The heart produces sounds during its normal operation, primarily the familiar “lub-dub” rhythm, composed of the first and second heart sounds, S1 and S2. S1 marks the beginning of the heart’s contraction phase (systole) as the mitral and tricuspid valves close. S2 signals the start of the relaxation phase (diastole) when the aortic and pulmonic valves snap shut. When the heart is struggling, extra sounds may become audible, known as S3 and S4 heart sounds. These additional low-frequency vibrations, often described as a gallop rhythm, signify underlying mechanical or structural problems and prompt medical investigation.
What S3 and S4 Sounds Represent
Both the S3 and S4 sounds occur during diastole, the period when the ventricles are relaxing and filling with blood. These sounds are not generated by the closure of heart valves like S1 and S2, but rather by the abrupt deceleration and vibration of the ventricular walls. S3 is known as a ventricular gallop and occurs early in diastole, immediately following the S2 sound, during the rapid passive filling phase. This sound is generated because blood rushes into a ventricle that is volume-overloaded or too compliant to handle the rapid inflow without vibrating.
The S4 sound is referred to as an atrial gallop and occurs much later, just before the S1 sound, during the final stage of ventricular filling. This late filling phase is when the atria contract forcefully to push the remaining blood into the ventricles. The S4 sound is created when the atrium must contract against a ventricle that is abnormally stiff and resistant to filling.
Causes of the S3 Gallop Sound
The S3 heart sound is fundamentally a sign of volume overload or systolic dysfunction, meaning the heart cannot effectively eject the blood it contains. Congestive heart failure with a reduced ejection fraction is a common cause, where the weakened ventricle is dilated and cannot empty completely during systole. When the next rush of blood enters the already partially filled chamber during early diastole, the walls vibrate, creating the S3 sound.
Conditions that increase the volume of blood the ventricle must handle are also frequent causes of the S3 gallop. Severe valvular regurgitation, such as mitral or tricuspid regurgitation, allows blood to flow backward, flooding the ventricle during the rapid filling phase. This excessive volume quickly overstretches the ventricular chamber, leading to the characteristic vibration. High-output states, such as severe anemia or hyperthyroidism, can also cause an S3 sound because the body’s increased metabolic demands force the heart to pump a much larger volume of blood per minute.
Causes of the S4 Gallop Sound
The S4 heart sound is a strong indicator of reduced ventricular compliance, often called diastolic dysfunction, caused by conditions that create high pressure within the heart. Unlike the volume problem that causes S3, S4 is a pressure problem related to a stiff, non-compliant ventricular wall. The sound is generated when the atrium contracts vigorously in late diastole, attempting to force blood into the rigid ventricle. The blood hits the unyielding wall, producing the low-frequency sound just before the next beat.
Long-standing systemic hypertension is the most common cause, as high blood pressure forces the left ventricle to work harder, leading to left ventricular hypertrophy (thickening of the muscle wall). This muscle thickening makes the ventricle stiff and less able to relax and accept blood. Other conditions that increase pressure and wall thickness, such as severe aortic stenosis, also frequently result in an S4 gallop. Acute myocardial ischemia can cause a transient S4 sound because the injured heart muscle becomes acutely stiff. The S4 sound is notably absent in patients with atrial fibrillation because chaotic electrical activity prevents the forceful atrial contraction necessary to generate the sound.
Clinical Importance and Diagnosis
The detection of an S3 or S4 heart sound in an adult is a significant clinical finding. The S3 sound, in particular, is a poor prognostic sign when heard in patients with heart failure, indicating a more advanced stage of disease and predicting a higher risk of re-hospitalization and mortality. The S4 sound is more indicative of chronic pressure-related issues like long-term hypertension and may be the first physical sign of ventricular hypertrophy.
A doctor hearing one of these gallops will proceed with diagnostic testing to confirm the cause and severity of the heart condition. Echocardiography, a non-invasive ultrasound of the heart, is the primary tool used to confirm the findings from the physical exam. This imaging technique allows doctors to visualize the heart’s structure, measure the ejection fraction, assess for chamber dilation, and quantify the degree of valve regurgitation or wall stiffness. The presence of a gallop sound guides subsequent management, as treating the underlying cause, whether volume overload or high-pressure stiffness, is the goal.