MR tagging demonstrates quantitative differences in regional ventricular wall motion in mice, rats, and men

Characterization of three-dimensional myocardial deformation in the mouse heart: an MR tagging study

PURPOSE

To develop a 3D MR tagging method that combines harmonic phase (HARP) and homogeneous strain analysis methods for quantification of regional myocardial wall motion in mice.

MATERIALS AND METHODS

3D tagged images were acquired from seven C57BL/6 mice. Intersecting tag points were reconstructed and 3D strains were quantified at apical, midventricular, and basal levels. Circumferential and radial strains quantified with 2D MR tagging were compared with those calculated from 3D tagged images.

RESULTS

Our data showed significant heterogeneity in radial, circumferential, and shear strains. Longitudinal strain was more homogeneous. The circumferential-longitudinal shear strain, a unitless measure of ventricular torsion, was positive throughout the left ventricle. There were strong correlations between 2D and 3D studies at the basal and midventricular levels.

CONCLUSION

This work demonstrates the feasibility of 3D characterization of cardiac function in mouse via the combination of HARP and homogeneous strain analysis.

Principal strain changes precede ventricular wall thinning during transition to heart failure in a mouse model of dilated cardiomyopathy

This study was performed to elucidate the relation between in vivo measurements of two-dimensional principal strains and the progression of left ventricle (LV) wall thinning during development of dilated cardiomyopathy in the protein kinase C-ε (PKC-ε) transgenic (TG) overexpressing mouse heart. Principal two-dimensional strains, E1 and E2, were determined in the LV wall of the anesthetized mouse using cardiac MRI tagging at 14.1 T. PKC-ε TG provided a model of pure dilated cardiomyopathy without evidence of hypertrophy (PKC-ε TG, n = 6). Ejection fraction, wall thickness, and principal strains were determined at 1-mo intervals in hearts of PKC-ε TG vs. age-matched, nontransgenic mice (NTG, n = 5) from age 6 to 13 mo. Through the study, PKC-ε TG displayed lower ejection fraction than NTG. At 7 mo, average principal strain E1 in PKC-ε TG hearts was lower compared with NTG (PKC-ε TG = 0.14 ± 0.03, NTG = 0.19 ± 0.03, P < 0.05). The greatest reductions in regional E1 occurred in the lateral segments. The principal strain E2 did not change significantly in either group. At 9 mo, LV wall thinning occurred in PKC-ε TG mice ( P < 0.01 vs. 8 mo) to 21% below values in NTG ( P < 0.001). Average E1 strain diverged between PKC-ε TG and NTG hearts by 25–43%. These E1 changes preceded LV wall thinning and predated the eventual transition from a compensated circumstance to the dilated phenotype. The findings indicate a near step function in E1 depression that precedes the onset of LV wall thinning and suggest E1 as a prognostic indicator of dilated cardiomyopathy.

Monitoring left ventricular function in small animals

Small animals such as mice and rats are extensively used to investigate the mechanisms and treatment of human cardiac diseases in vivo. The monitoring of left ventricular function is a key factor in this research. The measurement should be rapid, reproducible, and repeatable and allow the detection of subtle differences in function. Currently, echocardiography is most widely used in cardiac research laboratories for measuring left ventricular dimensions and function in small animals. Although the technique is rapid, the reproducibility of the calculations of left ventricular volumes is limited in some circumstances as a result of assumptions that do not necessarily hold true, such as in the setting of dilated, failing ventricles.

Mechanoelectrical feedback as novel mechanism of cardiac electrical remodeling

BACKGROUND

Altered electrical activation of the heart by pacing or disease induces profound ventricular electrical remodeling (VER), manifested electrocardiographically as T-wave memory and ultimately as deleterious mechanical remodeling from heterogeneous strain. Although T-wave memory is associated with altered expression of sarcolemmal ion channels, the biophysical mechanisms responsible for triggering remodeling of cardiac ion channels are unknown.

METHODS AND RESULTS

To test the hypothesis that mechanoelectrical feedback triggered by regional strain is a mechanism for VER, dogs (n=6) underwent 4 weeks of ventricular pacing to induce VER. Multisegment transmural optical action potential imaging of left ventricular wedges revealed profound and selective prolongation of action potential duration in late-activated (288+/-29 ms) compared with early-activated (250+/-9 ms) myocardial segments (P<0.05), providing the first experimental evidence that amplification of repolarization gradients between segments of left ventricle is the electrophysiological basis for T-wave memory. In vivo tagged magnetic resonance imaging revealed a 2-fold and preferential increase in circumferential strain in late-activated segments of myocardium, which exactly coincided with segments undergoing VER. VER could not be attributed to structural remodeling because it occurred without any histological evidence of cellular hypertrophy.

CONCLUSIONS

The mechanism responsible for triggering remodeling of ion channel function in VER was locally enhanced circumferential strain. These data suggest a novel mechanoelectrical feedback mechanism for inducing physiological and potentially deleterious electrical heterogeneities in the heart.