Focal myofiber disarray in a transgenic mouse model with overexpression of ras
caused significant increases in heterogeneity of regional function. Local surface shearing showed the greatest decrement, although reduced principal surface shortening correlated with areas of fiber disarray. Global function in terms of ventricular pressure and cardiac output were not altered in the hearts with myocyte disarray, and mean surface segment shortening also was not different. This result is consistent with previous findings of inhomogeneity of contractile function in human hypertrophic cardiomyopathy; these MR tagging studies in humans have indicated either normal or hyperkinetic overall function of the diseased heart (3
). Thus the changes in overall systolic septal mechanics from these transgenic mice are modest, consisting mostly of reduced septal torsion, but regional variations in function correlate to the underlying structural abnormalities. The effect of disarray alone may be to reduce surface function because previous studies of systolic surface function in a hypertrophy-only model show substantial increases in surface strains (20
There was a significant decrease in RV septal shear strain in transgenic mice compared with controls. Shear strain can be interpreted as a local indicator of ventricular torsion. The reduced shear in these animals suggests that disarray, which tends to concentrate in the septum, interferes with the ability of the myocardium to twist. Modeling studies on myocardial torsion have concluded that the normal transmural fiber angle distribution is responsible for the ability of the heart to twist (1
). The disruption of this ordered arrangement in a heart with focal disarray and the reduced RV septal shear are consistent with this conclusion. A MR study conducted by Young et al. (35
) indicated an increase in global ventricular torsion in humans associated with hypertrophic cardiomyopathy. Our results indicate that disarray in the septal wall contributes to reduced torsion in the septum; however, we did not examine the LV free wall for evidence of increased torsion, which might compensate for reduced torsion of the septum.
We tested the sensitivity of our strain analysis system by examining the effect of injury created by needle insertion. Local needle injury caused a reduction in maximum principal strain and an increase in heterogeneity of strain. From video camera images, the reduction in contraction corresponded with the area of the needle insertion. Staining of the tissue using TTC indicated an area of ~0.25 mm2 of necrotic tissue surrounding the needle insertion point. Thus we conclude that our technique is capable of detecting local variations in contractile function occurring on a length scale on the order of 0.25 mm.
Consistent with the finding of significant positive correlation between angular deviation and RV septal maximum principal strain, there was a decrease in E1 associated with areas of disarray compared with normal tissue. This indicates that RV septal function is directly influenced by disarray embedded in the septal wall. There was no significant reduction in overall maximum principal strain in transgenic animals (E1 = −0.10 ± 0.03 control, compared with −0.08 ± 0.03 transgenic). However, there was significantly reduced contractile function in the specific areas overlying disarray in the wall. There was also evidence of depth dependence in this effect as well, with no significant differences between strains associated with normal or disarrayed tissue when disarray was found at the midwall or close to the LV septum.
There was an overall reduction in average shear strain, E12
, on the RV septum in transgenic animals compared with control (average E12
= 0.05 ± 0.01 control, compared with 0.01 ± 0.03 transgenic). There is reduced shear associated specifically with disarray and particularly in areas with moderate to severe disarray. This result is consistent with modeling studies that describe the importance of transmural fiber distribution for the development of ventricular torsion during systole (28
During systolic contraction, the whole heart tends to twist in a left-handed helical fashion (positive twist). This is consistent with the fiber orientation found on the RV septal surface of the heart (fiber angle approximately − 60° with respect to the global circumferential axis). However, the fiber orientation at the LV septal surface is opposite (approximately +60° with respect to the global circumferential axis). According to Taber et al. (28
), this is due to the larger mechanical advantage of the superficial fibers as a result of a larger moment arm created by the larger radius. During ejection, the wall thickens and further enhances this mechanical advantage. The findings from the MR tagging study by Young et al. (35
), which found increased ventricular torsion, are consistent with this theory of mechanical advantage because the wall is thickened in human hypertrophic cardiomyopathy (this theory would predict increased torsion in any hypertrophic heart). Our results indicate the shear strain is reduced overall on the RV septal surface and particularly in areas overlying disarray. This is most likely due to the disruption of the transmural distribution of fiber orientation. Because disarray tends to concentrate in the septum in both human hypertrophic cardiomyopathy and the ras
overexpression model, there may be a reduced tendency to twist in the septum, whereas hypertrophy of the LV may result in larger twist overall.
This study presents a method for measuring two-dimensional nonhomogeneous deformation in the isolated ejecting mouse heart. Our hemodynamic measurements are similar to those reported from other isolated ejecting mouse heart preparations (9
). Our results describe the first measurement of regional nonhomogeneous strain in the mouse heart and offers a sensitive method for examining regional function in the mouse heart that may have use in other transgenic mouse models of disease. It would also be of interest to examine changes in diastolic regional function in this model, because diseases such as hypertrophic cardiomyopathy are associated with prolonged ventricular relaxation. This would best be done either with an arrested heart preparation or with high-speed imaging during diastole in a contracting heart because the time resolution of the current system is limited to 60 fields/s.
The potential impact of other structural alterations besides myocyte disarray on regional function have not been addressed in this study. For example, myocyte hypertrophy, interstitial fibrosis, and altered myocardial architecture could influence local function. Similarly, myocyte contractile properties could be altered and would influence local strain measures in addition to effects of altered myocyte orientations. Structural alterations may be prevalent beyond the interventricular septum and may affect global mechanics and function; we did not examine myocardium outside of the central septal region. Another limitation of the present study was the use of non-wild-type littermates for control hearts. We chose to use “normal” weight-matched mice (Swiss) for control. We performed disarray analysis on three C57/B16 control mice in addition to the Swiss mice. We found the average dispersion of fiber angle to be very similar to that of Swiss mice (13.2 ± 0.8° in C57/B16 vs. 12.6 ± 0.6° in Swiss). The correlation results in this study do not involve any of the normal Swiss mice. We also did not quantitatively assess cardiac hypertrophy in the individual animals, thus variations in the hypertrophic response could increase the variations seen at the function level.
We conclude that nonhomogeneous strain analysis is a sensitive indicator of regional dysfunction in the mouse heart. Myofiber disarray found in the septal wall in transgenic mice with ventricular expression of ras
causes local dysfunction. The most striking alteration in regional mechanics due to fiber disarray was the decrease in septal torsion. Torsion occurs due to shortening of muscle fibers arranged in a three-dimensional helical pattern, and in these transgenic hearts regions of fibers are disoriented. Because the principal strains were not affected as much as shear strains, there may be an uncoupling of the strain components due to fiber disarray. Hansen et al. (10
) suggest that LV torsion is a sensitive indicator of contractile mechanics. Additionally, modeling studies suggest that ventricular twist is very sensitive to alterations in transmural fiber angle distribution (28
) and crossover of fibers from epicardium to endocardium (4
). Alterations in ventricular torsion in situations with regional fiber disarray such as human hypertrophic cardiomyopathy may be caused directly by the disarray and a lack of normal torsional deformation may have implications for local stress and oxygen consumption of the tissue.