Why S3 Heart Sound is Normal in Children
An S3 heart sound is a normal physiological finding in children because their highly compliant ventricular myocardium and vigorous early diastolic filling create rapid deceleration of blood flow during early ventricular filling, producing an audible low-frequency sound without indicating cardiac pathology.
Physiological Mechanism
The S3 in healthy children results from specific hemodynamic characteristics of the young heart:
Rapid early diastolic filling velocity: Children demonstrate higher peak early diastolic transmitral velocities (approximately 67 cm/s) compared to adults, creating the conditions for an audible S3 1.
Increased deceleration rate: The abrupt deceleration of left ventricular inflow during early diastole generates the S3 sound, with children showing deceleration rates around 726 cm/sec² 2.
Enhanced ventricular compliance: The young myocardium has increased viscoelasticity that allows for rapid early filling followed by sudden intrinsic limitation of ventricular expansion, producing the characteristic low-frequency vibrations 3.
Acoustic Characteristics in Children
The physiological S3 in children has distinct phonocardiographic features:
Low-frequency distribution: Approximately 50% of the spectral energy is distributed in the 0-15 Hz frequency band, making it a low-pitched sound 3.
Audibility threshold: The S3 becomes audible when its frequency equals or exceeds 25 Hz and its amplitude reaches at least one-quarter of the first heart sound or one-third of the second heart sound 4.
Timing: The S3 occurs during early flow deceleration, close to peak early filling velocity, and its timing shifts earlier in the cardiac cycle with increasing age 3.
Age-Related Changes
The prevalence and characteristics of S3 change predictably with age:
High prevalence in youth: The S3 is common throughout childhood and remains present in approximately 23% of individuals approaching age 40 1.
Predictors of persistence: Lower body mass index and higher early diastolic left ventricular inflow velocity independently predict the presence of physiological S3 1.
Mechanism of disappearance: The S3 gradually disappears with age due to primary age-related alterations in left ventricular early diastolic function, not from increasing blood pressure or relative left ventricular hypertrophy as traditionally taught 1.
Clinical Distinction from Pathologic S3
The key difference between physiological and pathologic S3 lies in the clinical context, not the acoustic mechanism:
Shared mechanism: Both physiological and pathologic S3 result from rapid deceleration of early diastolic left ventricular inflow 2.
Distinguishing features in pathologic S3: Elevated left ventricular filling pressures (E/E' ratio), reduced ejection fraction, and decreased myocardial compliance measured by tissue Doppler imaging indicate pathology 5.
Clinical assessment: In children with normal cardiac examination findings, no family history of genetic cardiac disease, and absence of symptoms, the S3 requires no further evaluation 6.
Practical Clinical Approach
When encountering an S3 in a child:
Normal scenario: An isolated S3 in an asymptomatic child with otherwise normal cardiac examination findings is physiological and requires no intervention 6.
Red flags requiring echocardiography: Loud or pathological murmurs, cardiomegaly on chest radiograph, abnormal ECG findings, symptoms (dyspnea, exercise intolerance, syncope), or family history of genetic cardiac disease warrant echocardiographic evaluation 6.
Body habitus consideration: Leaner children are more likely to have an audible physiological S3, and the sound may become inaudible with weight gain or positional changes that reduce venous return 1, 2.