The third heart sound, or S3, is an extra heart sound sometimes heard during a cardiac exam. When heard alongside the normal S1 and S2 sounds, it creates a three-beat rhythm often described as a “gallop” rhythm, commonly referred to as an S3 gallop. In adults over 40, the presence of this sound frequently signals underlying heart dysfunction and requires immediate medical investigation. While not a disease itself, the S3 serves as an important acoustic marker of changes occurring within the heart’s pumping chambers.
The Physiological Mechanism of the S3 Sound
The two primary sounds of the heartbeat are S1, caused by the closure of the mitral and tricuspid valves at the beginning of ventricular contraction, and S2, caused by the closure of the aortic and pulmonic valves at the end of contraction. The S3 sound occurs shortly after S2, placing it in the very early phase of diastole, the period when the ventricles relax and fill with blood. This timing corresponds precisely with the moment of rapid ventricular filling, known as the protodiastolic phase.
The sound is generated by the sudden deceleration of blood rushing from the atria into the ventricles, causing vibrations within the ventricular walls that transmit to the chest wall. The S3 is typically a low-frequency, dull sound, distinguishing it from the higher-pitched S1 and S2. The mechanism is often described as blood striking a ventricular wall that is either overly compliant (stretched and dilated) or stiff and unable to accommodate the volume quickly.
The underlying physical event is the rapid inflow velocity of blood into the ventricle during early filling, causing the momentum of the incoming blood to decelerate abruptly. Research indicates that both physiological and pathological S3 sounds are related to this abnormally rapid deceleration rate of the early diastolic inflow. This rapid filling causes the ventricular structures, including the chordae tendineae and the ventricular wall, to tense and vibrate, creating the audible sound.
Underlying Conditions That Cause Pathological S3
In middle-aged and older adults, the S3 gallop is strongly associated with conditions that result in either ventricular dysfunction or excessive volume overload. The most common and serious cause is congestive heart failure, particularly systolic failure, where the heart muscle is weakened and the ventricle becomes dilated. A weakened ventricle struggles to empty completely during contraction, leading to an elevated residual volume that is then abruptly struck by the incoming flow of blood during diastole.
This mechanism of a volume-overloaded, poorly functioning ventricle explains why the S3 sound is considered a reliable sign of left ventricular failure. Conditions that lead to increased blood volume returning to the heart, such as severe mitral or tricuspid regurgitation, also frequently produce an S3 sound. In valvular regurgitation, the incompetent valve allows blood to flow backward into the atrium, resulting in a larger volume of blood rushing into the ventricle during the rapid filling phase.
Other causes relate to states where the blood flow itself is abnormally fast. High-output states, such as severe anemia or thyrotoxicosis, can increase the velocity of blood flow so the rapid filling phase becomes turbulent and audible. Although less common, acute conditions like a ventricular septal defect can also cause an S3 by allowing a large volume of blood to shunt into the ventricle, increasing the filling pressure. The appearance of an S3 in an adult suggests the heart is operating under significant hemodynamic stress, whether from volume issues or muscle weakness.
Detection and Clinical Interpretation
The S3 heart sound requires a specific technique for detection. Clinicians use a stethoscope, specifically the bell, which is better at picking up low-frequency sounds, to listen for the gallop. The sound is typically best heard over the apex of the heart, corresponding to the left ventricle, with the patient lying on their left side to bring the heart closer to the chest wall.
The clinical interpretation of the S3 sound relies heavily on the patient’s age and clinical context. An S3 sound heard in children, young adults under the age of 40, or during the later stages of pregnancy is often considered physiological, meaning it is a normal finding and not indicative of disease. In these cases, the sound is attributed to a healthy, supple ventricle that is simply undergoing very rapid, vigorous filling.
Conversely, an S3 sound identified in an adult over the age of 40 is considered pathological and a strong indicator of ventricular dysfunction or volume overload. In patients with known heart disease, the presence of an S3 is often a marker of worsening status and a predictor of adverse outcomes. Finding an S3 in an adult generally prompts further diagnostic testing, such as an echocardiogram, to assess the extent of heart muscle damage and overall function.