What is considered a loud second heart sound (S2)?

What Constitutes a Loud Second Heart Sound (S2)

A loud S2 is identified when the intensity of the second heart sound is abnormally increased compared to normal, with the specific component (A2 or P2) and location determining the underlying pathology—most commonly a loud P2 at the upper left sternal border indicates pulmonary hypertension. 1

Clinical Assessment of S2 Loudness

Location-Specific Evaluation

When evaluating a loud S2, you must first determine which component is accentuated and where it is best heard:

• Loud P2 (pulmonary component): Best heard at the upper left sternal border (pulmonic area), typically indicates elevated pulmonary artery pressure 1
• Loud A2 (aortic component): Best heard at the upper right sternal border (aortic area), may indicate systemic hypertension 1

Comparative Assessment

The loudness is determined by comparing S2 intensity across different locations:

• S2 in pulmonary area louder than in aortic area: Suggests pulmonary hypertension, though this finding has limited diagnostic accuracy (sensitivity 60%, specificity 22%) 2
• P2 > A2 in pulmonary area: Another sign of pulmonary hypertension, with sensitivity 57% and specificity 39% 2
• P2 audible in mitral area: Abnormal finding suggesting significant pulmonary hypertension (sensitivity 68%, specificity 41%) 2

Clinical Context and Pathophysiology

Pulmonary Hypertension

A loud P2 is the classic finding in pulmonary hypertension, as documented in multiple case reports:

• Physical examination in severe pulmonary arterial hypertension consistently reveals "a loud pulmonic component to the second heart sound" 3
• The increased pressure in the pulmonary artery causes more forceful closure of the pulmonary valve, generating a louder sound 4

Important Diagnostic Considerations

When assessing respiratory variation, you should:

• Listen during normal and deep breathing to evaluate splitting patterns 1
• Look for associated signs of right ventricular hypertrophy if pulmonary hypertension is suspected 1
• Assess for right ventricular lift and ECG findings (right axis deviation, incomplete RBBB) 5

Normal Physiologic Variations

Pregnancy

During pregnancy, a louder than normal S2 can be physiologic:

• A 50% increase in circulating blood volume and increased cardiac output can cause louder heart sounds 1
• The second heart sound may widen and appear fixed during later stages of pregnancy as a normal finding 1

Acoustic Characteristics

From a technical standpoint, the second heart sound has specific frequency characteristics:

• The major concentration of energy for S2 is below 150 Hz 6, 7
• S2 spectra have greater amplitude than S1 above 150 Hz, likely due to vibrations within the aorta and pulmonary artery 6
• The mean interval from valve closure to audible sound is 60 msec for the pulmonary component 8

Common Pitfalls

Critical diagnostic error to avoid: In congenitally corrected transposition, the second heart sound is loud due to the anterior position of the aortic valve, which can be mistaken for pulmonary hypertension 4. This makes it difficult to distinguish isolated congenitally corrected transposition from cardiac disease with elevated pulmonary artery pressure.

Limited semiological accuracy: The physical examination findings of a loud S2 for diagnosing pulmonary hypertension have relatively poor sensitivity and specificity, with likelihood ratios close to 1.0 2. Therefore, echocardiography should be obtained when pulmonary hypertension is suspected based on a loud P2, as recommended by the American College of Cardiology 1.