How is the S3 (third heart sound) best heard with a stethoscope?

How to Best Hear the S3 (Third Heart Sound)

The S3 is best heard using the bell of the stethoscope placed at the cardiac apex with the patient positioned in the left lateral decubitus position. 1

Stethoscope Technique

• Use the bell of the stethoscope rather than the diaphragm, as the S3 is a low-frequency, low-amplitude sound that requires the bell's ability to detect lower-pitched sounds 1
• Apply light pressure with the bell—excessive pressure will stretch the skin and convert it into a diaphragm, reducing sensitivity to low-frequency sounds 2
• Position the bell at the cardiac apex (point of maximal impulse) 1

Patient Positioning

• Place the patient in the left lateral decubitus position (lying on their left side) to optimize S3 detection 1
• This positioning brings the cardiac apex closer to the chest wall, which is critical for detecting the low-energy S3 1
• The patient should be relaxed and quiet during auscultation to minimize artifact 3

Clinical Context and Timing

• The S3 occurs in mid-diastole, shortly after the second heart sound (S2), during the rapid ventricular filling phase 4
• Use the "inching" technique: keep S2 in mind as a reference point, then move the stethoscope from the aortic area to the apex—if an extra sound occurs after S2, it is an S3 2
• The S3 indicates elevated left ventricular filling pressures and volume overload in most adults, making it highly specific for heart failure despite only intermediate sensitivity 1, 5

Important Exception

• In pregnancy, S3 is present in most patients as a normal physiologic finding due to the 50% increase in circulating blood volume and increased cardiac output 6, 1
• This represents a key exception where S3 does not indicate pathology 1

Common Pitfalls

• Do not confuse S3 with a split S2: A pathologic S3 can be misunderstood as an abnormal S2 with fixed splitting 7
• Distinguish S3 from S4: The S4 (atrial gallop) occurs before S1 in late diastole, while S3 occurs after S2 in early diastole; additionally, S4 is eliminated with firm pressure on the stethoscope, but S3 persists 2
• In patients with emphysematous chest or increased anteroposterior diameter, the S3 might be missed over usual precordial areas—try listening over the xiphoid or epigastric area 2