Third heart sounds, or S3s, are early diastolic, low frequency filling sounds best heard with the bell of the stethoscope. They occur approximately one hundred fifty msec after the second sound and are related to the acceleration and deceleration of blood during early passive filling of the ventricle. Third sounds may be normal, or physiologic, in children and young adults, as blood rapidly enters the ventricle then abruptly decelerates. Third heart sounds may be pathologic in heart failure as blood rapidly decelerates in a poorly functioning ventricle. They may also be pathologic in conditions that increase ventricular volume, such as valvular regurgitation and shunts.
Physiologic and pathologic third heart sounds may arise from either or both ventricular chambers. Right sided S3s are best heard at the tricuspid area and may augment with inspiration. Left sided S3s are best heard at the apex in the left lateral decubitus position.
Moving heart S3
This is a graphic example of the heart in a normal patient with a third heart sound at the apex. In the animation that follows, we can appreciate that the third heart sound is generated during early diastole and it is associated with rapid ventricular filling.
Volume curve S3
This is a graphic example of an S1 and an S2 followed by an S3. By simultaneously viewing the ventricular volume curve, one can appreciate that the third sound occurs just at the end of early, rapid diastolic filling. Let us listen together.
Another filling sound - S4
The other important filling sound that may be heard at the apex is a fourth heart sound. It occurs in late diastole when atrial contraction causes rapid blood flow into the ventricle. If the ventricle is less compliant than normal, rapid deceleration of blood occurs and causes low frequency vibrations analogous to those occurring with a third heart sound. In the elderly patient, it is not clear if an S4 invariably implies heart disease.
Graphic example
This is a graphic example of an S4 preceding an S1. Let us listen together.