Page 13, Cardiac Cycle, Dr. D. Penney
As stated earlier, S1 and S2 are normal heart sounds Table 1.03 . They both result from valve closure and reverberations within the walls of the ventricle. S1 is heard best over the apex (tip) of the heart, since the sound is projected downward. S2 is heard best above the base of the heart, the aortic component on the right and pulmonic component on the left. This apparent reversal of position is due to the fact that the two great vessels spiral around one another. As well as differences in the point in the cardiac cycle at which each sound is heard, they also differ in sound quality - S2 is a higher frequency, sharper sound than S1.
S3 and S4 are abnormal, thus cannot normally be heard with the standard stethoscope. S3 is commonly heard in cases where ventricular compliance is depressed, for example in heart failure, whereas S4 is usually associated with left atrial hypertrophy, as in mitral regurgitation.
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