Reduced or absent air entry
In one sentenceA sound of normal character that is weak or not heard at all. One of the most important findings is the absence of a sound.
Listen to the same sound in another patient. Breathing rate, chest wall, the pitch of the sound and the number of adventitious sounds vary within realistic limits.
What does it sound like?
The background sound becomes faint. It gains meaning when it is regional or one-sided.
Where is it heard?
Regional: effusion, pneumothorax, obstructive atelectasis, distal to an endobronchial lesion. Widespread: shallow breathing, obesity, hyperinflation, severe obstruction.
What causes it?
There are two possible reasons. Either little air reaches that region (the source is weak), or the sound cannot reach the chest wall (transmission is impaired). Pleural fluid or air separates the lung from the chest wall and blocks the sound. A single auscultation often cannot tell these two apart.
Quantitative criteria
- In COPD, the diminished breath sound heard on routine examination is partly the result of low flow. When flow is standardised, the reduction in sound intensity in emphysema may become less marked.
Clinical meaning
Low amplitude is not specific. Interpret it together with percussion (dullness / hyperresonance), chest movement, voice transmission tests and imaging. In severe asthma, loss of the wheeze with diminishing sounds ("silent chest") is an alarm sign.
Sounds it can be confused with
If the sound returns to normal when the patient breathes deeply, the cause is probably low flow.
Always compare symmetrical points, and ask the patient to breathe deeply through an open mouth. If a one-sided loss is new, request imaging.
See the reference list for sources.