The S3 gallop is an abnormal extra heart sound heard during a heart examination, following the normal S1 and S2 “lub-dub” sounds. This third sound creates a three-beat rhythm often compared to a horse galloping, hence the name “gallop.” The S3 signifies rapid ventricular filling during the early phase of the heart’s relaxation cycle. Although it is a low-frequency sound that can be subtle, its presence often indicates underlying heart dysfunction.
The Mechanism of the S3 Sound
The S3 sound results from the movement of blood as the heart chambers refill during early diastole. This is the period when the ventricles relax and begin to fill rapidly with blood from the atria.
The sound is believed to be caused by the sudden deceleration of rapid blood flow as it impacts a ventricular wall that is either overfilled or less compliant. Theories suggest the sound results from vibrations of blood bouncing between the ventricular walls or the sudden tensioning of the chordae tendineae. Regardless of the precise mechanism, the S3 is a consequence of unusually fast inflow of blood into the ventricle.
Primary Causes of Pathological S3 Gallop
A pathological S3 gallop indicates a problem with the heart’s ability to handle blood volume or its capacity to relax and fill properly. The most common cause in adults is Congestive Heart Failure, particularly heart failure with reduced ejection fraction. In this condition, the ventricle is dilated and weakened, struggling to pump blood effectively and accommodate incoming volume.
This leads to increased atrial pressure and an accelerated filling rate into the impaired ventricle, generating the S3 sound. Valvular heart diseases causing volume overload, such as severe mitral or tricuspid regurgitation, are also frequent causes. Regurgitation increases the volume the ventricle must manage, exacerbating the rapid filling.
Other causes relate to high-output states, where the body’s demand requires the heart to pump an unusually large volume. Conditions like severe anemia, thyrotoxicosis (an overactive thyroid), or a large systemic shunt can lead to this accelerated flow. These scenarios result in a rapid influx of blood into a ventricle that is structurally compromised or experiencing excessive volume load.
Distinguishing Normal from Pathological S3
The clinical significance of the S3 gallop depends heavily on the patient’s age and context. An S3 sound is considered physiological, or normal, in children, young adults under 40, and highly trained athletes. In these individuals, the sound is benign, reflecting a compliant and healthy ventricle that is filling rapidly.
However, the presence of an S3 in an adult over 40 is considered pathological and strongly indicates underlying heart disease. In older patients, the sound correlates with ventricular dysfunction or significant volume overload, often signaling heart failure. Clinicians differentiate between normal and pathological S3 by observing positional changes: a physiological S3 often diminishes when the patient sits up, while a pathological S3 persists.