The process of listening to the heart, known as auscultation, relies on recognizing the distinct sounds created by the heart’s valves opening and closing. These sounds provide important clues about the heart’s function and health. The fundamental rhythm of the heartbeat is composed of two primary sounds, S1 and S2. Sometimes a third (S3) or fourth (S4) sound can be detected, forming what is often called a gallop rhythm. These extra, low-frequency sounds can indicate underlying changes in the heart muscle or blood flow dynamics. The presence of S3 or S4 sounds, particularly in adults, signals a need for further medical evaluation, as they often correlate with significant cardiac conditions.
The Normal Heart Sounds (S1 and S2)
The recognizable “lub-dub” of a healthy heart represents the two normal heart sounds, S1 and S2. These sounds mark the beginning and end of the heart’s pumping phase, or systole. The first sound, S1, is produced by the nearly simultaneous closure of the atrioventricular (AV) valves—the mitral and tricuspid valves—at the start of systole. This closure prevents blood from flowing backward into the atria as the ventricles begin to contract and eject blood.
The second sound, S2, occurs at the end of systole and is caused by the closure of the semilunar valves, specifically the aortic and pulmonic valves. This sound prevents blood from flowing back into the ventricles as they begin to relax and refill, a phase called diastole. The S2 sound is naturally split because the aortic valve closes slightly before the pulmonic valve. This classic two-sound pattern establishes the baseline for the cardiac cycle, and any extra sounds heard outside of this rhythm are considered abnormal in most adults.
The S3 Gallop: Mechanism and Indication
The S3 heart sound, sometimes described as a ventricular gallop, is an extra, low-pitched sound that occurs early in diastole, immediately following S2. This sound is generated during the rapid filling phase of the ventricles when blood rushes in from the atria. The mechanism involves the sudden deceleration of blood flow as the ventricular wall quickly reaches its elastic limit, causing the wall itself and the attached structures to vibrate.
The presence of an S3 can be either physiological or pathological, depending on the person’s age and overall health. A physiological S3 may be heard in young, healthy individuals, trained athletes, or women in the later stages of pregnancy due to the heart’s more compliant walls and increased blood volume. In these cases, the sound is generally considered benign and not indicative of disease.
However, an S3 heard in an adult over the age of 40 is typically a pathological finding and is strongly associated with ventricular dysfunction and volume overload. This pathological S3 gallop suggests that the ventricle is failing to pump effectively, leading to an increased volume of blood remaining in the chamber and causing the walls to stretch excessively during the rapid filling phase. It is a highly specific marker for elevated ventricular filling pressures and is frequently a sign of congestive heart failure (CHF) or severe mitral regurgitation. The sound is often described as having a “Ken-tuck-y” cadence, representing the S1-S2-S3 rhythm.
The S4 Gallop: Mechanism and Indication
The S4 heart sound, also called an atrial gallop or presystolic gallop, is a low-pitched sound that occurs late in diastole, just before S1. Its timing places it during the final phase of ventricular filling, when the atria contract to push the last volume of blood into the ventricles. The sound is produced by the forceful contraction of the atrium against a ventricle that is stiff and non-compliant.
Unlike the S3, the S4 is a sign of a pressure-overloaded and rigid ventricle that resists blood flow. This increased stiffness results from the ventricular muscle thickening, known as hypertrophy, which makes the chamber less able to relax and accept blood. The forceful push of blood by the contracting atrium causes the stiff ventricular walls to vibrate, generating the S4 sound.
The S4 sound is nearly always considered pathological and is a clear indicator of decreased ventricular compliance. It is commonly observed in conditions that cause the heart muscle to work against high resistance, such as severe hypertension, aortic stenosis, or hypertrophic cardiomyopathy. Since the S4 requires an active atrial contraction, it is not present in individuals with atrial fibrillation, where the atria merely quiver instead of contracting effectively. The cadence of the S4 gallop is often compared to the word “Ten-nes-see.”