What causes the first heart sound (S1)?

The First Heart Sound (S1) Origin

The first heart sound (S1) is primarily caused by the closure of the mitral and tricuspid valves at the beginning of ventricular systole. The sound occurs when these atrioventricular valves close as ventricular pressure exceeds atrial pressure, creating vibrations in the valve leaflets and surrounding structures 1, 2, 3.

Components of S1

S1 consists of several components:

1. Initial low-frequency component:

   • Occurs at the beginning of ventricular systole
   • Represents vibrations caused by left ventricular contraction and deceleration of blood flow across the mitral valve 4
   • Precedes the high-frequency components

2. First major high-frequency component (M1):

   • Corresponds to mitral valve closure
   • Usually louder and occurs slightly earlier than T1
   • Coincides with mitral valve closure in 100% of patients 2

3. Second major high-frequency component (T1):

   • Corresponds to tricuspid valve closure
   • Usually occurs slightly after M1
   • Coincides with tricuspid valve closure in 70-100% of patients 2, 3

4. Additional component:

   • Some studies suggest a component that coincides with aortic valve opening 2

Timing and Physiological Basis

• S1 occurs after the QRS complex on ECG
• The timing corresponds to when ventricular pressure exceeds atrial pressure, forcing the AV valves to close
• The intensity of S1 is affected by:
  • PR interval (affects position of valve leaflets at onset of systole)
  • Valve mobility and structure
  • Ventricular contractility
  • Blood flow velocity

Clinical Significance

• Loud S1: May indicate:

  • Short PR interval (valves wide open at onset of systole)
  • Mitral stenosis (increased mobility of anterior leaflet)
  • Hyperkinetic states (pregnancy, anemia, thyrotoxicosis) 5
  • Early systolic mitral valve prolapse 6

• Soft S1: May indicate:

  • Long PR interval (valves partially closed at onset of systole)
  • Mitral regurgitation (incomplete valve closure)
  • Flail mitral valve leaflet 6
  • Left ventricular dysfunction

Auscultation Technique

• Best heard at the apex (mitral area) using the diaphragm of the stethoscope
• S1 is typically louder than S2 at the apex, while S2 is louder than S1 at the base 1
• The intensity and splitting of S1 should be assessed during both inspiration and expiration

Abnormal S1 in Pathological Conditions

• In mitral valve prolapse, the intensity of S1 varies based on the timing of prolapse:

  • Early systolic prolapse: Louder S1
  • Middle to late systolic prolapse: Normal S1
  • Flail mitral leaflet: Reduced S1 6

• Abnormal S1 characteristics can be analyzed using advanced techniques like matching pursuit method to diagnose mitral valve abnormalities with up to 93% accuracy 7

The first heart sound provides valuable diagnostic information about cardiac function and valve status when properly evaluated in the context of the complete cardiac examination.