Is an S3 (third heart sound) the same as a systolic murmur?

No, S3 and Systolic Murmurs Are Completely Different Cardiac Sounds

An S3 (third heart sound) is NOT the same as a systolic murmur—they are fundamentally different acoustic phenomena that occur at different times in the cardiac cycle and represent distinct pathophysiologic processes.

Timing Distinguishes These Sounds

S3 occurs in early diastole (after S2), while systolic murmurs occur between S1 and S2 during ventricular contraction. 1, 2

• S3 timing: The third heart sound occurs after the second heart sound (S2) during the rapid filling phase of early diastole 2, 3
• Systolic murmur timing: These sounds begin with or after S1 and end at or before S2, occurring entirely during ventricular systole 1, 4
• Using the "inching" technique with a stethoscope, keeping S2 as a reference point, clearly demonstrates that if a sound occurs after S2, it is an S3 (ventricular diastolic gallop), whereas if it appears before S2, it is a systolic murmur 2

Acoustic Characteristics Differ Completely

• S3 characteristics: A low-pitched, brief "gallop" sound best heard with the bell of the stethoscope at the apex, often described as sounding like "Ken-tuck-y" (S1-S2-S3) 2
• Systolic murmur characteristics: These are sustained sounds with varying pitch and quality depending on etiology—they can be midsystolic (crescendo-decrescendo), pansystolic (plateau-shaped throughout systole), early systolic, or late systolic 1, 4

Pathophysiology Is Fundamentally Different

S3 represents abnormally rapid deceleration of early diastolic ventricular filling, while systolic murmurs represent turbulent blood flow during ventricular contraction. 3, 5

S3 Mechanism:

• Generated by rapid deceleration of early diastolic left ventricular inflow (deceleration rate >700 cm/sec² in pathologic states) 5
• Related to myocardial viscoelasticity and ventricular compliance characteristics 6
• Occurs when blood rapidly decelerates as it fills the ventricle during early diastole 3, 5

Systolic Murmur Mechanism:

• Generated by turbulent blood flow across stenotic or regurgitant valves, through septal defects, or across outflow tract obstructions during systole 1, 4
• Pansystolic murmurs result from blood flowing between chambers with significant pressure differences throughout systole 4

Clinical Significance Differs

S3 indicates volume overload or ventricular dysfunction, while systolic murmurs indicate valvular or structural abnormalities. 3

S3 Clinical Meaning:

• In adults, S3 is a marker of severe hemodynamic alterations, indicating severe regurgitation (regurgitant fraction ≥40%), restrictive filling patterns, or ventricular dysfunction 3
• Associated with higher pulmonary pressures (mean 55 mmHg vs 41 mmHg without S3) and class III-IV heart failure symptoms 3
• Does NOT denote heart failure in the same way as S3; S4 is common in coronary disease and hypertension without decompensation 2

Systolic Murmur Clinical Meaning:

• Pansystolic murmurs almost always indicate pathological conditions (mitral regurgitation, tricuspid regurgitation, ventricular septal defect) requiring further evaluation 4
• Midsystolic murmurs may be innocent or pathological depending on grade, radiation, and associated findings 7

Common Pitfall to Avoid

Do not confuse an S3 with a short diastolic rumble that can occur when both S3 and S4 are present in close proximity. 2 When atrial (S4) and ventricular (S3) gallops occur together, they may create a short rumbling murmur that could be mistaken for a valvular lesion, but this is still a diastolic phenomenon, not a systolic murmur 2.