Ultrasonic imaging of propagation of electric excitation in heart wall

We have already shown that the pulsive vibration is excited on the myocardium 15 ms after the electrical stimulation to an isolated heart [1]. If the heart wall vibration caused by spontaneous electric excitation is visualized using ultrasound, the regional cell damage regarding electric potential due to diseases can be noninvasively detected. In this study, the spatial distribution of the minute vibration caused just around R-wave of ECG is visualized in in vivo experiments as follows: By the ultrasonic measurement of the myocardial motion [2], we detect pulsive waves spontaneously caused by electrical excitation or valve closure. Using a sparse sector scan [3], the waves are measured almost simultaneously at about 1,000 points set in the heart wall at a high frame rate (400 Hz). The consecutive spatial distributions of the interpolated phase of the waves reveal wave propagation along the wall. The propagation time is several ms, which is too short to be detected by conventional methods. The method was applied to 5 healthy subjects. The spontaneously driven pulsive waves and their propagations were clearly visible in all subjects in the longitudinal-axis, short-axis, and apical views. Just after the Q-wave of ECG, the propagation started from the apex, which is close to the papillary muscle (terminal of Purkinje fiber), to the base side of the heart. Its propagation speed was slow (1-2 m/s for 20-100 Hz), which shows the propagation of electrical excitation. Then, after R-wave of ECG, another pulsive wave started to propagate reversely from base to apex. Since its speed was several m/s for about 50 Hz but there was dispersion, this is the shear wave caused by the mitral-valve closure. The method noninvasively reveals the propagation of electrical conduction wave by measuring regional myocardial response to it in human heart, which will be a novel tissue characterization of the heart.