The genesis of the normally split first heart sound.

Abstract 1. 1. The intensity distribution along the anterior chest wall, timing, respiratory and poststraining variations of the physiologically split first heart sound, especially in the medium-high frequency range, has been investigated by means of multiple-filter phonocardiography in children who were 2 to 12 years of age. 2. 2. In 60 children from 2 to 12 years of age the first main component of the split first heart sound has on an average, its maximum intensity at the apex, where it is of greater amplitude than the second main group of vibrations. 3. 3. The second component of the split first heart sound has its point of maximum intensity (especially in the medium-high-frequency band) at the fourth left intercostal space and is the dominating vibration along the left sternal border in each age group. 4. 4. The reversal of the intensity relationship between both major components of the split first heart sound from the left sternal border to the apex is a constant feature in the whole group of children included in this study. 5. 5. The average time interval from the Q wave to the beginning of the first main component of the split first heart sound is 53.4 ± 0.33 msec. (S.D. 6.4 msec.); and to the onset of the second main component the interval is 75.2 ± 0.42 msec. (S.D. 7.8 msec.). 6. 6. Both main components of the split first heart sound are normally separated by no more than 21.5 ± 0.23 msec. (S.D. = 4.1 msec.). Therefore, in about 95 per cent of all normal children the splitting interval is shorter than 30 msec. 7. 7. The intensity of both components of the normally split first heart sound as well as the splitting interval increase with age. 8. 8. During inspiration the first component of the split first heart sound decreases in intensity, whereas the second component increases, if respiratory variations of sound conduction (transmission) can be eliminated. 9. 9. During the poststraining period of the Valsalva maneuver the second component of the split first heart sound reaches its maximum amplitude immediately after the release of pressure, whereas the initial group of vibrations is at first small and increases with some delay. 10. 10. The respiratory and poststraining variations of both splitting components reflect the different hemodynamic events of the left and right sides of the heart. 11. 11. The intensity distribution, timing, respiratory and poststraining behavior of the first component of the normally split first heart sound are in accordance with the concept of left-sided (mitral) origin of the vibration, whereas the corresponding data for the second main component of the normally split first heart sound leave no doubt that this vibration is, at least in the 2 to 12-year age group studied, of tricuspid and not of aortic origin.

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