Visualization in propagation of electric excitation in human heart

Conventional echocardiography visualizes cross-sectional images, motion, and torsional deformation during contraction of the heart. However, it is restricted to static configurations or large and slow motion. We have previously found that minute pulsive vibration occurs just after electrical stimulation of the extracted papillary muscle of a rat [Acoust Sci & Tech 2003;24:17]. By applying a novel ultrasound-based method [IEEE UFFC 1997;44:752] to human hearts, we were able to successfully measure the spontaneous response of the myocardium to electrical excitation [UMB 2009;35:936]. In the present study, we visualize the propagation of the myocardial response of the electric excitation in human hearts during systole. In the parasternal short-axis view of the left ventricle (LV), the RF reflective wave along each ultrasonic beam was acquired. The number of directions of the ultrasonic transmission was restricted to 16 to maintain a high frame rate (500 Hz), and then at all of about 25,000 points set in the heart wall, the velocity components toward the ultrasonic probe were simultaneously obtained as waveforms, and their instantaneous phases of 40-Hz components were color-coded (red: come close to). The instantaneous distribution of the phase was rearranged along the circumferential direction and set in an array consecutively at every 2 ms from the time of P-wave of the ECG, precisely revealing the propagation of the velocity components in the LV circumferential direction. This novel method was applied to healthy subjects. A velocity component corresponding to the contraction was generated at the septum at a time of R-wave of ECG, and propagated slowly (0.4 m/s) in clockwise direction along the LV circumferential direction. On the other hand, just from each radiation time of the first and second heart sounds, mechanical shear was generated at the septum and propagated in counterclockwise direction. These phenomena were observed for other subjects. The propagation of the contraction will correspond to one of the layers consisting of the LV. The subtle dynamic response of the myocardium to the arrival of the electrical stimulation accurately measured in the present study will show a potential for noninvasive assessment of myocardial damage due to heart failure.