AssessmentsNormal heart sounds
Normal heart sounds
Content Reviewers:Rishi Desai, MD, MPH
If you put a stethoscope over the chest, you’ll usually hear something that sounds like lub dub, lub dub, lub dub, which repeats over and over again, with each cardiac cycle, or heartbeat.
Now, the question is, where does this sound come from? Normally, blood is constantly moving through the four chambers of the heart- coming through the veins into the right atrium, going to the right ventricle, then shooting off via the pulmonary artery to the lungs and coming back from the pulmonary veins into the left atrium and the left ventricle, to be pumped into the aorta. So, in every step, some valves have to open and others have to close. Valves are just “communicating doors” that, when open, allow blood to pass through, and when closed, hold blood within a chamber. So, in total, our heart has four valves- two atrioventricular valves, which separate the atria from the ventricles and are the tricuspid valve, on the right side, and the mitral valve, on the left side, and two semilunar valves, which separate the ventricles from the large arteries coming off of them and are the pulmonary valve, on the right side, and the aortic valve, on the left side. And when these valves are closing, just like a door slamming shut, they are going to make a sound that is transmitted in the direction of the blood flow. The heart is positioned in such a way in the chest cavity, so that the sound of the closing of each of these valves is projected onto a small area of the anterior chest wall. So, let's start from the right side. If we place our stethoscope between the second and third rib, known as the right second intercostal space, just next to the upper border of the sternum, we’ll best hear the closing of the aortic valve. Then, right over on the left side, in the left second intercostal space, left upper sternal border, there is the pulmonary valve closing. Making our way down, between the fourth and fifth rib, next to the left lower border of the sternum, we’ll be closer to the tricuspid valve. Finally, let’s move a bit further to the left and downwards, between the fifth and sixth rib, so in the left fifth intercostal space, and in the midclavicular line. This means if we drew a line from the midpoint of the left clavicle, or the collarbone, straight down in a longitudinal fashion, we’ll find a point that would intersect with the fifth intercostal space. And that’s where the closing of the mitral valve is best heard. Now in reality, a lot of these things are happening at once, like a factory with lots of things happening in parallel. For example, the right and left atria are both full of blood, and that blood moves through them tricuspid and the mitral valve to get down into the ventricles. Initially, the blood flows passively into the ventricles, but near the end when there’s just a bit left, there’s an atrial contraction that gives the blood an extra hard push to help get it out. This part of the heartbeat, when blood is filling the relaxed ventricles is called diastole. Now once the ventricles have filled up, both of the atrioventricular valves snap shut, creating a long, loud sound that sounds a bit like “lub”. That’s the first heart sound, or S1. And because it’s basically the tricuspid and mitral valve closing, it’s best heard in the tricuspid valve area, so in the left fourth intercostal space, lower sternal border, as well as in the mitral valve area, so in the left fifth intercostal space, in the mid- clavicular line. So, at this point, the ventricles are full of a whole lot of blood and are ready to squeeze it out. And to do that, the aortic valve, on the left side, and the pulmonic valve, on the right side, quietly open up. Blood flows from the left ventricle into the aorta and from the right ventricle into the pulmonary artery. This part of the heartbeat, when blood is ventricles are contracted and pushing blood out is called systole. Eventually, the ventricles finish squeezing, so these two valves close down, making a short, sharp sound that sounds a bit like a “dub”. And this dub is called the second heart sound, or S2. This is heard louder in the aortic valve area, so in the right second intercostal space, right upper sternal border, and the pulmonary valve area, in the left second intercostal space, left upper sternal border. During inspiration, though, if you listen carefully with a stethoscope, this S2 sound actually splits into two separate sounds. That’s because the diaphragm muscle lowers during inspiration, and that creates negative pressure in the chest to bring in air, and that negative pressure also brings a bit more venous blood back to the right atrium and right ventricle. It takes a little bit longer for the right ventricle to squeeze this extra blood in the pulmonary artery and it takes a little bit longer for the pulmonary valve to close. So during inspiration, the closing of the pulmonary valve is heard slightly later than the aortic valve, and that’s called physiologic splitting of the S2. Now after both the aortic and pulmonary valves have shut down, the atrioventricular valves open again, letting the cycle start all over again.
Alright, now in addition to S1 and S2, there are two other "extra" sounds that are sometimes heard in the cardiac cycle, called S3 and S4. S3 and S4 are heard in different parts of diastole. In early diastole, right after S2, which signifies the closing of the aortic and pulmonic valves, the atrioventricular valves are open and blood is flowing into the ventricles. If there’s a lot of blood coming in, the ventricles fill up quickly, and send fluid waves that bounce off of the walls of the ventricles which makes them vibrate and create a third heart sound, or S3. S3 sounds kind of like “lub-dub-ta”. In trained athletes and also in pregnancy this is totally normal and just means that the ventricles are handling some extra blood volume. But an S3 can also be a sign of volume overload, like in congestive heart failure, where there is too much volume coming into that ventricle. Now, at the end of diastole, just before S1, the atria are contracting to get that last bit of blood into the ventricles. If the ventricles are too stiff, meaning that they can’t easily relax, the atria will have to contract extra hard to push that blood in, creating the fourth heart sound, or S4. So, S4 sounds kind of like "ta-lub-dub". In most cases, this stiffness is because the ventricular muscles have hypertrophied, or increased in size, in order to pump against high blood pressure in the aorta or pulmonary artery. In other words, S4 is usually a sign of pressure overload, or severe hypertension.
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