First off, let's discuss what we mean by the "splitting" of the second heart sound (S2). Think of S2 as two distinct sounds: A2 and P2, marking when the aortic and pulmonic valves close. The aortic valve typically shuts a bit sooner than the pulmonic valve, creating a gap of about 30 milliseconds. This gap shifts slightly when you breathe in and out.
So, why does this split happen at all? It's all about the pressures in the ventricles and the associated major blood vessels. When the ventricles start to relax (entering diastole), the pulmonic and aortic valves close only when the pressure in their respective ventricles drops below the pressure in the major vessels they're linked to.
Think of the pulmonic valve. It shuts when the pressure in the right ventricle is lower than the pressure in the pulmonary artery. Similarly, the aortic valve closes when the pressure in the left ventricle falls beneath the pressure in the aorta.
Now, let's add some figures to this explanation. The right ventricle works at pressures of around 20-25/4 mmHg (25 systolic, 4 diastolic), while the pulmonary artery operates at 25/10 mmHg. The right side of the heart works at a lower pressure than the left side, which means the right ventricle takes longer to reach a pressure lower than that of the pulmonary artery. This causes the pulmonic valve to close (P2 sound) slightly later than the aortic valve (A2 sound).
Compare this to the left side of the heart. The left ventricle operates at 120/10 mmHg, while the aorta works at 120/80 mmHg. This large difference in diastolic pressures (80 in the aorta versus 10 in the left ventricle) means the left ventricle reaches a pressure lower than that of the aorta earlier, causing the aortic closure sound to occur before the pulmonic one.
Now, onto why the split in S2 changes with respiration. During inhalation, more blood returns to the right heart, filling the right ventricle. This extra blood takes longer to push into the pulmonary artery, so P2 moves away from A2, widening the split. When you breathe out, less blood returns to the right heart, so the right ventricle empties more quickly. This causes P2 to move closer to A2, narrowing the split.
Now, let's explore some medical conditions that can affect the split in S2.
Narrowing split S2
In aortic stenosis, the aortic valve is narrowed, and the left ventricle takes more time to pump blood through the constricted valve, causing A2 to delay and narrowing the split in S2. This logic also applies to subvalvular aortic stenosis and conditions like systemic hypertension and left bundle branch block, where the left ventricle's efficiency is reduced.
Wide variable split S2
Conditions like mitral regurgitation and ventricular septal defect can widen the split in S2. This is because the left ventricle empties blood into two places (the aorta and either the left atrium or right ventricle), so the aortic valve closes sooner (an early A2), creating a wider split. Similarly, the right bundle branch block causes a delay in P2, resulting in a wider split in S2.
Wide fixed split S2
Atrial septal defect (ASD), which shows a wide, fixed split in S2. During inhalation, the pressure in the right and left atria is similar, so the ASD doesn't function. However, during exhalation, the right atrial pressure is lower than the left atrial pressure, allowing blood to flow from the left to the right atria through the ASD. This means the amount of blood entering the right ventricle doesn't change with respiration, resulting in a wide but unvarying split in S2.
Written by Dr Lavanya Narayanan, MBBS, MRCP (UK), FRCP (Lond)
An internist, diabetologist and educationist, also the Lead Content Manager at MEDIT