Physiology
It occurs at the beginning of diastole, approximately 0.12 to 0.18 seconds after S2. This produces a rhythm classically compared to the cadence of the word "Kentucky" with the final syllable ("-ky") representing S3. One may also use the phrase "SLOSH’-ing-in" to help with the cadence (SLOSH S1, -ing S2, -in S3), as well as the pathology of the S3 sound., or any other number of local variants.
S3 may be normal in people under 40 years of age and some trained athletes but should disappear before middle age. Re-emergence of this sound late in life is abnormal and may indicate serious problems like heart failure. The sound of S3 is lower in pitch than the normal sounds, usually faint, and best heard with the bell of the stethoscope.
It has also been termed a ventricular gallop or a protodiastolic gallop because of its place in early diastole. It is a type of gallop rhythm by virtue of having an extra sound; the other gallop rhythm is called S4. The two are quite different, but they may sometimes occur together forming a quadruple gallop. If the heart rate is also very fast (tachycardia), it can become difficult to distinguish between S3 and S4 thus producing a single sound called a summation gallop. S3 is a dull, low-pitched sound best heard with the bell placed over the cardiac apex with the patient lying in the left lateral decubitus position. This heart sound when present in a child or young adult implies the presence of a supple ventricle that can undergo rapid filling. Conversely, when heard in a middle-aged or older adult, an S3 is often a sign of disease, indicating increased ventricular filling due to congestive heart failure or severe mitral or tricuspid regurgitation.
Read more about this topic: Third Heart Sound
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