*The first heart sound is otherwise called as S1.
*Its duration is 0.14 seconds.
*Its frequency is 25-45 Hz.
*Though S1 and S2, both are high pitched sounds, S1 has a lower frequency than S2. (50 Hz).
*The sound produced is a slightly prolonged "LUB".
*The cause of the first heart sound is the sudden closure of the mitral and tricuspid valves.
*The timing of the S1, is at the start of the ventricular systole.
*The first and second heart sounds, and the opening snaps are best heart with the diaphragm of the stethoscope, and low pitched sounds, such as the third and fourth heart sounds are best heard with the bell of the stethoscope.
*S3 and S4, can often not be heard with stethoscope (ear) and require phonocardiogram. S3 often and S4 sometimes can be recorded in phonocardiogram.
*CAUSES OF SOFT S1 :
1. Poor conductance of sound through chest wall. eg: Obesity, Ephysema, Pleural effusion and Pericardial effusion.
2. Decrease in rate of LV pressure development. eg: Myxoedema, Cardiomyopathy, Acute MI and MR.
3. PR interval and velocity of valve closure. eg: Prolonged PR interval - First degree heart block.
4. Mobility of the valve. eg: Severe calcification of the valve or reduced mobility of the valve - Long standing MS is associated with severe calcification of mitral valve and soft S1.
*CAUSES OF LOUD S1 :
1. Tachycardia : S1 is louder if diastole is shortened due to Tachycardia. eg: Anemia, anxiety and fever.
2. Increased AV flow due to high cardiac output. eg: AV fistula and thyrotoxicosis.
3. Increased AV flow due to left to right shunt. eg: ASD, PDA and other conditions.
4. Prolonged AV flow due to stenosis. eg: MS and TS.
5. Short PR interval (velocity of valve closure) - At short PR intervals, the mitral valve leaflets are maximally separated by atrial contraction. At the onset of LV systole, the mitral valve leaflets therefore close with a high velocity and with a large excursion resulting in loud S1.
*First Heart sound (M1,T1) is produced by the closure of the Mitral valves (M1) and Tricuspid valves (T1).
*The Mitral valves close slightly before the Tricuspid valves .
*This normal splitting (physiological splitting) cannot be detected by auscultation because both components are low pitched and separated by only 20-30 milliseconds.
*Therefore splitting of the first heart sound whenever audible, is considered pathological.
*CAUSES OF PATHOLOGICAL NORMAL SPLITTING OF S1 (T1 follows M1 after a long gap):
- RBBB (Right Bundle Branch Block)
- LV pacing
- Ectopic beats and idioventricular rhythms originating in left ventricle
- Ebstein's anamoly
*CAUSES OF PATHOLOGICAL REVERSE SPLITTING OF S1 (T1 precedes M1) :
- LBBB
- RV pacing
- Ectopic beats and idioventricular rhythms originating in right ventricle
- Severe MS and atrial myxoma
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