What is a palpable first heart sound (S1)?

Palpable First Heart Sound (S1): Clinical Significance and Characteristics

A palpable first heart sound (S1) refers to a first heart sound that is strong enough to be felt as a precordial impulse or thrill during cardiac examination, indicating increased force of valve closure and heightened cardiac contractility. A palpable S1 is characterized by a hyperkinetic precordial impulse that can be felt during palpation of the chest wall, reflecting increased force of mitral and tricuspid valve closure during early systole.

Physiological Basis of S1

The first heart sound (S1) is primarily produced by:

• Closure of the mitral (M1) and tricuspid (T1) valves at the beginning of systole
• Sudden deceleration of blood columns associated with valve closure
• Resulting vibrations in the cardiohemic system 1

The normal S1 consists of two high-frequency components:

• M1 (mitral component): Occurs approximately 0.06 seconds after the Q wave on ECG
• T1 (tricuspid component): Follows at approximately 0.09 seconds after the Q wave 1

Causes of a Palpable S1

A first heart sound becomes palpable when its intensity is significantly increased. Common causes include:

1. Physiological conditions with increased cardiac output:

   • Pregnancy
   • Anemia
   • Thyrotoxicosis
   • Arteriovenous fistula 2

2. Pathological conditions:

   • Early systolic mitral valve prolapse (MVP) with normal leaflet coaptation 3
   • Mitral stenosis (with delayed but forceful valve closure)
   • Shortened PR interval (allowing valve closure against higher ventricular pressure)
   • Hyperkinetic states

Clinical Assessment

Physical Examination Findings

• Palpable precordial impulse or thrill during S1
• Hyperkinetic precordial impulse 2
• S1 may be louder than normal with prominent splitting 2
• In normal hearts, S1 is typically louder than S2 at the apex, but a palpable S1 represents an exaggeration of this normal finding 4

Associated Findings

• In mitral valve prolapse, a palpable S1 is often associated with:

  • Early systolic prolapse coincident with initial leaflet coaptation
  • Midsystolic click
  • Late systolic murmur 3

• In mitral stenosis:

  • Delayed Q-M1 interval (approximately 0.10 seconds vs normal 0.06 seconds)
  • Sometimes reversed splitting of S1 1

Diagnostic Evaluation

When a palpable S1 is detected, further evaluation may include:

1. Echocardiography:

   • Confirms underlying structural abnormalities
   • Assesses valve morphology and function
   • Evaluates chamber size and function 4
   • Can determine timing of valve closure in relation to cardiac cycle 1

2. Electrocardiogram (ECG):

   • Identifies conduction abnormalities
   • Assesses for chamber enlargement patterns
   • Evaluates PR interval 4

3. Combined echophonocardiography:

   • Can precisely correlate valve events with heart sounds
   • Helps distinguish between M1 and T1 components 1, 5

Clinical Significance

The presence of a palpable S1 has important diagnostic implications:

• In normal pregnancy, a palpable S1 is a common physiological finding 2
• In mitral valve prolapse, the amplitude of S1 provides clues to the type and timing of prolapse:
  • Early systolic prolapse: Significantly increased S1 intensity (ratio of S1 to A2 approximately 6.2 ± 3.1)
  • Middle to late prolapse: Normal S1 intensity (ratio 1.3 ± 0.6)
  • Flail mitral valve: Reduced S1 intensity (ratio 0.3 ± 0.5) 3

Common Pitfalls

1. Misinterpreting normal physiological variations:

   • During pregnancy, a palpable S1 is a normal finding and should not be misinterpreted as pathological 2

2. Overlooking subtle findings:

   • Signs of PH on physical examination, including a palpable S1, can be subtle and often overlooked 2

3. Confusing with other cardiac findings:

   • A palpable S1 should be distinguished from a palpable left parasternal lift (which suggests right ventricular hypertrophy) 2
   • It should also be differentiated from a palpable S2, which may indicate pulmonary hypertension

4. Failing to correlate with other cardiac findings:

   • A palpable S1 should be interpreted in the context of other cardiac findings such as murmurs, gallops, and jugular venous pulsations 2

By understanding the significance of a palpable first heart sound, clinicians can better recognize important cardiac conditions and guide appropriate diagnostic evaluation.