How do you differentiate between reverse and fixed splitting of the S2 (second heart sound)?

Differentiating Reverse vs Fixed Splitting of S2

Reverse splitting occurs when P2 closes before A2 (opposite of normal), causing S2 to split during expiration and become single during inspiration, while fixed splitting maintains a constant split interval throughout the respiratory cycle without respiratory variation. 1

Key Distinguishing Features

Reverse (Paradoxical) Splitting

Respiratory Pattern:

• S2 splits during expiration and becomes single during inspiration 2
• This is the opposite of normal physiologic splitting 2

Mechanism:

• Delayed aortic valve closure (A2) causes P2 to occur first 2
• During inspiration, increased venous return further delays P2, causing the two components to merge 2

Associated Conditions:

• Left bundle branch block (LBBB) - most common cause 2, 3
• Severe aortic stenosis with soft or absent A2 1, 4
• Volume overload of left ventricle 2
• Conduction defects in left ventricle 2

Clinical Pitfall:

• Approximately one-third of patients with classical LBBB may have normal splitting rather than reversed splitting, suggesting bilateral bundle branch block when this occurs 3
• QRS duration is significantly longer in LBBB patients with reversed splitting compared to those with normal splitting 3

Fixed Splitting

Respiratory Pattern:

• S2 remains split with a constant interval during both inspiration and expiration 1
• No respiratory variation in the splitting interval 5

Mechanism:

• Right ventricular volume overload causes persistent delay in pulmonic valve closure 1
• The split interval does not vary with respiratory changes in venous return 5

Associated Conditions:

• Atrial septal defect (ASD) - classic finding 1, 5
• Pulmonary stenosis 5
• Right ventricular dysfunction 1

Clinical Context:

• In ASD, fixed splitting is accompanied by a precordial lift, systolic pulmonary flow murmur, and right ventricular volume overload on imaging 1
• Fixed splitting is not invariable in ASD and may be absent in some cases 1

Diagnostic Approach

Physical Examination:

• Listen carefully during normal respiration at the pulmonic area (second left intercostal space) 1
• Have the patient breathe deeply to accentuate respiratory variation 1
• Note whether splitting widens during inspiration (normal), expiration (reverse), or remains unchanged (fixed) 1

Associated Findings to Assess:

• For reverse splitting: check for delayed carotid upstroke (aortic stenosis), assess for LBBB on ECG 1, 2
• For fixed splitting: assess for right ventricular lift, look for right axis deviation and incomplete RBBB on ECG (ASD) 1

Confirmatory Testing:

• Transthoracic echocardiography is the first-line diagnostic test for evaluating abnormal heart sounds 4
• ECG to identify conduction abnormalities (LBBB for reverse splitting, RBBB for fixed splitting) 1, 2
• Advanced wavelet transform analysis can quantitatively measure splitting intervals if available 5, 6

Clinical Significance

Reverse splitting indicates:

• Delayed left ventricular systole requiring investigation for structural heart disease or conduction abnormalities 2
• In severe aortic stenosis, A2 may become so soft that single S2 is heard rather than reversed splitting 1, 4

Fixed splitting indicates:

• Right ventricular volume overload requiring evaluation for intracardiac shunts 1
• Warrants echocardiography to rule out ASD or other structural abnormalities 1, 4