The S3 heart sound is an extra sound that occurs in the cardiac cycle, often referred to as a “Ventricular Gallop.” This third sound is a low-frequency, brief vibration that can be challenging to hear against the normal “lub-dub” rhythm of the heart.
Identifying the Timing and Mechanism of S3
The S3 sound occurs during the early phase of ventricular filling, a period known as early diastole, which immediately follows the second heart sound (S2). This timing places the sound approximately 0.12 to 0.18 seconds after the S2 sound has finished. The characteristic rhythm created by the three sounds in quick succession—S1, S2, and S3—is often compared to the cadence of the word “Kentucky”.
The sound is produced by the rapid deceleration of blood as it rushes from the atria into the ventricles during this early filling phase. This high-velocity inflow hits a physical limit, causing the ventricular walls and the column of blood to vibrate. This mechanism is most pronounced when the ventricle is either overloaded with volume or has become less compliant and unable to accommodate the sudden influx of blood easily.
The resulting vibration is a dull, low-pitched thud, which is significantly quieter than the higher-pitched S1 and S2 sounds. The low frequency, typically in the range of 25 to 50 Hertz, explains why the S3 sound is so difficult to detect.
Primary Auscultation Point
The S3 sound primarily originates from the left ventricle, the heart’s main pumping chamber. Therefore, the optimal place to listen is the cardiac apex, often known as the mitral area, as it lies closest to the left ventricle.
This anatomical location is typically found in the fifth intercostal space, along the mid-clavicular line on the left side of the chest. This is the same general area where the point of maximal impulse (PMI) can often be felt. Placing the stethoscope directly over this impulse ensures maximum proximity to the source of the sound.
Because S3 is a low-frequency sound, using the correct part of the stethoscope is important for detection. The bell of the stethoscope is specifically designed to transmit low-pitched sounds, whereas the diaphragm filters them out. The bell should be applied lightly to the skin, barely creating a seal, to best amplify these subtle vibrations.
If the S3 sound originates from the right ventricle, a less common finding, it would be best heard along the lower left sternal border. However, the S3 sound from the left ventricle is almost exclusively sought at the apex.
Optimizing Detection Through Maneuvers
Specific patient positioning and breathing instructions are necessary to bring the heart closer to the chest wall and minimize competing noise. The single most effective maneuver is placing the patient in the left lateral decubitus position. This shifts the heart closer to the chest surface, concentrating the apical impulse.
When the patient is in this position, the examiner should place the bell of the stethoscope lightly on the cardiac apex. A firm press stretches the skin, causing it to act like a diaphragm and filter out the low-pitched S3 sound. Instructing the patient to exhale fully and briefly hold their breath (suspended expiration) also helps. This maneuver pushes the lungs away from the heart, reducing respiratory noise and maximizing the chances of detection.
Clinical Significance of S3
Once detected, the interpretation of the S3 heart sound depends heavily on the patient’s age and overall health status. In children, young adults up to the age of 40, and pregnant women, an S3 sound can be a normal finding, referred to as a physiological S3. In these individuals, it simply reflects a healthy, compliant ventricle capable of rapid filling.
The presence of an S3 in an adult over the age of 40 is typically a sign of underlying heart pathology. The pathological S3 strongly suggests ventricular dysfunction or volume overload, often because the ventricle is stiff or unable to relax properly. This finding is recognized as one of the most sensitive indicators of ventricular dysfunction, such as that seen in heart failure. The detection of a pathological S3 in an older adult warrants further investigation into conditions like severe mitral regurgitation.