Heart Sounds and Splitting of S2
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Direct answer
Splitting of S2 is the audible separation between closure of the aortic valve (A2) and closure of the pulmonary valve (P2). Normally A2 comes first and the gap widens on inspiration, because extra venous return delays P2 closure. Wide, fixed and paradoxical patterns, together with S3, S4 and the opening snap, help localise the lesion at the bedside.
What you must remember
- Physiological split: A2 precedes P2, and the gap widens on inspiration and narrows on expiration, heard best at the left upper sternal border.
- Wide split: a delayed P2 from right bundle branch block or pulmonary stenosis, with the gap still varying with breathing.
- Fixed split: the gap stays constant in both phases of respiration, the classic sign of an atrial septal defect.
- Paradoxical split: P2 comes before A2 in expiration, the split narrows on inspiration, and the pattern points to left bundle branch block, aortic stenosis or left ventricular failure.
- S3: a low-pitched early diastolic sound after S2 from rapid ventricular filling, abnormal in older adults with heart failure or mitral regurgitation, but normal in children and athletes.
- S4: a late diastolic sound just before S1, caused by atrial contraction into a stiff ventricle, and always pathological in adults.
- Opening snap: a high-pitched early diastolic sound in mitral stenosis, best at the apex, with a shorter gap from S2 signalling more severe stenosis.
Approaching S2 at the bedside
A 28-year-old woman with a past history of rheumatic fever has a loud first sound and an apical diastolic rumble. The approach to the second sound follows a fixed order, and each step narrows the diagnosis.
- Listen at the base. Place the bell or diaphragm over the second intercostal spaces on both sides of the sternum and compare the two closure sounds.
- Time it against breathing. Note whether the gap widens on inspiration, as in the physiological or wide split, or is widest on expiration and narrows on inspiration, as in the paradoxical split.
- Ask whether it is fixed. A split that stays the same through the respiratory cycle points to an atrial septal defect with a left-to-right shunt.
- Name the reversal. If P2 comes before A2 during expiration, consider left bundle branch block, aortic stenosis or severe left ventricular dysfunction.
- Add the extra sounds. Look for S3 in early diastole, S4 just before S1, and an opening snap after S2 at the apex, which together complete the picture.
Where the exam tries to catch you
Sound questions often ask which lesion produces a given pattern, so the traps below repeat the key pairs.
- Trap: a wide split is attributed to right bundle branch block whatever the breathing pattern. Correct: right bundle branch block gives a wide split that still varies with respiration, while a fixed split is the atrial septal defect signature.
- Trap: paradoxical splitting is read as a benign finding in an older patient. Correct: a paradoxical split signals delayed aortic valve closure from left bundle branch block, aortic stenosis or left ventricular failure.
- Trap: a mid-to-late systolic click is taken for an ejection click. Correct: a mid-to-late systolic click points to mitral valve prolapse, while an early ejection click points to aortic or pulmonary stenosis.
Frequently asked questions
Which heart sound follows S2 in mitral stenosis?
The opening snap is the early diastolic sound that follows S2 in mitral stenosis, and it is heard best at the apex. The shorter the interval between S2 and the opening snap, the more severe the mitral stenosis is considered to be.
What causes fixed splitting of the second heart sound?
Atrial septal defect causes fixed splitting of the second heart sound. The left-to-right shunt keeps right ventricular output high, so the gap between A2 and P2 stays the same whatever the phase of breathing.
Is S3 always a sign of heart disease?
An S3 is not always abnormal, because it is a physiological finding in children, young adults and athletes. In older adults it suggests heart failure, volume overload or mitral regurgitation and needs further evaluation.
Why is S4 always considered pathological in adults?
An S4 is produced when the atria contract against a stiff, poorly relaxing left ventricle. It occurs in hypertension, left ventricular hypertrophy, aortic stenosis and recent myocardial infarction, so it is never regarded as a normal finding in adults.
What causes paradoxical splitting of S2?
Paradoxical splitting of S2 is caused mainly by left bundle branch block, aortic stenosis or severe left ventricular dysfunction. Delayed aortic valve closure places A2 after P2 in expiration, and the split narrows and may disappear on inspiration.