What did we hear at the apex or mitral area? We heard a very distinct cadence, not just "lub dub, lub dub", but we heard "lub dub boom, lub dub boom, lub dub boom, lub dub boom" - first sound, second sound, and very aptly named - a third sound.
What is the cause of the third sound? We already know why the first and second sound; the third sound is a filling sound due to blood rushing into the ventricle and accelerating and then decelerating. It can be a normal sound, and in this case, that third heart sound occurred about .015 or .016 after the second sound. Let's everybody listen and appreciate "lub dub boom, lub dub boom, lub dub boom". That third sound, again, can be normal, can be abnormal. You judge it by the company it keeps. Let's listen with the bell for that low frequency sound especially.
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.
Graphic example
This is a graphic example of an S2 followed by an S3. Let us listen together. Note that the third heart sound is low-frequency.
Third heart sound
Third heart sounds are early diastolic, low-frequency filling sounds, best heard with the bell of the stethoscope. They occur approximately fifteen hundredths of a second after the second sound, and are related to the acceleration and deceleration of blood during early, passive filling of the ventricle.
Left ventricular third sounds may be normal or physiologic in children and young adults, as blood rapidly accelerates into the ventricle. They may also be pathologic in heart failure, as blood rapidly decelerates due to a stiff, non-compliant ventricle.
Left ventricular third sounds are best heard at the apex. Right ventricular third sounds are best heard at the tricuspid area and may augment with inspiration.