● The 1st heart sound is due to closure of the AV valves (mitral and tricuspid):

- It is louder in mitral stenosis, with a hyperdynamic circulation, and softer with mitral incompetence and severe heart failure.

● The 2nd heart sound is due to closure of the semilunar valves (aortic and pulmonary):

- It is louder in systemic or pulmonary hypertension and softer with aortic or pulmonary valve stenosis.
- Splitting of the 2nd heart sound may be caused by the following:
  - Physiological: effect of respiration.
  - Fixed: increased volume load on right ventricle such as atrioseptal defect or pulmonary hypertension.
  - Paradoxical: delayed or prolonged left ventricular systole (e.g. LBBB).