Mediscuss! Medical Forum

Most of us are well aware of the mechanism of production of heart sounds. But when it comes to the mechanism of production of breath sounds, many are at loss.

Breath sounds are produced in the major airways, that is trachea and major bronchi.

It is a common misconnception that these sounds are produced in the alveoli. They are not. The velocity of air in the alveoli is not significant enough to produce turbulance and sounds.

Now that we know where the sounds are produced, what is the character of the breath sound that is produced in the major airways?

Most will answer this as vesicular. Again, not so!

The sound that is produced in the major airways is bronchial in character (you will notice that if you auscultate on the trachea). However, the sound that is normally heard on the chest wall, where we usually auscultate, is vesicular.

The bronchial breath sounds produced at the major airways have to travel all across the tissues (bronchi, bronchioles, alveolar walls, blood vessels, ribs, muscles, subcutaneous tissue, skin) to reach the body surface from where they are auscultated.

While they are being transmitted through these tissues, some frequencies of sound are absorbed and the character of the sound changes. This changed sound is termed vesicular breath sound.

1. Any abnormal condition where sounds are better transmitted to the chest wall from the major airways.

Consolidation:

• Here, inflammatory exudates would have filled the alveoli, thereby replacing the usual air. Inflammatory exudate is liquid to semisolid in consistency.
  Liquid & Solids are better conductors of sound than air.
  As air is replaced by a better conducting medium, the bronchial sound produced at the trachea is conduced better and transmitted as it is.
  Therefore, we hear bronchial breath sound on an area of consolidation.

Upper lobe fibrosis
Upper lobe collapse

• In both the above conditions, as there is loss of lung volume, the traches is pulled towards the side of lesion. When the trachea is pulled, the distance between the trachea and the chest wall reduced on the affected side,.
  The sound has to travel for a lesser distance than earlier, there is less filtration of frequencies, which ensures that the sound remains bronchial.

Upper level of pleural effusion

• In pleural effusion, fluid generally accumulates below upwards. As it accumulates, the lung tissue is compressed and displaced upwards.
  This compressed lung tissue is of a higher density than earlier and can conduct sounds better.
  

2. In addition to these, in lung cavities, a type of bronchial breath sound, called 'cavernous' is produced at the cavity.

Cavity

• When a lung cavity is connected to a bronchiole, bronchial sound is produced in the cavity, due to air entering a hollow chamber.
  To understand how this happens, take an empty bottle and blow gently over the top of the open bottle. You will hear a 'hollow' sound. Thsi is the characted of bronchial breathing.
  If the patient also has fibrosis in adition to the cavity, a common combination in post tubercular lesions, fibrosis also contributes to bronchial breathing.

Hope this is useful in understanding lung sounds better.