Heart sounds are vibrations created by the mechanical activity of the heart, which doctors listen to using a stethoscope in a process called auscultation. A healthy heart produces a rhythmic “lub-dub” sound as its valves open and close to control blood flow. These sounds provide clues about the heart’s condition. When extra sounds occur outside of the normal rhythm, they can signal underlying issues with the heart’s structure or function.
The Baseline: Understanding S1 and S2
The familiar “lub-dub” rhythm is produced by the two normal heart sounds, S1 and S2. S1 marks the beginning of ventricular contraction, known as systole. This sound is generated by the simultaneous closure of the atrioventricular valves (the mitral and tricuspid valves) as the ventricles begin to squeeze blood out.
S2 marks the end of systole and the beginning of the relaxation phase, known as diastole. S2 is caused by the closure of the semilunar valves (the aortic and pulmonic valves) as the ventricles finish ejecting blood. These two sounds establish the timing reference for any extra sounds, as systole occurs between S1 and S2, and diastole occurs between S2 and the next S1.
Defining the S3 Gallop
The third heart sound, S3, is a low-frequency sound that occurs early in the diastolic phase, right after S2. When present, S3 creates a triple rhythm often compared to the cadence of the word “Kentucky,” earning it the name “ventricular gallop.” The mechanism is the sudden deceleration of blood as it rushes into a volume-overloaded or overly compliant ventricle.
This rapid, turbulent filling causes the ventricular walls to vibrate. In children and young adults, S3 may be a normal, temporary finding due to a highly flexible ventricle. However, in a middle-aged or older adult, S3 is often a significant sign of severe conditions like Congestive Heart Failure (CHF). S3 is highly suggestive of elevated ventricular filling pressures and a high volume state within the heart.
Defining the S4 Gallop
The fourth heart sound, S4, occurs much later in diastole, just before S1. This sound is referred to as an “atrial gallop” or “presystolic gallop” and creates a rhythm compared to the cadence of the word “Tennessee.” The timing of S4 coincides with the contraction of the atria, known as the “atrial kick,” which pushes the final volume of blood into the ventricles.
The mechanism for S4 involves the atria forcefully contracting to push blood into a ventricle that is abnormally stiff and resistant to filling. This stiffness, or reduced compliance, often results from conditions that cause the ventricular muscle to thicken, such as severe hypertension or aortic stenosis. The sound is caused by the vibration of the rigid ventricular wall as it receives the final, forceful wave of blood. The key difference from S3 is that S4 indicates a stiff, non-compliant ventricle, while S3 indicates an overfilled, compliant ventricle.
Clinical Implications and Detection
The presence of an S3 or S4 gallop is a significant clinical finding, marking underlying cardiac dysfunction. Detecting either sound suggests the heart is working under abnormal conditions: struggling with excessive volume (S3) or attempting to fill a stiff, resistant chamber (S4). Physicians detect these sounds during a physical examination using a stethoscope, a technique called cardiac auscultation.
Because both S3 and S4 are low-frequency sounds, they are best heard using the bell side of the stethoscope, which picks up lower-pitched vibrations. Patients may be asked to lie on their left side or perform other maneuvers to make the faint sounds more audible. Once an S3 or S4 is detected, the next step in diagnosis is typically an Echocardiogram (Echo). The Echo uses sound waves to create a moving image of the heart, visually confirming the mechanical problem, such as ventricular dilation or hypertrophy, suggested by the extra heart sound.