Heart sounds

The human heart produces distinct noises during its rhythmic beating. These sounds are not caused by the muscle itself contracting. Instead, they are generated by the flow of blood through the heart. Specifically, heart sounds reflect the turbulence created when heart valves snap shut. When these valves close suddenly, they cause a reverberation within the blood. Medical professionals use a stethoscope to listen to these sounds. This process, known as cardiac auscultation, provides vital auditory data about heart health.

In a healthy adult, there are two primary sounds. These are often described as a "lub" and a "dub." The first sound, called S1, forms the "lub." It is composed of two parts: the mitral valve closure (M1) and the tricuspid valve closure (T1). Usually, the mitral closure happens slightly before the tricuspid closure. S1 occurs at the beginning of ventricular contraction, or systole. As the ventricles contract, they squeeze against the closed semilunar valves. This pressure sends blood rushing toward the atria. The parachute-like leaflets of the atrioventricular valves catch this rush and snap shut.

To prevent these valves from turning inside out, the heart uses a special mechanism. The papillary muscles in each ventricle contract at the same time as the ventricles. These muscles are attached to the valve leaflets via chordae tendineae, often called heart strings. When the muscles contract, the chordae tendineae become tense. This prevents the valves from evertng, which means they won't bulge too far into the atria. This tension ensures that blood cannot flow backward into the lower-pressure atria. The S1 sound itself is the result of the blood reverberating after this sudden block of flow.

The second heart sound is called S2, or the "dub." This sound marks the end of ventricular systole and the start of ventricular diastole, which is the relaxation phase. S2 is composed of the aortic valve closure (A2) and the pulmonary valve closure (P2). Normally, the aortic closure precedes the pulmonary closure. As the left ventricle empties, its internal pressure falls below the pressure in the aorta. This causes blood to flow quickly backward toward the ventricle. The pocket-like cusps of the aortic valve catch this blood and snap shut. A similar process happens with the pulmonary valve as pressure falls in the right ventricle.

Sometimes, the S2 sound can be "split." A physiological split is normal and often occurs during inhalation. During inhalation, a decrease in intrathoracic pressure increases the time needed for pulmonary pressure to exceed right ventricular pressure. However, a wide and fixed split can indicate an atrial septal defect. Other conditions, such as pulmonary stenosis or a right bundle branch block, can cause a wide and variable split. In cases of pulmonary hypertension or pulmonary embolism, the pulmonary component (P2) may sound much louder than usual. Conversely, aortic stenosis can make the S2 sound much softer.

Beyond the normal sounds, doctors may hear extra sounds called gallop rhythms. An S3 sound, or "ventricular gallop," sounds like "lub-dub-ta." It occurs at the beginning of diastole when blood rushes into the ventricles. While S3 can be benign in young people or athletes, it may signal heart failure in older adults. An S4 sound, or "atrial gallop," sounds like "ta-lub-dub." This sound occurs when blood is forced into a stiff or hypertrophic ventricle. S4 is generally a sign of a pathologic state, such as systemic hypertension. If S3 and S4 are both present, it is called a quadruple gallop.

Another common finding is a heart murmur. Murmurs are whooshing sounds produced by turbulent blood flow. For a murmur to be audible, there must be a pressure difference of at least 30 mm of Hg between chambers. Turbulence can be caused by stenosis, where a valve opening is too narrow, or insufficiency, where a valve fails to close properly. If turbulence occurs outside the heart, it is called a bruit or vascular murmur. Doctors grade murmurs from 1 to 6. A Grade 1 murmur is very faint, while a Grade 6 is so loud it can be heard even when the stethoscope is partly off the chest.