The origins of breath sounds are still unclear. According to leading theories, turbulent air flow in the central airways produces the tracheal and bronchial breath sounds. As these sounds pass through the lungs to the periphery, lung tissue filters out their higher-pitched components and only the soft and lower-pitched components reach the chest wall, where they are heard as vesicular breath sounds. Normally, tracheal and bronchial sounds may be heard over the trachea and main-stem bronchi; vesicular breath sounds predominate throughout most of the lungs.
When lung tissue loses its air, it transmits high-pitched sounds much better. If the tracheobronchial tree is open, bronchial breath sounds may replace the normal vesicular sounds over airless areas of the lung. This change is seen in lobar pneumonia when the alveoli fill with fluid, red cells, and white cells—a process called consolidation. Other causes include pulmonary edema or hemorrhage. Bronchial breath sounds usually correlate with an increase in tactile fremitus and transmitted voice sounds. These findings are summarized below.
Airless Lung, as in Lobar Pneumonia
Breath Sounds Transmitted Voice Sounds
Spoken words muffled and indistinct Spoken "ee" heard as "ee"
Whispered words faint and indistinct, if heard at all Normal
Bronchial or bronchovesicular over the involved area
Spoken words louder, clearer (bronchophony)
Spoken "ee" heard as "ay" (egophony)
Whispered words louder, clearer (whispered pectoriloquy)
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