third heart sound
The third heart sound, written S3, is a soft, low-pitched thud heard in early diastole — just after the second heart sound — as the ventricles begin to fill. If the usual two sounds are lub-dub, the S3 adds a gentle dub close behind, giving the cantering cadence that, when prominent, is called a gallop. It is dull and quiet, easy to overlook unless one listens for it deliberately.
The sound is thought to arise as blood rushing into the ventricle in early diastole is suddenly decelerated by the ventricular wall and the structures within. In a stiff or volume-loaded ventricle this rapid filling produces a more audible vibration. In adults, a new S3 commonly accompanies the high filling pressures of heart failure, so it is one of the more specific physical signs of a struggling, overloaded ventricle.
Context decides its meaning. In children, athletes, and pregnant women a physiological S3 is normal, reflecting brisk, healthy filling. The same sound in a middle-aged or older patient with breathlessness and other signs of congestion is far more concerning and supports a diagnosis of ventricular volume overload or failure.
A heart-failure patient admitted with worsening breathlessness has an audible S3 at the apex; after diuretic treatment unloads the ventricle, the sound fades on later examination.
An S3 that appears with congestion and fades after diuresis tracks the ventricle's filling pressure.
An S3 is the low extra sound that creates a gallop rhythm; the two terms describe the same finding from different angles — the sound itself versus the cadence it produces.