Causes of Split S1 Heart Sound
The split S1 heart sound is primarily caused by asynchronous closure of the mitral and tricuspid valves, with the mitral valve typically closing first (M1) followed by the tricuspid valve (T1). 1, 2
Physiological Causes of Split S1
- Normal physiological splitting: A small degree of splitting (less than 30 ms) may be normal in some individuals
- Respiratory variation: Split S1 may be more noticeable during inspiration when right ventricular filling is increased
- Exercise: Can temporarily increase the splitting of S1
Pathological Causes of Split S1
Right-sided Delays (Most Common)
- Right bundle branch block (RBBB): Delays right ventricular activation, leading to delayed tricuspid valve closure 3
- Ebstein's anomaly: Abnormally large anterior tricuspid leaflet with increased excursion causes delayed tricuspid valve closure 4
- Tricuspid stenosis: Delays tricuspid valve closure due to prolonged right atrial emptying
- Atrial septal defect (ASD): Associated with delayed tricuspid closure due to right ventricular volume overload 3
- Pulmonary stenosis: Can delay right ventricular contraction and tricuspid valve closure
Left-sided Delays (Less Common)
- Mitral stenosis: Delays mitral valve closure, which can lead to reversed splitting (T1 before M1) 2
- Left bundle branch block (LBBB): Delays left ventricular activation, leading to delayed mitral valve closure
- Pre-excitation syndromes: Can alter the timing of ventricular activation and valve closure
Diagnostic Approach
Physical Examination
- Best heard at the lower left sternal border or apex with the diaphragm of the stethoscope
- Characterized by two distinct high-frequency components separated by a brief interval
- Dynamic auscultation can help differentiate split S1 from other cardiac sounds:
- Split S1 is not eliminated with pressure on the stethoscope (unlike S4) 5
- Splitting may vary with respiration (typically increases with inspiration for right-sided causes)
Diagnostic Testing
- Electrocardiogram (ECG): To identify conduction abnormalities like RBBB or LBBB
- Echocardiography: The gold standard for confirming the cause of split S1 by visualizing:
- Valve morphology and function
- Chamber size and function
- Presence of congenital anomalies like ASD or Ebstein's anomaly 3
Clinical Pearls
Split S1 must be differentiated from:
- S1 + ejection sound (ejection sound occurs slightly later in systole)
- S1 + S4 (S4 is eliminated with pressure on the stethoscope) 5
- Early systolic click (occurs after S1)
In patients with mitral stenosis, the Q-M1 interval may be delayed to approximately 0.10 seconds (compared to normal 0.06 seconds), which can sometimes result in reversed splitting of S1 2
Echophonocardiographic studies have confirmed that the two high-frequency components of S1 correspond to closure of the mitral and tricuspid valves, with the average time from the Q wave to M1 being 0.06 seconds and to T1 being 0.09 seconds in normal individuals 2, 6
Wide splitting of S1 (>0.03 seconds) is generally considered pathological and warrants further investigation, especially when associated with other cardiac findings 2