Is mitral valve regurgitation (mitral valve regurg) more closely associated with changes in the first heart sound (S1) or the second heart sound (S2)?

Mitral Valve Regurgitation and Heart Sounds

Mitral valve regurgitation is primarily associated with changes in the first heart sound (S1), not the second heart sound (S2).

Clinical Auscultatory Findings in Mitral Regurgitation

The characteristic auscultatory finding in mitral regurgitation is a holosystolic (pansystolic) murmur that begins with S1 and extends through systole, generated by the pressure gradient between the left ventricle and left atrium throughout the entire systolic period 1. This murmur is typically heard best at the apex and may radiate to the axilla 1, 2.

First Heart Sound (S1) Changes

The intensity and characteristics of S1 are directly affected by mitral valve pathology causing regurgitation:

• In flail mitral leaflet (a common cause of severe mitral regurgitation), S1 is characteristically diminished or absent because the leaflets fail to coapt normally at their free margins 3
• The S1 amplitude ratio (S1 to aortic component of S2) is markedly reduced to 0.3 ± 0.5 in patients with flail valves, compared to 1.4 ± 0.7 in normal subjects 3
• In mitral valve prolapse with late systolic regurgitation, S1 intensity may be normal (1.3 ± 0.6) when prolapse occurs in middle to late systole 3
• A third heart sound (S3) is commonly present in severe mitral regurgitation due to rapid early diastolic filling from the volume-overloaded left ventricle 1
• A diastolic flow rumble may be audible at the apex in severe mitral regurgitation, reflecting increased flow across the mitral valve during diastole 1

Second Heart Sound (S2) Relationship

S2 is not characteristically altered by mitral regurgitation itself 1. The holosystolic murmur of mitral regurgitation extends up to or through S2, but the sound itself remains normal unless there is concurrent pulmonary hypertension (which would cause an accentuated pulmonic component of S2) 1.

Clinical Correlation and Diagnostic Pitfalls

The combination of a holosystolic murmur, diminished S1, and an S3 at the apex strongly suggests severe mitral regurgitation 1. However, critical pitfalls exist:

• In acute severe mitral regurgitation (such as from papillary muscle rupture), the murmur may be soft or even absent despite hemodynamic catastrophe, because the non-compliant left atrium rapidly equalizes pressure with the left ventricle 4
• Eccentric jets that impinge on the atrial wall lose energy and may appear mild on color Doppler despite severe regurgitation—the physical examination showing a loud holosystolic murmur and S3 should raise suspicion that echocardiography has underestimated severity 1
• In mitral valve prolapse with late systolic regurgitation only, the murmur is confined to late systole and S1 remains normal, which can lead to underestimation of clinical significance 1, 3

Algorithmic Approach to Assessment

When evaluating suspected mitral regurgitation based on heart sounds:

1. Assess S1 intensity: Diminished or absent S1 suggests flail leaflet or severe regurgitation 3
2. Characterize the murmur timing: Holosystolic indicates severe regurgitation with persistent pressure gradient; late systolic suggests mitral valve prolapse 1
3. Listen for S3: Presence indicates volume overload and suggests at least moderate-to-severe regurgitation 1
4. Evaluate for diastolic rumble: Suggests severe regurgitation with increased diastolic flow 1
5. If physical examination suggests severe disease but echocardiography shows mild regurgitation, the echocardiogram is likely wrong—pursue additional imaging 1

The energy ratio of S2 decreases with increasing severity of mitral regurgitation in signal analysis studies, but this is a research finding not clinically applicable at the bedside 5.