Speeds of Contraction Responses Propagating along Septum at Pre-ejection Period are Different Between Radial and Longitudinal Directions

Abstract

Measuring myocardial contractile movement caused by electrical excitation leads to early detection of abnormalities of cardiomyocytes due to disease. However, the detailed mechanism of the transition process in the myocardium from dilation to contraction at the pre-ejection period is still unclear. In the present study, we acquired ultrasonic RF signals with a high frame rate of 1.16 ms using the parallel beamforming [1] by transmitting a plane wave with a sector probe. Velocity waveform in the 2D direction, the beam direction (the radial direction in the heart wall), and the direction orthogonal to the beam (the longitudinal direction in the heart) were simultaneously estimated by applying the speckle tracking along the heart wall. The cross-correlation coefficient was interpolated so that the spatial resolutions in displacement estimation along the radial and longitudinal directions were increased to 1.0 and 2.2 μm, respectively. We then detected the propagation speeds of contraction response due to electrical excitation along the radial and longitudinal directions at the pre-ejection period. The radial component of 2D velocity waveforms was measured at 112 points along the septum around the R wave of ECG. The velocity component to RV (LV expansion) and that of LV (contraction) were obtained. In the period around R wave, two components propagated along the septum from the basal to apical sides with speeds of 3 m/s and 6 m/s. On the other hand, for the longitudinal component, three components propagated with speeds of 1 m/s, 3 m/s and 12 m/s. Though these all velocity components were simultaneously measured for the same points in the septum, the situations of 2D velocity waveforms were quite different, which would be useful for understanding what occurs at the pre-ejection period.