| Feature | S1 ("Lub") | S2 ("Dub") | | :--- | :--- | :--- | | | Closure of Mitral & Tricuspid valves | Closure of Aortic & Pulmonic valves | | Phase | Marks onset of Systole | Marks onset of Diastole | | Sound Quality | Lower pitch, longer duration | Higher pitch, shorter, sharper | | Best Heard At | Cardiac Apex (5th ICS, MCL) | Cardiac Base (2nd ICS, RSB) |
The aortic and pulmonic valves (semilunar valves) snap shut to prevent blood from flowing back into the ventricles from the major arteries. Comparison at a Glance Pitch Duration Valves Closing Mitral & Tricuspid Aortic & Pulmonic Cycle Phase Start of Systole Start of Diastole Why It Matters lub-dub sound