[Experimental studies on the genesis of low-frequency vibrations (M-sound) of the first heart sound using a miniature accelerometer].

We studied eight mongrel dogs, weighing 15 to 35 kg, in which an initial low-frequency vibration of the first heart sound (M-sound) was recognized on the chest wall. A miniature accelerometer weighing 0.5 gm was used to record surface velocity signals and surface acceleration signals as well as phonocardiograms over the cardiac apex of the closed chest wall and over the pericardium or epicardium. The frequency response of accelerometer was essentially flat (+/- 1.5 dB) from 1 to 200 Hz. The accelerometer mounted was such that its sensitive axis was perpendicular to the recording surface. Equisensitive phonocardiograms were obtained to compare the signal size of M-sound on the epicardium at several positions including the cardiac apex, the left ventricular anterior wall near the interventricular septum, the left ventricular antero-lateral wall and the right ventricular anterior wall. Intraventricular phonocardiograms and pressure curves were obtained by a Millar catheter directly inserted through the left ventricular wall near the apex to keep the tip near the apex. Furthermore, the relationship between M-sound and the shortening of the myocardium at the apex was investigated by means of ultrasonic dimension system and phonocardiography. Studies of M-sound were performed not only in sinus rhythm, but also in atrio-ventricular dissociation and ventricular pacing after crushing sinus node or electrical vagus stimulation. The results were as follows: The M-sounds over the chest wall, pericardium and epicardium were recorded coincidentally with the onsets of the left ventricular pressure curve, its dP/dt, low-frequency vibration of the first heart sound of intraventricular phonocardiogram, positive velocity, and acceleration of myocardial surface. The M-sound on the epicardium was maximal in intensity at or near the cardiac apex, in comparison with those recorded on the left ventricular anterior wall near the interventricular septum, left ventricular antero-lateral wall, and right ventricular anterior wall. The M-sound was observed between A and C points of the mitral valve echogram. There was no effect of atrial contraction on the M-sound in cases of atrio-ventricular dissociation. The onset of the M-sound on the epicardium at the apex was not always coincident with the onset of shortening of the myocardium at the same position.(ABSTRACT TRUNCATED AT 400 WORDS)