Where S3 is Physiologically Heard
The physiological third heart sound (S3) is best heard at the cardiac apex using the bell of the stethoscope with the patient in the left lateral decubent position.
Anatomical Location
The S3 is heard at the cardiac apex, which is typically located in the fifth intercostal space at the midclavicular line 1, 2. This location corresponds to the point of maximal impulse (PMI) where left ventricular activity is most prominent.
Key Auscultatory Characteristics
Normal Physiological S3
- Present in most pregnant patients as a normal finding 1, 2
- Occurs during early diastole when rapid ventricular filling takes place
- Best appreciated with the bell of the stethoscope applied lightly to the skin
- Enhanced by having the patient in the left lateral position
Distinguishing Features
The physiological S3 in pregnancy is part of the normal cardiovascular adaptation, where:
- Third heart sounds are present in most patients 1, 2
- Associated with increased blood volume (50% increase) and cardiac output
- Accompanied by a hyperkinetic precordial impulse and louder first heart sound with prominent splitting 1, 2
Mechanism
The S3 results from sudden intrinsic limitation of longitudinal expansion of the left ventricular wall during early diastolic filling, creating a negative jerk transmitted to the chest wall surface 3, 4. This occurs when rapid ventricular filling is abruptly decelerated 5.
Right-Sided S3 (Important Caveat)
A right-sided S3 may be present and increases with inspiration, as do nearly all right-sided heart sounds 6. This would be heard best at the left lower sternal border rather than the apex, and the inspiratory augmentation helps distinguish it from a left-sided S3.
Clinical Context
In young, healthy individuals and pregnant women, the S3 is a benign finding reflecting vigorous ventricular filling. The key is recognizing the apical location and understanding that in physiological states, this represents normal hemodynamics rather than pathology.