What are heart sounds?
The heart sounds are the normal sounds that is made by the activity of the heart and heard with a stethoscope placed on the chest wall (auscultation). This sounds like a “lub, dub” and is associated with the beating heart (contraction and relaxation) and the flow of blood through the heart and great blood vessels. There are four heart sounds and it is the result of different parts of the cardiac cycle. Contrary to popular belief, the heart sounds are not a result of the heart muscle contracting and relaxing but rather the effects associated with the beating heart. Sound arises from vibration and any part of the cardiac cycle that causes reverberation will lead to a heart sound.
First Heart Sound
When the heart contracts, the valves between the atria and ventricles (AV valves – mitral and tricuspid valves) close tightly in order to prevent backward flow of blood. Blood rushes against the valves causing it to momentarily bulge into the respective atria and the chordae tendinae connected to these valves also stretch. The blood is then forced forward again, back into the ventricle to be pushed out into the respective arteries – pulmonary artery and aorta.
The first heart sound is caused by the blood hitting against the closed AV valves, vibration of the valves and chordae tendinae bouncing back and the blood being pushed back into the ventricles where it hits against the ventricular wall. This sound is not a result of the valve leaflets slapping together as the valve closes. The first heart sound, the “lub” of “lub dub”, is best heard at the cardiac apex.
Second Heart Sounds
The second heart sound is a result of the semilunar valves (aortic and pulmonary valves) closing as the heart approaches the end of systole (contraction). These valves are located at the junction of the ventricles and large arteries, the blood vessels that carry blood out of the heart. The semilunar valves prevent blood flowing back into the ventricles when the heart contraction is completed and the heart then relaxes.
These valves close by a small amount of blood in the vessel flowing backwards and filling up the valve cusps. This happens quite suddenly and for an instant the cusps of the semilunar valves stretch back into the heart valve before recoiling forward into the vessel again. This causes the vibrations that is responsible for the second heart sound or the “dub” of “lub dub”. It is best heard at the upper left sternal border.
Third Heart Sounds
The third heart sound is a soft rumbling sound that is not always audible. It is also known as the ventricular gallop and occurs at around the middle of diastole (relaxation of the heart). The exact cause of this sound is not clear but it is believed to be due to the flow of blood from the atria into the partially filled ventricles. This does not happen right at the beginning of diastole because the ventricles are empty and some quantity of blood in the ventricles are required to create this vibration. The third heart sound, if audible, is best heard at the cardiac apex.
Fourth Heart Sound
The fourth heart sound, also known as the atrial gallop, cannot be heard with a stethoscope. It is a weak sound of very low frequency and requires the aid of a phonocardiogram. This heart sound is a result of the blood being forced into ventricles when the atria contracts and is a continuation of the third heart sound.