The reasoning: place the sound on the cycle
| Sound | Timing | Mechanism | Meaning | Best heard |
|---|---|---|---|---|
| S3 | early diastole, ~0.12–0.16 s after A2 | rapid ventricular filling into a compliant, volume‑loaded ventricle | normal in the young / pregnancy; in older adults → systolic HF, volume overload, MR | bell, apex, left lateral |
| S4 | late diastole, just before S1 | atrial contraction into a stiff ventricle | LVH, hypertension, ischaemia, HCM, AS; never in AF | bell, apex |
| Opening snap | early diastole, ~0.04–0.12 s after A2 | abrupt opening of a stenotic but mobile mitral valve | rheumatic MS | diaphragm, apex to left sternal edge |
| Pericardial knock | early diastole, a little before where an S3 would fall | sudden halt of filling by a rigid pericardium | constrictive pericarditis | left sternal edge |
| Ejection click | early systole, just after S1 | opening of an abnormal semilunar valve or dilated great vessel | bicuspid aortic valve, pulmonary stenosis | base |
| Mid‑systolic click | mid systole | sudden tensing of prolapsing mitral leaflets/chordae | mitral valve prolapse | apex |