Gallops, and Continuous Murmurs
You are listening to a typical example of a third heart sound, or S3. Shortly after S2, the closing of the semilunar valves, the AV valves open and diastole begins. Diastole is itself further divided into several stages, the first being that of rapid filling, where 80% of the blood stored in the atria during systole is transferred to the ventricles. At the end of this stage, about 140-160 msec after S2, an S3 may be heard if the volume which has been transferred is abnormally large, as in mitral regurgitation. It can be thought of as a sound which is generated when the ventricle is forced to dilate beyond its normal range because the atrium has overloaded volume. An S3 is usually heard best with the bell of the stethoscope placed at the apex while the patient is in the left lateral decubitus position. The presence of an S3 is usually normal in children and young adults, but pathologic in those over the age of 40.
A good mnemonic to remember the cadence and underlying pathology of an S3 is:
S1 S2 S3 S1 S2 S3
Try to repeat the above phrase in time to the rhythm that you hear now.
Compare this to a
fourth heart sound.