The contribution of left ventricular muscle bands to left ventricular rotation: Assessment by a 2-dimensional speckle tracking method

Background: Torsion is an essential component of left ventricular (IV) function. Systolic rotation, as a component of torsion, winds the heart muscle up like a spring, setting up recoil for early diastole. We used a new 2-dimensional speckle tracking strain method to study differences in twisting in subendocardial and subepicardial layers of the IV in open-chest pigs. Our aim was to identify the relative contributions of the inner or outer layers of the IV wall to rotation and, hence, systole. Methods. A total of 23 juvenile pigs were imaged in the short axis, epicardially, to obtain images at a level just below the papillary muscles with high-frequency (14 MHz) ultrasound. Speckle tracking software using scanline files was used to measure the torsional contribution of septum, anterior, posterior, and inferior IV wall segments. Two zones on the septum were evaluated separately: one with apparent circumferential fiber orientation in the inner layer and one with a speckle pattern suggesting longitudinal fiber orientation on the right ventricular aspect of the septum. Pressure rate changes (dP/dt) during the cardiac cycle were measured as an index of IV function and correlated with the regional torsion. Results: Mean peak rotations measured by speckle tracking echocardiography at the apex showed counterclockwise rotation of IV septal wall (10.68 +/- 2.67 degrees for the inner layer and 8.27 +/- 1.73 degrees for the outer layer). The time difference for time to peak rotation was 213.22 +/- 77.95 and 241.17 +/- 54.67 milliseconds for inner and outer layers, respectively. Significant differences were shown between the inner and outer layer of the IV for both rotation (P = .000) and timing of rotation (P = .02). The dP/dt measurements correlated well with the inner rotation magnitude of the IV and with the difference of short-axis rotation between inner and outer layers of the IV wall. Conclusions: Inner and outer layers of the IV wall, especially at the septum, have different rotational behaviors. When used with very high-resolution imaging, this method could contribute to the understanding of functional contributions of the IV wall and their relative contribution to cardiac segmental twisting.