The S4 heart sound is produced when the atria contract and push blood into a ventricle that has become stiff or less compliant than normal. This stiffness forces the blood to decelerate suddenly against the resistant ventricular wall, generating low-frequency vibrations (around 20 to 30 Hz) that a trained ear can pick up with a stethoscope. The most common causes are high blood pressure, aortic valve narrowing, and heart muscle disease, though it can also appear in older adults without any clear cardiac problem.
How the S4 Sound Is Produced
A normal heartbeat has two audible sounds: S1 (the “lub”) when the heart contracts, and S2 (the “dub”) when the valves snap shut after blood is ejected. The S4 is an extra sound that occurs just before S1, during the final phase of the heart’s filling cycle. At that moment, the atria give a last squeeze, sometimes called the “atrial kick,” to push the remaining blood into the ventricles.
When the ventricle wall is thickened, scarred, or otherwise resistant to stretching, that incoming blood slams to a halt more abruptly than usual. The vibrations from that sudden deceleration are what create the S4. Because the sound originates inside the ventricle rather than the atrium itself, it disappears in people with atrial fibrillation, where the atria no longer contract in an organized way and there is no effective atrial kick to push blood forward.
High Blood Pressure and Aortic Stenosis
The two most common conditions behind an S4 are chronic high blood pressure and aortic valve stenosis (narrowing of the valve that opens into the aorta). Both force the left ventricle to pump against higher-than-normal resistance. Over time, the heart muscle responds by growing thicker, a process called left ventricular hypertrophy. A thicker wall is a stiffer wall, and that reduced compliance is exactly what sets up the S4.
Once hypertrophy is established, the ventricle doesn’t relax and fill as easily during diastole. This is the early stage of what clinicians call diastolic heart failure: the heart still squeezes adequately, but it can’t fill properly. Symptoms like shortness of breath with exertion or fluid retention can follow. An S4 heard during a physical exam is one of the earliest clues that this process is underway.
Ischemic Heart Disease
When part of the heart muscle isn’t getting enough blood flow, whether during a heart attack or from chronic coronary artery disease, the affected tissue becomes temporarily or permanently stiffer. Oxygen-starved heart muscle loses its ability to relax normally between beats. The result is the same mechanical problem: the atrial kick meets a resistant ventricle, and an S4 appears. In acute chest pain scenarios, a new S4 can be a bedside sign that ischemia is actively affecting the heart’s ability to fill.
Cardiomyopathy
Several types of heart muscle disease produce an S4. Hypertrophic cardiomyopathy, a genetic condition where the heart wall grows abnormally thick, is a classic cause in younger patients. The thickened septum and ventricular walls dramatically reduce compliance. Restrictive cardiomyopathy, where the heart muscle becomes stiff from infiltrative diseases like amyloidosis, also commonly produces an S4. In patients with restrictive cardiomyopathy who remain in normal sinus rhythm, the fourth heart sound is actually more common than the third heart sound.
Dilated cardiomyopathy, by contrast, tends to produce an S3 rather than an S4, because the problem is a stretched-out, floppy ventricle rather than a stiff one. This distinction is one way clinicians use heart sounds to narrow down what type of heart problem might be present.
How S4 Differs From S3
Both S3 and S4 are low-pitched extra heart sounds, but they occur at different points in the filling cycle and point to different problems. The S3 happens in early diastole, about 120 to 180 milliseconds after S2, when blood rushes passively from the atrium into a ventricle that is often dilated. It is most associated with volume overload and congestive heart failure with a weak pump.
The S4 happens later, in the presystolic portion of diastole, just before S1. It reflects a stiffened ventricle rather than a dilated one. Conditions like hypertension, aortic stenosis, ischemia, and hypertrophic cardiomyopathy are its typical causes. When both sounds are present simultaneously, the combination is called a “summation gallop” and generally indicates severe cardiac dysfunction.
S4 in Healthy People
Not every S4 means something is wrong. The ventricle naturally stiffens with age, and S4 sounds become increasingly common in people over 50. The reported prevalence in older adults varies widely, from 11% to 75% depending on the study and whether sensitive recording equipment (phonocardiography) was used versus a stethoscope alone. One study of asymptomatic adults aged 18 to 94 found the S4 in about 15.6% of participants. A Framingham Heart Study analysis found phonocardiographic evidence of an S4 in roughly 73% of both healthy subjects and those with cardiovascular disease, suggesting that the sound alone, particularly in older people, doesn’t reliably separate the sick from the well.
Athletes can also have an audible S4 without any underlying disease. Their slower resting heart rates and larger filling volumes create conditions where the atrial contraction produces enough force against a normal ventricle to generate the sound. Hyperkinetic states like anemia and an overactive thyroid can do the same: vigorous atrial contractions push blood into a ventricle that is handling higher-than-usual volumes.
How the S4 Is Detected
Because the S4 is a very low-frequency sound, it is best heard using the bell of the stethoscope pressed lightly against the chest. Pushing too hard actually stretches the skin into a flat diaphragm, which filters out low frequencies. The ideal listening spot is along the lower left edge of the breastbone, with the patient lying on their left side. This position brings the heart closer to the chest wall and makes the faint vibrations easier to pick up.
Even with proper technique, the S4 is easy to miss. It sits at the lower edge of human hearing, and in a noisy clinic or with a fast heart rate that shortens diastole, it can blend into the background. Phonocardiography and echocardiography can confirm its presence when clinical suspicion is high but the stethoscope exam is inconclusive.