Kino M, Shahamatpour A, Spodick D H
Am J Cardiol. 1976 May;37(6):848-52. doi: 10.1016/0002-9149(76)90108-9.
A clinically heterogeneous group of 48 subjects (aged 19 to 76 years) were searched for a fourth heart sound (S4) by three independent "blind" auscultators. Phonocardiograms recorded immediately after auscultation were measured by another "blind" observer. An S4 was identified in 32 subjects (67 per cent) in phonocardiograms with nominal filter peaks of both 70 and 35 hertz. Results in these subjects revealed two groups by auscultatory performance: those with an "easily heard" and those with an "easily missed" S4. Subjects with an "easily missed" S4 were significantly younger than those with an "easily heard" S4 (31.2 +/- 2.8 years versus 50.0 +/- 4.0 years, P less than 0.001). P-R and P-S4 intervals and relative amplitude of S4 (ratio of fourth to first heart sound [S1] amplitude) were not significantly different in the two groups. Splitting of the first heart sound (S1) was observed more frequently in the phonocardiogram of patients with an "easily missed" S4, but this trend did not reach statistical significance. The interval between S4 and the low frequency component of S1 was significantly short in those with an "easily heard" S4 (49.4 +/- 4.1 msec versus 70.0 +/- 5.0 msec, P less than 0.005). The mean S4-S1 (low frequency component) interval for the group with an "easily heard" S4 approximated 1 cycle length for S4 vibrations, a finding consistent with temporal acoustic summation. An alternate hypothesis is modification of S1.
