Abstract
The relation between auricular systole and the first sound, the role of the atrio-ventricular valves, and myocardial contraction have been considered, together and separately, as factors contributing to the production of the first heart sound. 1-7 Some observers have felt that closure and tensing of the atrio-ventricular valves at the onset of systole are the sole elements in causing the first sound, while others have adhered to the theory that the initial heart sound is partly muscular and partly valvular in origin. We investigated the problem, applying a method designed to prevent atrio-ventricular valve movement.
The instrument here used to record the heart sounds was a cathoderay stethograph, furnished by the Burdick Corporation. The instrument is aperiodic to most vibrations produced by the heart, and the lags and overshoots, so characteristic of most string galvanometers used for this work, are lacking. Therefore, the sound tracings are more nearly exact reproductions of the cardiac vibrations produced. The instrument is so constructed that sound waves below fifty cycles per second, which are not appreciated by the normal ear, may be heard.
Dogs were anesthetized with veterinary nembutal. The thoraces were opened and respiration was maintained by a respiratory pump. To prevent movements of the mitral and tricuspid valves, small balloons were affixed to the ends of brass tubes 3 mm in diameter; these were introduced into the ventricular cavities through small openings made in the auricles and then through the mitral and tricuspid apertures. The inflow of blood into the heart was then cut off by clamping the venae cavae, so as to empty the heart of blood, and the balloons were inflated by water or air to a pressure of 70 mm of Hg against a manometer.
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