The first heart sound`s analysis, part 3: splitting of S1

The first heart sound consists of two components: mitral  and tricuspid.  These two components are separated by a short interval which creates an audible narrow splitting of S1.  Here, you can find more information and a good example of the normal splitting of S1.

Mitral component of S1 is loud and clearly heard over the whole precordial area, while tricuspid component is heard only over a small area along the left lower sternal border and to a smaller degree at the apex.  Tricuspid component almost never heard at the base of the heart.  

So, search for splitting of S1 along the left lower sternal border.    


There is only one abnormality of S1's splitting - wide splitting.  When could it happen?


Some electrical abnormalities can cause delayed activation and contraction of the right ventricle.  As the result, tricuspid component of S1 becomes delayed, the interval between mitral and tricuspid components becomes longer and splitting of S1 becomes wider. 


Whenever the right ventricle's activation is delayed, S1 is widely split. The most frequent cause of wide splitting of S1 is right bundle branch block.  Some other causes are idioventricular rhythm from the left ventricle, left ventricle ectopic beats and left ventricular pacing.   

Listen to a young pregnant woman with partial AV-dissociation (there are two pacemakers: sinus node pacing and additional pacemaker somewhere in the left ventricle).  The frequencies of these pacemakers are nearly equal.  Note wide splitting of first and second heart sounds.





There are two abnormal heart sound close to S1. Fourth heart sound (S4) precedes S1.  Ejection sounds can be appreciated slightly after S1.  These sounds are present in some frequent pathologic situations and can create impression of wide splitting of S1.

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