Vesicular Breath Sounds : Mechanism of formation

Vesicular breath sounds refers to the breath sounds heard over the various lung areas. The term ‘vesicular breath sounds’ was coined by Lennac. It was named so due to the belief that they are produced by air flowing through the alveoli. But it a misnomer. Vesicular breath sounds are in fact produced by the air flowing through the bronchi and the bronchioles. But they have a different character (low intensity, low pitch, expiratory phase lasting 1/3 of inspiration with no gap between expiration and inspiration) from the bronchial breath sounds (high intensity, high pitch, expiratory phase lasting as much as inspiration) normally heard over the trachea.

So why does vesicular breath sounds differ from bronchial breath sounds even though both of them are produced by air flow through tubes(bronchi/trachea)? That’s because the air in the alveoli act as a muffler which modifies the characters of the vesicular breath sounds. It allows only the low frequency sounds to be conducted to the surface. Thus vesicular breath sounds are low intensity and low pitch. The expiratory phase is short because the breath sounds produced in the later 2/3 of expiration is mainly composed of high pitch sounds which are cut off.

Bronchial breath sounds can be heard over the lung areas in certain pathological conditions in which the air in the alveoli lost. It can be either due to collapse of lung with patent airways or due to filling of alveoli with pus (pneumonia), blood (pulmonary hemorrhage) or serum (pulmonary edema). In both these cases the muffling action of the air in  the alveoli is lost and bronchial breath sounds are heard over lung areas. The cause of bronchial breath sounds can be identified by looking for the presence of adventitious sounds. If there is fluid in the alveoli, the bronchial breath sounds are asociated with crackles. Whereas in case of a collapse, there are no associated crackles heard.