Why is the first heart sound (S1) loud in atrial septal defect (ASD)?

Why S1 is Loud in Atrial Septal Defect

The first heart sound (S1) in atrial septal defect (ASD) is characterized by an attenuated mitral component and an accentuated tricuspid component, with the overall S1 being louder due to the prominent tricuspid component resulting from right ventricular volume overload. 1

Pathophysiological Basis

• The loudness of S1 in ASD is primarily due to the accentuated tricuspid component (IT) of the first heart sound, which results from increased right ventricular filling and volume overload 1
• The left-to-right shunt across the ASD causes increased blood flow through the right side of the heart, leading to right ventricular volume overload and pulmonary overcirculation 2
• This increased flow creates a relative disproportion between the tricuspid valve orifice size and the diastolic blood flow volume, affecting valve closure dynamics 3

Hemodynamic Changes

• The tricuspid valve closing excursion (Tx) is significantly larger in ASD patients compared to those without ASD, contributing to the louder tricuspid component of S1 1
• Increased tricuspid inflow velocity during atrial contraction in ASD patients leads to greater closing energy of the tricuspid valve 1
• The mitral component (IM) of S1 is actually attenuated in ASD patients, but the overall S1 is perceived as louder due to the dominant tricuspid component 1

Clinical Correlation

• On auscultation, ASD typically presents with a fixed splitting of the second heart sound (S2) and a systolic pulmonary flow murmur 2
• With large shunts, a diastolic flow rumble across the tricuspid valve may also be present due to increased flow 2
• The mid-diastolic murmur heard in ASD is a right-sided finding that typically increases with inspiration 3

Changes After ASD Closure

• Following surgical closure of ASD, the mitral component of S1 is augmented while the tricuspid component is attenuated 1
• This change occurs because closure of the defect normalizes the hemodynamics, reducing right ventricular volume overload 1
• Postoperatively, the mitral valve closing excursion (Mx) significantly increases while the tricuspid valve closing excursion (Tx) significantly reduces 1

Diagnostic Implications

• The characteristic loud S1 with accentuated tricuspid component can be a helpful diagnostic clue in identifying ASD 1
• This finding should prompt further evaluation with imaging techniques to demonstrate shunting across the defect and evidence of right ventricular volume overload 2
• Patients with unexplained right ventricular volume overload should be referred to an Adult Congenital Heart Disease (ACHD) center for further diagnostic studies 2

Clinical Pearls

• The loud S1 in ASD should be distinguished from other causes of increased S1 intensity such as mitral stenosis or short PR interval 4
• The intensity of heart sounds and murmurs in ASD may change during pregnancy due to increased cardiac output 2
• In older patients with ASD, the development of pulmonary hypertension may alter the auscultatory findings 2, 5