Changes in regional myocardial volume during the cardiac cycle: implications for transmural blood flow and cardiac structure

On the structural origin of the anisotropy in the myocardium: Multiscale modeling and analysis

Due to structural heterogeneities within the tissue, the myocardium displays an orthotropic material behavior. However, the link between the microstructure and the macroscopic mechanical properties is still not fully established. In particular, if it is admitted that the cardiomyocyte organization induces a transversely isotropic symmetry, the relative role in the observed orthotropic symmetry of cardiomyocyte orientation variation and perimysium collagen "sheetlet" structure, two mechanisms occurring at different scales, is still a matter of debate. In order to shed light on this question, we designed a multiscale model of the myocardium, bridging the cell, sheetlet and tissue scales. More precisely, we compared the macroscopic anisotropy obtained by homogenization of different mesostructures consisting in cardiomyocytes and extracellular collageneous layers, also taking into account the variation of cardiomyocyte and sheetlet orientations on the macroscale, to available experimental data. This study confirms the importance of sheetlets layers in assuring the tissue's anisotropic response, as cardiomyocytes-only mesostructures cannot reproduce the observed anisotropy. Moreover, our model shows the existence of a size effect in the myocardial tissue shear properties, which will require further experimental analysis.

Left ventricular T1-mapping in diastole versus systole in patients with mitral regurgitation

Cardiovascular magnetic resonance T1-mapping enables myocardial tissue characterisation, and is capable of quantifying both intracellular and extracellular volume. T1-mapping is conventionally performed in diastole, however, we hypothesised that systolic readout would reduce variability due to a reduction in myocardial blood volume. This study investigated whether T1-mapping in systole alters T1 values compared to diastole and whether reproducibility alters in atrial fibrillation compared to sinus rhythm. We prospectively identified 103 consecutive patients recruited to the Mitral FINDER study who had T1 mapping in systole and diastole. These patients had moderate or severe mitral regurgitation and a high incidence of ventricular dilatation and atrial fibrillation. T1, ECV and goodness-of-fit (R2) values of the T1 times were calculated offline using Circle cvi42 and in house-developed software. Systolic T1 mapping was associated with fewer myocardial segments being affected by artefact compared to diastolic T1 mapping [217/2472 (9%) vs 515/2472 (21%)]. Mean native T1 values were not significantly different when measured in systole and diastole (985 ± 26 ms vs 988 ± 29 respectively; p = 0.061) and mean post-contrast values showed similar good agreement (462 ± 32 ms vs 459 ± 33 respectively, p = 0.052). No clinically significant differences in ECV, native T1 and post-contrast T1 were identified between diastolic and systolic T1 maps in males versus females, or in patients with permanent atrial fibrillation versus sinus rhythm. A statistically significant improvement in R2 value was observed with systolic over diastolic T1 mapping in all analysed maps (n = 411) (96.2 ± 1.4% vs 96.0 ± 1.4%; p < 0.001) and in subgroup analyses [Sinus rhythm: 96.1 ± 1.4 vs 96.3 ± 1.4 (n = 327); p < 0.001. AF: 95.5 ± 1.3 vs 95.9 ± 1.2 (n = 80); p < 0.001] [Males: 95.8 ± 1.4 vs 96.1 ± 1.3 (n = 264); p < 0.001; Females: 96.2 ± 1.3 vs 96.4 ± 1.4 (n = 143); p = 0.009]. In conclusion, myocardial T1 mapping is associated with similar T1 and ECV values in systole and diastole. Furthermore, systolic acquisition is less prone to gating artefact in arrhythmia.

Changes in left ventricle myocardial volume during stress test using cadmium-zinc-telluride cardiac imaging: Implications in coronary artery disease

BACKGROUND

Cadmium-zinc-telluride (CZT) SPECT allows the estimation of left ventricle myocardial volume (LVMV). We tested the clinical relevance of rest-stress LVMV changes (Δ LVMV) in detecting coronary artery disease (CAD, coronary stenosis > 70%), using CZT-SPECT.

METHODS

We prospectively enrolled 512 consecutive patients with known or suspected CAD (mean age: 70.3 ± 9.2 years, 72% male) for stress-rest myocardial perfusion imaging (MPI, single-day stress-rest protocol). We quantified summed stress scores (SSS), summed rest scores, and summed difference scores, together with LVMV and ejection fraction (EF) after stress and at rest. All patients underwent coronary angiography within 30 days.

RESULTS

Two hundred seventy-two patients had CAD at coronary angiography. ΔLVMV ≤ 5 mL, corresponding to 6% of change from rest LVMV, was the best predictor of CAD (AUC = 0.831, 79% sensitivity, 82% specificity), irrespective of the stress protocol (dipyridamole or exercise stress) and independently of MPI-SSS, LV EF, and clinical history (P = 0.004). Integrated discrimination improvement (IDI) and net reclassification improvement (NRI) were significant for the addition of ΔLVMV ≤ 5 mL (IDI = 6.1%, P < 0.0001; NRI = 29.7%, P = 0.02) to MPI-SSS, whereas the other parameters were not.

CONCLUSIONS

The evaluation of ΔLVMV using CZT-SPECT can improve the diagnostic accuracy in predicting the presence of CAD when added to conventional MPI.

An under-recognized phenomenon: Myocardial volume change during the cardiac cycle

BACKGROUND

Myocardial volume is assumed to be constant over the cardiac cycle in the echocardiographic models used by professional guidelines, despite evidence that suggests otherwise. The aim of this paper is to use literature-derived myocardial strain values from healthy patients to determine if myocardial volume changes during the cardiac cycle.

METHODS

A systematic review for studies with longitudinal, radial, and circumferential strain from echocardiography in healthy volunteers ultimately yielded 16 studies, corresponding to 2917 patients. Myocardial volume in systole (MVs) and diastole (MVd) was used to calculate MVs/MVd for each study by applying this published strain data to three models: the standard ellipsoid geometric model, a thin-apex geometric model, and a strain-volume ratio.

RESULTS

MVs/MVd<1 in 14 of the 16 studies, when computed using these three models. A sensitivity analysis of the two geometric models was performed by varying the dimensions of the ellipsoid and calculating MVs/MVd. This demonstrated little variability in MVs/MVd, suggesting that strain values were the primary determinant of MVs/MVd rather than the geometric model used. Another sensitivity analysis using the 97.5th percentile of each orthogonal strain demonstrated that even with extreme values, in the largest two studies of healthy populations, the calculated MVs/MVd was <1.

CONCLUSIONS

Healthy human myocardium appears to decrease in volume during systole. This is seen in MRI studies and is clinically relevant, but this study demonstrates that this characteristic was also present but unrecognized in the existing echocardiography literature.

Myocardial Strain Measured by Epicardial Transducers-Comparison Between Velocity Estimators

This study describes results from an experimental ultrasound system with miniature transducers sutured directly onto the epicardial surface and used to measure heart contractions continuously. This system was used to find velocity distributions through the myocardium. The resulting velocities were used to track the motion of four layers at different depths through the myocardium and to find the regional strain in each of the four layers. Velocities inside the myocardium vary from the epicardial to the endocardial borders. Conventional velocity estimators based on Doppler and on time delay estimation were modified to better handle these variations. Results from four different velocity estimators were tested against a simulation model for ultrasound echoes from moving tissue and on ultrasound recordings from five animals. We observed that the tested velocity estimators were able to reproduce the myocardial velocity distributions, track the myocardial layer motion and estimate strain at different positions inside the myocardium for both simulated and real ultrasound recordings. The most accurate results were obtained when the digitized ultrasound scanlines were upsampled by a factor of 10 before applying cross-correlation to estimate time delays. A modified Doppler algorithm allowing the velocity to vary linearly with time throughout the duration of the pulse packet (constant acceleration Doppler) was found to be better at capturing rapidly changing velocities compared with conventional Doppler processing. The best results were obtained using upsamling and time delay estimation, but the long computation time required by this method may make it best suited in a laboratory setting. In a real-time system, the computationally quicker constant acceleration Doppler may be preferred.

A thermodynamic transient cross-bridge model for prediction of contractility and remodelling of the ventricle

Cardiac hypertrophy is an adaption of the heart to a change in cardiovascular loading conditions. The current understanding is that progression may be stress or strain driven, but the multi-scale nature of the cellular remodelling processes have yet to be uncovered. In this study, we develop a model of the contractile left ventricle, with the active cell tension described by a thermodynamically motivated cross-bridge cycling model. Simulation of the transient recruitment of myosin results in correct patterns of ventricular pressure predicted over a cardiac cycle. We investigate how changes in tissue loading and associated deviations in transient force generation can drive restructuring of cellular myofibrils in the heart wall. Our thermodynamic framework predicts in-series sarcomere addition (eccentric remodelling) in response to volume overload, and sarcomere addition in parallel (concentric remodelling) in response to valve and signalling disfunction. This framework provides a significant advance in the current understanding of the fundamental sub-sarcomere level biomechanisms underlying cardiac remodelling. Simulations reveal that pathological tissue loading conditions can significantly alter actin-myosin cross-bridge cycling over the course of the cardiac cycle. The resultant variation in sarcomere stress pushes an imbalance between the internal free energy of the myofibril and that of unbound contractile proteins, initiating remodelling. The link between cross-bridge thermodynamics and myofibril remodelling proposed in this study may significantly advance current understanding of cardiac disease onset.

Continuous Estimation of Acute Changes in Preload Using Epicardially Attached Accelerometers

OBJECTIVE

A miniaturized accelerometer can be incorporated in temporary pacemaker leads which are routinely attached to the epicardium during cardiac surgery and provide continuous monitoring of cardiac motion during and following surgery. We tested if such a sensor could be used to assess volume status, which is essential in hemodynamically unstable patients.

METHODS

An accelerometer was attached to the epicardium of 9 pigs and recordings performed during baseline, fluid loading, and phlebotomy in a closed chest condition. Alterations in left ventricular (LV) preload alter myocardial tension which affects the frequency of myocardial acceleration associated with the first heart sound ( f_S1). The accuracy of f_S1 as an estimate of preload was evaluated using sonomicrometry measured end-diastolic volume (EDV[Formula: see text]). Standard clinical estimates of global end-diastolic volume using pulse index continuous cardiac output (PiCCO) measurements (GEDV[Formula: see text]) and pulmonary artery occlusion pressure (PAOP) were obtained for comparison. The diagnostic accuracy of identifying fluid responsiveness was analyzed for f_S1, stroke volume variation (SVV[Formula: see text]), pulse pressure variation (PPV[Formula: see text]), GEDV[Formula: see text], and PAOP.

RESULTS

Changes in f_S1 correlated well to changes in EDV[Formula: see text] ( r²=0.81, 95%CI: [0.68, 0.89]), as did GEDV[Formula: see text] ( r²=0.59, 95%CI: [0.36, 0.76]) and PAOP ( r²=0.36, 95%CI: [0.01, 0.73]). The diagnostic accuracy [95%CI] in identifying fluid responsiveness was 0.79 [0.66, 0.94] for f_S1, 0.72 [0.57, 0.86] for SVV[Formula: see text], and 0.63 (0.44, 0.82) for PAOP.

CONCLUSION

An epicardially placed accelerometer can assess changes in preload in real-time.

SIGNIFICANCE

This novel method can facilitate continuous monitoring of the volemic status in open-heart surgery patients and help guiding fluid resuscitation.

Cardiac MRI demonstrates compressibility in healthy myocardium but not in myocardium with reduced ejection fraction

BACKGROUND

The professional guidelines assume that the myocardial volume in systole (MVs) is equal to that in diastole (MVd), despite some limited evidence that points to the contrary. The aim of this manuscript is to determine whether this is true in healthy myocardium using gold standard cardiac MRI, as well as transthoracic echocardiography (TTE). The secondary aim is to determine whether there are similar MV changes in patients with heart failure with reduced ejection fraction (HFrEF).

METHOD

A prospectively derived cohort at Mayo Clinic of 115 adult subjects (mean age 42.8 years, 58% female) with no cardiac risk factors was identified. Cardiac MRI was obtained on all 115 patients, 51 of whom also consented to a TTE. MRI from a retrospectively derived cohort of 50 HFrEF patients was also collected. MVs and MVd was calculated using standard approaches with inclusion of the papillary muscles.

RESULTS

In the healthy population, MRI demonstrated MVs/MVd = 0.87 (SD 0.04) and TTE demonstrated MVs/MVd = 0.79 (SD 0.07), suggesting compressibility (p < 0.0001). In the 51 healthy patients who received both imaging modalities, MVs/MVd was 8.0% higher in MRI than TTE (p < 0.0001), but both modalities had MVs/MVd < 1 (p < 0.0001). A Bland-Altman plot demonstrated that as the mean MVs/MVd increases, the difference in MVs/MVd MRI-TTE declines (r = -0.53, p < 0.0001). However, in HFrEF populations, MVs/MVd = 1.01 (0.03), suggesting myocardial incompressibility.

CONCLUSION

Contrary to currently accepted standards, healthy myocardium is compressible but HFrEF myocardium is incompressible. The ratio MVs/MVd merits further study in an expanded normal cohort and in disease states.

Systolic-to-diastolic myocardial volume ratio as a novel imaging marker of cardiomyopathy

BACKGROUND

In patients with normal left ventricular ejection fraction, it may be difficult to distinguish between the normal and diseased heart. Novel assessments of ventricular function, such as extracellular volume imaging, myocardial perfusion imaging and myocardial contraction fraction are emerging to better assess disease burden in these cases. This study endeavored to determine whether the ratio of myocardial volume in systole to myocardial volume in diastole (MVs/MVd), differs between normal hearts and those with disease states characterized by normal ejection fraction.

METHOD

Consecutive patients from 2008 to 2018 with hypertrophic cardiomyopathy (HCM), cardiac amyloidosis, and heart failure with preserved ejection fraction (HFpEF) who underwent cardiac magnetic resonance imaging (MRI) were selected for inclusion, along with a sex- and age-matched cohort of normal volunteers who also underwent cardiac MRI. Manual tracings were performed on each MRI to calculate MVs/MVd, which was then compared across subgroups.

RESULTS

Included were 50 patients with HCM, 50 patients with cardiac amyloidosis, 26 patients with HFpEF, and 30 normal subjects. Age was 54.1 years (SD 16.7); mean MVs/MVd was 0.88 (SD 0.04) in the normal subgroup, 1.03 (SD 0.06) in HCM patients, 1.03 (SD 0.06) in cardiac amyloidosis patients, and 0.97 (SD 0.02) in HFpEF patients, with all pathology subgroups different from the normal subgroup (p < .0001 for each). The ratio of MVs/MVd discriminated diseased from normal with c statistic 0.989 (p < .001).

CONCLUSIONS

This study suggests that a novel and easily-captured metric of ventricular function, MVs/MVd, can differentiate normal ventricular function from multiple cardiomyopathies with normal ejection fractions.

On the in vivo systolic compressibility of left ventricular free wall myocardium in the normal and infarcted heart

Although studied for many years, there remain continued gaps in our fundamental understanding of cardiac kinematics, such as the nature and extent of heart wall volumetric changes that occur over the cardiac cycle. Such knowledge is especially important for accurate in silico simulations of cardiac pathologies and in the development of novel therapies for their treatment. A prime example is myocardial infarction (MI), which induces profound, regionally variant maladaptive remodeling of the left ventricle (LV) wall. To address this problem, we conducted an in vivo fiduciary marker-based study in an established ovine model of MI to generate detailed, time-evolving transmural in vivo volumetric measurements of LV free wall deformations in the normal state, as well as up to 12 h post-MI. This was accomplished using a transmural array of sonomicrometry crystals that acquired fiducial positions at ∼250 Hz with a positional accuracy of ∼0.1 mm, covering the entire infarct, border, and remote zones. A convex-hull method was used to directly calculate the Jacobian J(t)=Δv(t)/ΔVED from sonocrystal positions over the entire cardiac cycle, where ΔV is the volume of each convex polyhedral at end diastole (ED) (typically ∼1 cc). We demonstrated significant in vivo compressibility in normal functioning LV free wall myocardium, with JES=0.85±0.07 at end systole (ES). We also observed substantial regional variations, with the largest reduction in local myocardial tissue volume during systole in the base region accompanied by substantial transmural gradients. These patterns changed profoundly following loss of perfusion post-MI, with the apical region showing the greatest loss of volume reduction at ES. To verify that the sonocrystals did not affect local volumetric measurements, JES measures were also verified by non-invasive magnetic resonance imaging, exhibiting very similar changes in regional volume. We note that while our estimates of regional compressibility were in close agreement with the values previously reported for large animals, ranging from 5% to 20%, the direct, comprehensive measurements of wall compressibility presented herein improved on the limitations of previous reports. These limitations included dependency on the small local volumes used for analysis and often indirect measurement of compressibility. Our novel findings suggest that proper accounting for the myocardial effective compressibility at the ∼1 cc volume scale can improve the accuracy of existing kinematic indices, such as wall thickening and axial shortening, and simulations of LV remodeling following MI.

Myocardial volume change during cardiac cycle derived from three orthogonal systolic strains: towards a quality assessment of strains

BACKGROUND

The relative modification of the myocardial volume between end-systole and end-diastole ( Vs/d=Vend-systole/Vend-diastole ) has already been assessed with different methods and falls in a range of 0.9-0.97 (mean value = 0.93).

PURPOSE

To estimate Vs/d from the three longitudinal ( ɛl) , circumferential ( ɛc ), and radial ( ɛr ) strains of the left ventricle using the formula: Vs/d=(1+ɛc)(1+ɛr)(1+ɛl) and to test whether this estimate of Vs/d can be used as a marker of the echocardiography quality.

MATERIAL AND METHODS

Two hundred manuscripts, including a total of 34,690 patients or healthy volunteers, were identified in the Medline database containing values of ɛl , ɛc , and ɛr measured from echocardiography.

RESULTS

The median value of was 0.93, in accordance with the literature, with no significant difference between patients or healthy volunteers ( P = 0.38). The proportion of studies with Vs/d=0.93±0.1 was 79%. When only considering groups of healthy volunteers, the studies failing this test had higher standard deviations for the three individual strains: 0.038 vs. 0.029 ( P = 0.02) for ɛl ; 0.060 vs. 0.034 ( P < 10^-6) for ɛc , and 0.243 vs. 0.101 ( P < 10^-14) for ɛr .

CONCLUSION

The median ratio of the left ventricular myocardial volumes between end-systole and end-diastole in the investigated studies was Vs/d=0.93 . The formula (1+ɛc)(1+ɛr)(1+ɛl)∉[0.83;1.03] could be used to detect studies with inaccurate strain measurements.

Compressibility and Anisotropy of the Ventricular Myocardium: Experimental Analysis and Microstructural Modeling

While the anisotropic behavior of the complex composite myocardial tissue has been well characterized in recent years, the compressibility of the tissue has not been rigorously investigated to date. In the first part of this study, we present experimental evidence that passive-excised porcine myocardium exhibits volume change. Under tensile loading of a cylindrical specimen, a volume change of 4.1±1.95% is observed at a peak stretch of 1.3. Confined compression experiments also demonstrate significant volume change in the tissue (loading applied up to a volumetric strain of 10%). In order to simulate the multiaxial passive behavior of the myocardium, a nonlinear volumetric hyperelastic component is combined with the well-established Holzapfel–Ogden anisotropic hyperelastic component for myocardium fibers. This framework is shown to describe the experimentally observed behavior of porcine and human tissues under shear and biaxial loading conditions. In the second part of the study, a representative volumetric element (RVE) of myocardium tissue is constructed to parse the contribution of the tissue vasculature to observed volume change under confined compression loading. Simulations of the myocardium microstructure suggest that the vasculature cannot fully account for the experimentally measured volume change. Additionally, the RVE is subjected to six modes of shear loading to investigate the influence of microscale fiber alignment and dispersion on tissue-scale mechanical behavior.

Relationship between the left ventricular size and the amount of trabeculations

Contemporary imaging modalities offer noninvasive quantification of myocardial deformation; however, they make gross assumptions about internal structure of the cardiac walls. Our aim is to study the possible impact of the trabeculations on the stroke volume, strain, and capacity of differently sized ventricles. The cardiac left ventricle is represented by an ellipsoid and the trabeculations by a tissue occupying a fixed volume. The ventricular contraction is modeled by scaling the ellipsoid whereupon the measurements of longitudinal strain, end-diastolic, end-systolic, and stroke volumes are derived and compared. When the trabeculated and nontrabeculated ventricles, having the same geometry and deformation pattern, contain the same amount of blood and contract with the same strain, we observed an increased stroke volume in our model of the trabeculated ventricle. When these ventricles contain and eject the same amount of blood, we observed a reduced strain in the trabeculated case. We identified that a trade-off between the strain and the amount of trabeculations could be reached with a 0.35- to 0.41-cm dense trabeculated layer, without blood filled recesses (for a ventricle with end-diastolic volume of about 150 mL). A trabeculated ventricle can work at lower strains compared to a nontrabeculated ventricle to produce the same stroke volume, which could be a possible explanation why athletes and pregnant women develop reversible signs of left ventricular noncompaction, since the trabeculations could help generating extra cardiac output. This knowledge might help to assess heart failure patients with dilated cardiomyopathies who often show signs of noncompaction.

Improved identifiability of myocardial material parameters by an energy-based cost function

Myocardial stiffness is a valuable clinical biomarker for the monitoring and stratification of heart failure (HF). Cardiac finite element models provide a biomechanical framework for the assessment of stiffness through the determination of the myocardial constitutive model parameters. The reported parameter intercorrelations in popular constitutive relations, however, obstruct the unique estimation of material parameters and limit the reliable translation of this stiffness metric to clinical practice. Focusing on the role of the cost function (CF) in parameter identifiability, we investigate the performance of a set of geometric indices (based on displacements, strains, cavity volume, wall thickness and apicobasal dimension of the ventricle) and a novel CF derived from energy conservation. Our results, with a commonly used transversely isotropic material model (proposed by Guccione et al.), demonstrate that a single geometry-based CF is unable to uniquely constrain the parameter space. The energy-based CF, conversely, isolates one of the parameters and in conjunction with one of the geometric metrics provides a unique estimation of the parameter set. This gives rise to a new methodology for estimating myocardial material parameters based on the combination of deformation and energetics analysis. The accuracy of the pipeline is demonstrated in silico, and its robustness in vivo, in a total of 8 clinical data sets (7 HF and one control). The mean identified parameters of the Guccione material law were [Formula: see text] and [Formula: see text] ([Formula: see text], [Formula: see text], [Formula: see text]) for the HF cases and [Formula: see text] and [Formula: see text] ([Formula: see text], [Formula: see text], [Formula: see text]) for the healthy case.

Improved identifiability of myocardial material parameters by an energy-based cost function

Myocardial stiffness is a valuable clinical biomarker for the monitoring and stratification of heart failure (HF). Cardiac finite element models provide a biomechanical framework for the assessment of stiffness through the determination of the myocardial constitutive model parameters. The reported parameter intercorrelations in popular constitutive relations, however, obstruct the unique estimation of material parameters and limit the reliable translation of this stiffness metric to clinical practice. Focusing on the role of the cost function (CF) in parameter identifiability, we investigate the performance of a set of geometric indices (based on displacements, strains, cavity volume, wall thickness and apicobasal dimension of the ventricle) and a novel CF derived from energy conservation. Our results, with a commonly used transversely isotropic material model (proposed by Guccione et al.), demonstrate that a single geometry-based CF is unable to uniquely constrain the parameter space. The energy-based CF, conversely, isolates one of the parameters and in conjunction with one of the geometric metrics provides a unique estimation of the parameter set. This gives rise to a new methodology for estimating myocardial material parameters based on the combination of deformation and energetics analysis. The accuracy of the pipeline is demonstrated in silico, and its robustness in vivo, in a total of 8 clinical data sets (7 HF and one control). The mean identified parameters of the Guccione material law were [Formula: see text] and [Formula: see text] ([Formula: see text], [Formula: see text], [Formula: see text]) for the HF cases and [Formula: see text] and [Formula: see text] ([Formula: see text], [Formula: see text], [Formula: see text]) for the healthy case.

Systolic myocardial volume gain in dilated, hypertrophied and normal heart. CMR study

AIM

To investigate changes in myocardial tissue volume during the cardiac cycle to verify the hypothesis of non-compressibility of the myocardium in healthy individuals (HI) as well as in patients with hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), and aortic stenosis (AS).

MATERIALS AND METHODS

The study group included 30 HI, and patients with HCM (n=110), DCM (n=89), and AS (n=78). Left ventricular (LV) function, end-diastolic, and end-systolic volumes were calculated based on cardiac magnetic resonance imaging (CMR) for all participants.

RESULTS

End-systolic myocardial volumes were higher than end-diastolic in both controls (91.2±26.6 versus 85.1±24.3 ml, p<0.001) and in all patient groups: HCM (214.3±81.6 versus 176±64.2 ml, p<0.01), DCM (128.4±43.1 versus 115.4±42.9 ml, p<0.001) and AS (155.1±37.1 versus 129.4±34.6 ml, p<0.001). HCM and AS patients had significantly higher systolic volume gain than HI (21.5±8.3 versus 10.6±6.3%, p<0.01 and 18.3±5.7 versus 10.6±6.3% p=0.013, respectively). Conversely, DCM patients had lesser increases in myocardial systolic volume than HCM patients (11.2±4.8% versus 21.5±8.3, p=0.01) and AS patients (11.2±4.8% versus 18.3±5.7, p=0.02). No differences were found in systolic volume gain between AS and HCM patients (p=ns) or between DCM patients and HI (p=ns).

CONCLUSION

End-systolic myocardial volume was significantly higher than end-diastolic volume in all subsets of patients. The systolic volume gain was greater in individuals with hypertrophy than in those without.

Transmural gradients of myocardial structure and mechanics: Implications for fiber stress and strain in pressure overload

Although a truly complete understanding of whole heart activation, contraction, and deformation is well beyond our current reach, a significant amount of effort has been devoted to discovering and understanding the mechanisms by which myocardial structure determines cardiac function to better treat patients with cardiac disease. Several experimental studies have shown that transmural fiber strain is relatively uniform in both diastole and systole, in contrast to predictions from traditional mechanical theory. Similarly, mathematical models have largely predicted uniform fiber stress across the wall. The development of this uniform pattern of fiber stress and strain during filling and ejection is due to heterogeneous transmural distributions of several myocardial structures. This review summarizes these transmural gradients, their contributions to fiber mechanics, and the potential functional effects of their remodeling during pressure overload hypertrophy.

Normal diastolic and systolic myocardial T1 values at 1.5-T MR imaging: correlations and blood normalization

PURPOSE

To introduce blood normalization for myocardial T1 values at magnetic resonance (MR) imaging and to evaluate regional differences between systolic and diastolic myocardial T1 values in healthy subjects.

MATERIALS AND METHODS

This prospective study (ClinicalTrials.gov identification number, NCT01728597) was approved by the institutional review board, and volunteer informed consent was obtained. Forty healthy subjects (20 women; age range, 20-35 years) underwent electrocardiographically gated 1.5-T MR imaging. A modified Look-Locker inversion recovery sequence was used to acquire myocardial T1 maps in systole and diastole. Regional T1 values were evaluated in 16 myocardial segments; blood T1 was derived from the blood pool in the center of the left ventricular cavity. Linear regression slopes between myocardial and blood T1 values were used to normalize myocardial T1 to the mean blood T1 of the study population. Mean T1 values were compared by using the t test, with P < .05 considered to indicate a significant difference.

RESULTS

Mean myocardial T1 (984 msec ± 28 [standard deviation] in diastole, 959 msec ± 21 in systole) and all segmental T1 values between diastole and systole differed significantly (P < .001). Blood T1 correlated well with segmental myocardial T1 (R = 0.73 for diastole, R = 0.72 for systole). After normalization to blood T1, significant sex differences in myocardial T1 disappeared and variances in mean myocardial T1 decreased. Blood-normalized diastolic and systolic myocardial T1 values correlated strongly with each other on segmental (r = 0.72) and global (r = 0.89) levels. Subregional myocardial T1 distribution characteristics in diastole were similar to those in systole.

CONCLUSION

In normal myocardium, diastolic and systolic myocardial T1 values differ significantly but correlate strongly. Blood normalization eliminates sex differences in myocardial T1 values and reduces their variability.

Contribution of myocardium overlying the anterolateral papillary muscle to left ventricular deformation

Previous studies of transmural left ventricular (LV) strains suggested that the myocardium overlying the papillary muscle displays decreased deformation relative to the anterior LV free wall or significant regional heterogeneity. These comparisons, however, were made using different hearts. We sought to extend these studies by examining three equatorial LV regions in the same heart during the same heartbeat. Therefore, deformation was analyzed from transmural beadsets placed in the equatorial LV myocardium overlying the anterolateral papillary muscle (PAP), as well as adjacent equatorial LV regions located more anteriorly (ANT) and laterally (LAT). We found that the magnitudes of LAT normal longitudinal and radial strains, as well as major principal strains, were less than ANT, while those of PAP were intermediate. Subepicardial and midwall myofiber angles of LAT, PAP, and ANT were not significantly different, but PAP subendocardial myofiber angles were significantly higher (more longitudinal as opposed to circumferential orientation). Subepicardial and midwall myofiber strains of ANT, PAP, and LAT were not significantly different, but PAP subendocardial myofiber strains were less. Transmural gradients in circumferential and radial normal strains, and major principal strains, were observed in each region. The two main findings of this study were as follows: 1) PAP strains are largely consistent with adjacent LV equatorial free wall regions, and 2) there is a gradient of strains across the anterolateral equatorial left ventricle despite similarities in myofiber angles and strains. These findings point to graduated equatorial LV heterogeneity and suggest that regional differences in myofiber coupling may constitute the basis for such heterogeneity.

Interstitial fluid flow and cyclic strain differentially regulate cardiac fibroblast activation via AT1R and TGF-β1

Cardiac fibroblasts are exposed to both cyclic strain and interstitial fluid flow in the myocardium. The balance of these stimuli is affected by fibrotic scarring, during which the fibroblasts transition to a myofibroblast phenotype. The present study investigates the mechanisms by which cardiac fibroblasts seeded in three-dimensional (3D) collagen gels differentiate between strain and fluid flow. Neonatal cardiac fibroblast-seeded 3D collagen gels were exposed to interstitial flow and/or cyclic strain and message levels of collagens type I and III, transforming growth factor β1 (TGF-β1), and α-smooth muscle actin (α-SMA) were assessed. Flow was found to significantly increase and strain to decrease expression of myofibroblast markers. Corresponding immunofluorescence indicated that flow and strain differentially regulated α-SMA protein expression. The effect of flow was inhibited by exposure to losartan, an angiotensin II type 1 receptor (AT1R) blocker, and by introduction of shRNA constructs limiting AT1R expression. Blocking of TGF-β also inhibited the myofibroblast transition, suggesting that flow-mediated cell signaling involved both AT1R and TGF-β1. Reduced smad2 phosphorylation in response to cyclic strain suggested that TGF-β is part of the mechanism by which cardiac fibroblasts differentiate between strain-induced and flow-induced mechanical stress. Our experiments show that fluid flow and mechanical deformation have distinct effects on cardiac fibroblast phenotype. Our data suggest a mechanism in which fluid flow directly acts on AT1R and causes increased TGF-β1 expression, whereas cyclic strain reduces activation of smad proteins. These results have relevance to the pathogenesis and treatment of heart failure.

Oblique 3D MRI tags for the estimation of true 3D cardiac motion parameters

Aim of this study is to demonstrate the advantages of oblique 3D tags in cardiac magnetic resonance imaging (MRI) and the potential to accurately describe the complex motion of the myocardial wall. 3D cardiac Cine data were densely tagged with 3D oblique tags. The latter were tracked using Gabor analysis and active geometries. From the tag intersections, common 2D parameters such as long axis shortening, radial shortening and rotation were evaluated on a global as well as detailed local level. Finally, the same data were used to estimate left ventricular volume change and myocardial stress/strain. We have successfully tracked dense 3D tags and evaluated common parameters on a detailed local level. In addition, inherently 3D parameters could be estimated. Global motion data are in accordance with previously published data. Oblique tags allow for unambiguous localization of the tag plane in all MRI slices and in any time frame. In contrast to HARP, our tag tracking methodology allows for tracking of the tags even when they are dense. Motion parameters can be extracted in greater detail. Moreover, the intersections of dense oblique 3D tags provide a natural basis for a finite element model of the heart. Straight forward access to the 3D characteristics of the cardiac motion is provided.

Hemodynamic improvement in cardiac resynchronization does not require improvement in left ventricular rotation mechanics: three-dimensional tagged MRI analysis

BACKGROUND

Earlier studies have yielded conflicting evidence on whether or not cardiac resynchronization therapy (CRT) improves left ventricular (LV) rotation mechanics.

METHODS AND RESULTS

In dogs with left bundle branch block and pacing-induced heart failure (n=7), we studied the effects of CRT on LV rotation mechanics in vivo by 3-dimensional tagged magnetic resonance imaging with a temporal resolution of 14 ms. CRT significantly improved hemodynamic parameters but did not significantly change the LV rotation or rotation rate. LV torsion, defined as LV rotation of each slice with respect to that of the most basal slice, was not significantly changed by CRT. CRT did not significantly change the LV torsion rate. There was no significant circumferential regional heterogeneity (anterior, lateral, inferior, and septal) in LV rotation mechanics in either left bundle branch block with pacing-induced heart failure or CRT, but there was significant apex-to-base regional heterogeneity.

CONCLUSIONS

CRT acutely improves hemodynamic parameters without improving LV rotation mechanics. There is no significant circumferential regional heterogeneity of LV rotation mechanics in the mechanically dyssynchronous heart. These results suggest that LV rotation mechanics is an index of global LV function, which requires coordination of all regions of the left ventricle, and improvement in LV rotation mechanics appears to be a specific but insensitive index of acute hemodynamic response to CRT.