First Heart Sound (S1): Produced mostly by mitral valve closure; equal to/louder than S2 at apex, softer than S2 at base, synchronous with apical impulse, just precedes carotid impulse; accentuated with tachycardia (anxiety, exercise, anemia, hyperthyroidism, etc) and in mitral stenosis (mitral valve is still open widely at onset of ventricular systole); can be diminished in first-degree AV block; variable with heart block and very irregular heart rhythm (eg, Afib and other chaotic rhythms); splitting may be present and is usually best heard in tricuspid area (fifth LICS just lateral to sternum); differentiate split S1 from S4 or early systolic click by timing, variation, character of sounds
Second Heart Sound (S2): Usually louder than S1 at base; physiological splitting accentuated during inspiration, disappears during exhalation; wide and persistent splitting can be heard with early A2 (MR, VSD, WPW), electrical delay (RBBB, LVPVC, WPW) or mechanical delay (pulmonary stenosis or subvalvular pulmonary obstruction, large PE, RV dysfunction) of P2, dilated pulmonary artery, ASD; paradoxical splitting may accompany TR, electrical delay (LBBB, RVPVC, RV pacemaker, CAD) of mechanical delay (AS, hypertrophic cardiomyopathy, CAD, PDA, AI) of A2
Third Heart Sound (S3): Low-pitched sound occurring in early diastole during rapid ventricular filling; may arise from either ventricle; typically best heard at apex in left lateral decubitus position; may be normal finding in children and young adults; pathological finding inpts > 40 yr; associated with ventricular dysfunction of any cause, increased early diastolic filling (hyperkinetic states, mitral/tricuspid regurgitation, left to right shunts), restrictive/constrictive pericarditis
Fourth Heart Sound (S4): Low-pitched sound occurring just before S1 at/medial to apex; can occur in children as benign finding; in disease is associated with increased resistance to ventricular filling (acute ischemia, LV aneurysm, hypertensive heart disease, dilated or hypertrophic cardiomyopathy, aortic stenosis, systemic or pulmonary HT), increased late diastolic filling (anemia, thyrotoxicosis, AV fistula, acute mitral or tricuspid regurgitation), or delayed AV conduction
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