term safety and efficacy of CCM with any cell type.

Early postoperative changes in regional systolic and diastolic left ventricular function after transmyocardial laser revascularization: a comparison of holmium:YAG and CO2 lasers

OBJECTIVES

The purpose of this study was to determine the short-term effects of transmyocardial laser revascularization (TMR) on regional left ventricular systolic and diastolic function, myocardial blood flow (MBF) and myocardial water content (MWC).

BACKGROUND

Clinical studies of TMR have noted a significant incidence of cardiac complications in the early postoperative period. However, the early post-treatment effects of laser therapy on the myocardium and their potential contribution to postoperative cardiac morbidity are unknown.

METHODS

Swine underwent holmium:yttrium-aluminum-garnet (holmium:YAG) (n = 12) or carbon dioxide (CO2) (n = 12) laser TMR. Regional systolic function for the lased and nonlased regions was quantitated using preload recruitable work area (PRWA) and regional diastolic function with the ventricular stiffness constant alpha.

RESULTS

Preload recruitable work area was significantly decreased in the lased regions both 1 (59.8+/-13.0% of baseline, p = 0.02) and 6 h (64.2+/-9.4% of baseline, p = 0.02) after holmium:YAG TMR. This decreased PRWA was associated with a significant reduction in MBF to the lased regions (13.2% reduction at 1 h, p = 0.02; 18.4% decrease at 6 h post-TMR, p = 0.01). These changes were not seen after CO2 laser TMR. A significant increase in MWC (1.4+/-0.3% increase with holmium:YAG, p = 0.004; 1+/-0.2% increase with CO2, p = 0.002) and alpha (217.4+/-44.2% of baseline 6 h post-holmium:YAG TMR, p = 0.05; 206+/-36.7% of baseline 6 h post-CO2 TMR, p = 0.03) was seen after TMR with both lasers.

CONCLUSIONS

In the early postoperative setting, impaired regional systolic function in association with regional ischemia is seen after TMR with a holmium:YAG laser. Both holmium:YAG and CO2 lasers are associated with increased MWC and impaired diastolic relaxation in the lased regions. These changes may explain the significant incidence of early postoperative cardiac morbidity. The impact of these findings on anginal relief and long-term outcome are not known.

Myogenic cell transplantation improves in vivo regional performance in infarcted rabbit myocardium

BACKGROUND

Although cardiac transplantation is an ideal treatment for end-stage heart disease, inadequate donor availability has stimulated efforts to manage terminally injured myocardium by other innovative methods. Autologous skeletal myoblast transplantation, or cellular cardiomyoplasty, is one method to potentially mediate myocardial repair within chronically injured hearts. However, few investigators have documented the ability of myogenic cells to alter load-insensitive indices of systolic and diastolic performance in vivo. In this study, both systolic and diastolic regional myocardial function were evaluated following left ventricular cryoinjury and compared with function after myogenic cell transplantation.

METHODS

Left ventricular pressure and segment length were determined in 9 rabbits by micromanometry and sonomicrometry 1 week following cryoinjury and 3 weeks after myoblast transplantation. At study termination, the extent of myoblast engraftment was determined by histologic analysis. Systolic performance was based on the linear regression of stroke work and end-diastolic segment length. Diastolic properties were evaluated by the curvilinear relationships between left ventricular pressure and strain, and left ventricular pressure and end-diastolic segment length.

RESULTS

Although mean indices of systolic performance were unchanged after cell transplantation, systolic performance improved in 3 animals. In contrast, myoblast engraftment was associated with significantly improved diastolic properties (strain and dynamic stiffness) in all animals.

CONCLUSIONS

These data quantify temporal changes in regional myocardial performance and suggest that cellular cardiomyoplasty improves diastolic compliance prior to affecting systolic performance. Cellular cardiomyoplasty, a potential therapeutic option for ischemic heart disease, appears to reverse diastolic creep and thus may represent a clinical alternative to transplantation in the near future.

Cellular cardiomyoplasty improves diastolic properties of injured heart

BACKGROUND

Acute myocardial infarction leads to loss of functional myocytes and structural integrity that often decreases diastolic compliance and increases resting myocardial segment length (diastolic creep). Successfully engrafting autologous skeletal myoblasts could improve compliance and potentially reverse creep. Thus, we transplanted myoblasts into cryoinjured rabbit heart (n = 15, CRYO) and measured regional diastolic properties in the presence (n = 9, +ENG) or absence (n = 6, -ENG) of engraftment.

MATERIALS AND METHODS

Left ventricular (LV) pressures (P) and myocardial segment lengths (SL) were measured in vivo by micromanometry and sonomicrometry after cryoinjury (CRYO) and again 3 weeks following transplantation of myoblasts. Performance was estimated from the relationships between end-diastolic (ED) P and strain (epsilon) or between EDP and EDSL. Compliance was characterized by strain (epsilon(8)) and dynamic stiffness (dP/dL(8)) at 8 mm Hg. Creep was characterized by resting myocardial segment length (EDSL(0)) and static stiffness at 8 mm Hg (m(stat8)).

RESULTS

Successful myoblast engraftment was determined via histologic examination. In nine +ENG animals, diastolic properties improved. Regional strain (epsilon(8)) increased (0.06 +/- 0.02 CRYO vs 0.10 +/- 0.04 +ENG; P = 0.0009) while dynamic stiffness (dP/dL(8)) decreased (43 +/- 23 mm Hg/mm CRYO vs 23 +/- 14 mm Hg/mm +ENG; P = 0.009). Static stiffness (m(stat8)) was unaffected (0.78 +/- 0.2 mm Hg/mm CRYO vs 0.72 +/- 0. 1 mm Hg/mm +ENG; P = 0.08), and creep did not occur (EDSL(0) = 10.3 +/- 2.8 CRYO vs 10.4 +/- 2.3 +ENG; P = 0.74). In the absence of myoblast engraftment (n = 6, -ENG), strain decreased (epsilon(8) = 0. 06 +/- 0.02 CRYO vs 0.05 +/- 0.02 -ENG; P = 0.048), but dynamic stiffness (dP/dL(8)) did not (36 +/- 19 mm Hg/mm CRYO vs 28 +/- 12 mm Hg/mm -ENG; P = 0.20). Furthermore, static stiffness decreased (0. 78 +/- 0.3 mm Hg/mm CRYO vs 0.65 +/- 0.2 mm Hg/mm -ENG; P = 0.05) and creep was obvious (EDSL(0) = 10.8 +/- 3.6 mm CRYO vs 13.0 +/- 4. 4 mm -ENG, P = 0.04).

CONCLUSIONS

Myoblast engraftment may partially overcome the loss of myocytes and structural integrity that often follow chronic myocardial ischemia. Improved compliance and reversal of diastolic creep suggest regeneration of viable muscle within once infarcted myocardium.

A novel arresting solution for study of postmortem pressure--volume curves of the rat left ventricle

OBJECTIVE

We have found diastolic properties of the rat heart extremely sensitive to the method used to induce arrest. Accordingly, we sought to develop a reliable solution for measuring the LV pressure-volume relationship (LVPVR) in the rat.

MATERIALS AND METHODS

The study had five phases: (i) K120Na100, consisting of KCl (120 mEq/L) in NaCl (100 mEq/L) diluted with distilled water was developed in preliminary experiments. (ii) ACI rats were arrested with 3 cc of 4 degreesC KCl (n = 6) or K120Na100 (n = 6) infused into the aortic root. The LVPVR was expressed as normalized volume (Vn) at standardized pressures. Myocardial water content (%MWC) was determined. (iii) Six hearts were arrested with K120Na100 and LVPVRs observed over 1 h. (iv) Four hearts were instrumented with sonomicrometry crystals to compare in vivo and postmortem pressure diameter data. (v) The relation between body weight and dry heart weight was determined in 48 animals.

RESULTS

In hearts arrested with KCl, mean Vn at pressures of 10, 15, and 20 mmHg (206 +/- 26, 306 +/- 21, and 336 +/- 25 microl, respectively) was significantly reduced vs K120Na100 hearts (345 +/- 11, 407 +/- 12, and 472 +/- 18 microl) (P < 0.05). Mean %MWC changed insignificantly. Vn at 20 mm Hg became significantly smaller vs initial data 60 min after arrest with K120Na100 (P < 0.05, ANOVA). No differences between in vivo and postmortem mean normalized diameter were observed. The correlation coefficient for the relation between body weight and dry heart weight was 0.80. Conclusions. K120Na100 at 4 degreesC reliably preserves LV diastolic properties in the rat heart for 30 min. Normalization of LV volume to body weight is justified.

Effect of myocardial hypertrophy on systolic and diastolic function in children: insights from the force-frequency and relaxation-frequency relationships

OBJECTIVE

The objective of this study was to evaluate the effect of myocardial hypertrophy on systolic and diastolic properties of the left ventricle in children.

BACKGROUND

In children with myocardial hypertrophy, ejection phase indices are invariably increased. However, indices of force-generation, e.g., end-systolic elastance and invasive indices of diastolic properties, have been studied infrequently in children with myocardial hypertrophy.

METHODS

We studied 10 children with congenital aortic stenosis or coarctation of aorta and nine control patients. Systolic properties were assessed from shortening fraction, end-systolic fiber elastance (Ef(es)) measured at resting heart rates, and force-frequency relationship measured at heart rates increasing from 110 to 160 beats per minute. Diastolic properties were assessed from time constant of relaxation (tau) at matched heart rates, chamber stiffness constant, myocardial stiffness constant, and relaxation-frequency relationship measured at gradually increasing heart rates.

RESULTS

Ef(es) remained unchanged by myocardial hypertrophy, however, tau was prolonged (tauL: 27.3+/-2.3 vs. 21.8+/-2.2 ms, p < 0.001; and tauD: 43.2+/-3.1 vs. 34.3+/-3.3 ms, p < 0.001). Both chamber and myocardial stiffness constants remained unchanged. Incremental increases in heart rate produced incremental improvement in both contraction and relaxation. Slopes of force-frequency and relaxation-frequency relationships remained unchanged in the experimental group. However, the relaxation-frequency relationship manifested a parallel shift upward.

CONCLUSIONS

In conscious, sedated children with myocardial hypertrophy, systolic function assessed by an index of force generation remains unchanged. However, relaxation is prolonged but passive diastolic properties remain unaffected. The combined effect of hypertrophy and heart rate does not alter the force-frequency and relaxation-frequency relationships.

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.

Effect of altering filling pattern on diastolic pressure-volume curve

BACKGROUND

The early-to-late ventricular filling ratio (E:A) is widely used to index diastolic function. While filling patterns reflect diastolic properties, they can also modulate chamber pressures due to myocardial viscoelasticity. We hypothesized that such feedback can potentially temper effects of delayed relaxation and/or volume loading on diastolic pressures.

METHODS AND RESULTS

Six isolated blood-perfused canine left ventricles were studied with ejection and filling controlled by an intracavitary volume servo-pump. Diastolic filling was determined by a simulated atrial pressure source that was either constant or varied to yield dual-phase filling at a specified E:A ratio. E:A ratio was randomly set to 3:1, 1:3, or 1:1, and data were recorded at each ratio at three different preloads. With principally early filling (E:A=3:1), diastolic pressure rise from viscosity increased in proportion with the relaxation time constant (r=.91, P<.0001). However, this dependence was lost as E:A ratio declined (eg, P=.63 for E:A 1:3). Furthermore, E:A=3:1 yielded 37% to 50% lower end-diastolic pressures at similar volumes (versus E:A=1:3) as initial viscous forces decayed. Offsetting early and late filling effects led to little net change in mean diastolic pressure independent of E:A ratio or preload.

CONCLUSIONS

Diastolic filling pattern itself influences chamber pressures early and late in diastole due to viscoelasticity, with larger net effects on end-diastolic pressure. Since E:A ratio normally falls with delayed relaxation but rises with higher preload or reduced compliance, the present results suggest that changes in filling pattern may modulate direct effects of such factors on elevating diastolic pressure.

Decrease in forces responsible for diastolic suction during acute coronary occlusion

BACKGROUND

The production of left ventricular (LV) restoring forces generated during contraction, which are responsible for diastolic suction, is dependent on end-systolic volume (ESV) and systolic transmural and 3D deformation. We tested the hypothesis that acute coronary occlusion would result in loss of forces that cause suction.

METHODS AND RESULTS

Ten open-chest dogs were subjected to a 10-minute acute coronary occlusion (proximal left anterior descending coronary artery). A servomotor connected to the left atrium (LA) was used to rapidly clamp LA pressure during systole below the level of the succeeding LV diastolic pressure, resulting in nonfilling diastoles during which the LV fully relaxed at its ESV. LA clamps at multiple ESVs (conductance catheter) allowed delineation of positive and negative portions of the fully relaxed LV pressure-volume relation (FRPVR). A negative fully relaxed pressure (FRP) indicates the presence of restoring forces. After 10 minutes of acute coronary occlusion, there was an upward shift of the FRPVR. Thus, for example, at matched ESVs before and during coronary occlusion, FRP was -1.1+/-1.1 (+/-SD) mm Hg before versus 0.2+/-1.2 mm Hg after 10 minutes of coronary occlusion (P<.05).

CONCLUSIONS

Acute coronary occlusion results in a rapid decrease in forces responsible for suction. This phenomenon is independent of the level of ESV and may contribute to ischemic diastolic dysfunction.

Elastic energy as an index of right ventricular filling

BACKGROUND

Right ventricle (RV) preload assessment remains controversial because the complexity of RV geometry is an obstacle to wall stress modeling. We developed a method to evaluate end-diastolic RV elastic energy (EL), a variable that integrates all the stretching effects of venous return and that can be easily estimated at the bedside from the area under the diastolic RV pressure-volume curve. The purpose of this study was to compare the clinical utility of EL and of the two conventional variables used to assess RV filling, ie, right atrial pressure (Pra) and RV end-diastolic volume (EDV).

METHOD

We studied 26 postoperative patients who required a rapid fluid challenge. Energetics were evaluated by constructing the RV pressure-volume loop at the bedside using right heart catheterization with RV ejection fraction (EF) derivation. Correlations between RV filling and RV performance (ejection and mechanical efficiency) were studied. RV filling indexes were Pra, EDV, and EL. Indexes of RV ejection were stroke volume (SV), RV stroke work (RVSW), mechanical energy expenditure during ejection (EM), and total energy expenditure of contraction (ET). Indexes of RV mechanical efficiency were EF and the EM/ET ratio.

RESULTS

Three important results were obtained. First, among RV ejection indexes, those that correlated best with RV filling indexes were EM and ET. Second, we found significant linear relationships between improved RV filling, as assessed by changes in EDV and EL, and improved RV ejection, as assessed by changes in SV, RVSW, EM, or ET. Third, changes in EDV and EL also predicted improved mechanical efficiency, as assessed by changes in EF and EM/ET. In, all situations, changes in EL yielded the strongest correlations.

CONCLUSIONS

Derivation of EL is simple and appears to be the best clinical means of assessing Starling's law of the heart for the RV.

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.

Role of atrial contraction in diastolic pressure elevation induced by rapid pacing of hypertrophied canine ventricle

The mechanism of diastolic pressure elevation induced by acute rapid pacing in pressure-load hypertrophied left ventricles (LVs) remains incompletely understood. It has been ascribed to abnormalities of coronary flow, metabolism, and calcium cycling. However, rapid pacing also alters the timing of atrial and ventricular stimulation relative to the diastolic filling period, and this could also influence diastolic pressures. To test the role of such mechanical factors, LV pressure-volume hemodynamics were measured in closed-chested anesthetized dogs during and after abrupt cessation of rapid atrial pacing. Twenty-one dogs were studied: 6 dogs with LV hypertrophy (LVH) induced by perinephritic hypertension, 5 sham-operated normotensive dogs, and 10 acute normotensive control dogs. In LVH dogs, but not in sham-operated or control dogs, end-diastolic pressure rose progressively with increasing heart rate from 5.6 +/- 3.1 mm Hg at baseline to 22.6 +/- 8.1 mm Hg at 220 beats per minute. In all hearts, rapid pacing shifted the timing of left atrial contraction so that it occurred near the onset of LV filling rather than at end diastole. However, in LVH hearts, early LV diastolic pressure and peak atrial pressure were also markedly elevated. Most striking, immediately after terminating the pacing, diastolic pressure declined to near baseline. This rapid pressure decline occurred just when atrial systole would have ensued and before ventricular activation would have followed had pacing continued. Thus, diastolic pressure elevation resolved before a change in ventricular pacing rate. The role of atrial contraction was further explored by simultaneous atrioventricular pacing. This shifted the time of atrial systole so that it occurred during LV isovolumic contraction, while maintaining the identical LV pacing rate. This change eliminated the diastolic pressure elevation found previously. Further analysis revealed that the pressure increase during rapid pacing was not due simply to partial LV filling imposed on a relaxing ventricle or to hypertension or an intact pericardium. These data indicate that mechanical effects of atrioventricular interaction play an important role in tachycardia-induced diastolic dysfunction in this model of LVH and can be more causative than ischemia or metabolic factors in this setting.

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.

Heart rate and force-frequency effects on diastolic function of the left ventricle in exercising dogs

BACKGROUND

Previous studies from our laboratory have shown pronounced augmentation of the force-frequency relation on myocardial contraction during exercise, but the influence of this effect on diastole has not been investigated.

METHODS AND RESULTS

Accordingly, the effect of changing heart rate on left ventricular (LV) relaxation, filling dynamics, and pressure-volume relations during exercise was studied in eight conscious dogs. The exercise heart rate was slowed from 208 +/- 21 (SD) to 163 +/- 11 beats per minute by injection of a specific sinus node inhibitor (UL-FS 49, or zatebradine, 0.6 mg/kg) during continuous exercise. Heart rate was then abruptly restored to the predrug level by atrial pacing during continued exercise. LV volume was calculated by use of implanted ultrasonic crystals, and LV pressure was determine with an implanted micromanometer. Comparing conditions after pacing back to a heart rate of 210 beats per minute with those obtained when the heart rate was slowed by atrial pacing, LV dP/dtmax was increased by 27% at the higher rate (P < .01), despite a marked decrease in LV end-diastolic pressure (24 +/- 4 versus 10 +/- 5 mm Hg, P < .01) and the time constant of isovolumic LV pressure decay (tau) was significantly shortened (19 +/- 5 versus 14 +/- 4 milliseconds, P < .01). The peak rapid filling rate in early diastole (PFR) was not significantly changed by increasing the heart rate, since it was maintained at the slower rate. During exercise, at the slowed heart rate the early portion of the diastolic pressure-volume curve was significantly shifted upward and to the right compared with that at the physiological heart rate, but the late portion of the curve was unchanged.

CONCLUSIONS

These data indicate that the negative inotropic effect of the force-frequency relation when heart rate was slowed during exercise caused pronounced impairment of LV relaxation and early filling dynamics. Conversely, an important component of the pronounced improvement of diastolic ventricular function during normal exercise was shown to result from exercise-induced enhancement of the positive inotropic effects of the force-frequency relation on myocardial contraction and relaxation.

Revascularization for acute regional infarct: superior protection with warm blood cardioplegia

Continuous retrograde warm blood cardioplegia was compared with two widely used hypothermic myocardial protection techniques in a canine model of acute regional myocardial ischemia with subsequent revascularization. Animals (n = 30) underwent 45 minutes of left anterior descending coronary artery occlusion then cardioplegic arrest (60 minutes), followed by separation from cardiopulmonary bypass and data collection. The cold oxygenated crystalloid cardioplegia group (CC; n = 8) and the cold blood cardioplegia group (CC; n = 10) had cardiopulmonary bypass at 28 degrees C, antegrade arrest, and intermittent retrograde delivery. The warm blood cardioplegia group (WB; n = 12) had normothermic cardiopulmonary bypass, antegrade arrest, and continuous retrograde delivery. Overall ventricular function (preload recruitable stroke work relationship; ergs x 10(3)/mL) was significantly (p < 0.001) better for WB (WB, 80 +/- 11; CB, 67 +/- 13; CC, 57 +/- 12). Systolic function (maximum elastance relationship; mm Hg/mL) was also significantly (p < 0.001) better for WB (WB, 11.6 +/- 3.6; CB, 8.6 +/- 2.7; CC, 6.2 +/- 1.3). Diastolic function (stress-strain relationship; dynes x 10(3)/cm2) revealed significantly (p < 0.001) decreased compliance for CC (WB, 20 +/- 6; CB, 19 +/- 7; CC, 27 +/- 11). Left anterior descending coronary artery regional adenosine triphosphate/adenosine diphosphate ratios were significantly (p = 0.02) worse for CC (WB, 10.2 +/- 2.3; CB, 9.4 +/- 2.6; CC, 5.6 +/- 1.5). Myocardial edema significantly (p = 0.03) increased over time only in the CC animals (WB, 0.4% +/- 2.3%; CB, -0.3% +/- 3.6%; CC, 5.5% +/- 2.3%). In this model of acute regional myocardial ischemia and revascularization, continuous retrograde warm aerobic blood cardioplegia provided superior myocardial protection compared with cold oxygenated crystalloid cardioplegia with intermediate results for cold blood cardioplegia.

Regional remodeling and nonuniform changes in diastolic function in patients with left ventricular dysfunction: modification by long-term enalapril treatment. The SOLVD Investigators

OBJECTIVES

The purpose of the present study was to assess the process of late regional remodeling and the changes in regional diastolic function at the base and apex of the left ventricle in patients with chronic systolic dysfunction.

BACKGROUND

Remodeling has been suggested to play an important role in the progression of left ventricular dysfunction and heart failure. However, the regional difference in the process of late remodeling and its relation to diastolic function remain unclear.

METHODS

In 32 patients with previous myocardial infarction and left ventricular ejection fraction < or = 35%, left ventricular hemodynamic and angiographic data were studied before and 1 year after randomization to conventional therapy with placebo (n = 12) or enalapril, 10 mg twice daily (n = 20). Left ventricular regional wall dynamics were analyzed in the basal and apical regions by the area method.

RESULTS

In the placebo group, left ventricular end-diastolic and end-systolic regional areas increased significantly over time at the base but were unchanged at the apex. At the base, the diastolic left ventricular pressure-regional area relation shifted rightward and the regional stiffness constant decreased (6.9 +/- 4.3 to 5.0 +/- 3.1 x 10(-3) mm-2, p < 0.05), indicating an increase in regional distensibility. At the apex, however, the diastolic pressure-regional area relation shifted upward slightly, and the regional stiffness constant increased from 11.5 +/- 4.4 to 14.4 +/- 5.6 x 10(-3) mm-2 (p = 0.08). The regional peak filling rate was maintained at the base but decreased at the apex (1,014 +/- 436 to 762 +/- 306 mm2/s, p < 0.05); further, the changes in regional peak filling rate during follow-up were inversely related to the changes in the regional stiffness constant (r = -0.78, p < 0.001) at the apex. In contrast, in the enalapril group, end-diastolic and end-systolic regional areas significantly decreased over time both at the base and at the apex. Diastolic pressure-regional area relations shifted leftward, but the regional stiffness constant and regional peak filling rate did not change significantly either at the base or at the apex.

CONCLUSIONS

These findings suggest that in patients with severe systolic left ventricular dysfunction, there was a regional difference in the process of late remodeling between the base and apex of the left ventricle, which was associated with nonuniform changes in regional diastolic function in the placebo group. The data also suggest that the nonuniform progression of regional remodeling and diastolic dysfunction was prevented by long-term enalapril treatment.

Application of finite-element analysis with optimisation to assess the in vivo non-linear myocardial material properties using echocardiographic imaging

An application of finite-element analysis with an optimisation technique to assess the myocardial material properties in diastasis in vivo is described. Using the data collected from an animal model, the three-dimensional geometry of the left ventricular chamber, at several times in diastole, was reconstructed. From the measurement of the ventricular chamber pressure during image acquisition, finite-element analysis was performed to predict the expansion during diastasis. Initially, by restricting the motion of the epicardial nodes and computing the reaction forces, an 'equivalent pericardial pressure' was determined and applied in subsequent analysis. The duration of diastasis was divided into three or four intervals and the analysis was performed at each interval to assess the material properties of the myocardium. Using such a step-wise linear approach, the non-linear material properties of the myocardium during passive expansion was determined. Our results demonstrated that the computed 'equivalent pericardial pressure' increased with and was smaller than the corresponding left ventricular chamber pressure. The passive myocardium exhibited a linear tangent modulus against chamber pressure relationship which is equivalent to an exponential stress/strain relationship, similar to those suggested by in vitro studies.