In one sentencePleural air separates the vibrating lung from the chest wall. The result is a sharp loss of sound on one side.
- Typical finding: a sharp reduction or loss of sound on one side.
- Percussion is hyperresonant. Over an effusion, by contrast, it is dull.
- A small pneumothorax can be missed on auscultation. If in doubt, image the chest.
Not the main change.
Yes. Flow falls in the collapsed lung.
Yes, this is the main change. The layer of air between the lung and the chest wall blocks sound almost completely.
Marked one-sided reduction or loss of breath sounds. Hyperresonance on percussion.
For the reasoning: Where does sound come from? · How does sound reach the chest wall? · How do adventitious sounds arise? · From sound to differential diagnosis
What do you hear?
On the affected side, breath sounds are clearly diminished or absent. The opposite side is normal. The difference is sharp. So the quickest clue comes from listening to both sides at the same level, one after the other.
Why does it sound like this?
Air in the pleural space separates the lung from the chest wall. Sound from the lung crosses this layer of air poorly, and very little energy reaches the chest wall.
Distinguishing clues
- Percussion: Hyperresonance suggests a pneumothorax. Dullness suggests an effusion.
- Fremitus: It is reduced in both, so it does not tell them apart.
- Clinical picture: Sudden chest pain and breathlessness. Hypotension, marked tachycardia and hypoxaemia suggest a tension pneumothorax.
Pitfalls
- A small pneumothorax can be missed on auscultation.
- The degree of reduction depends on the amount and location of the air and on where you listen. In a supine patient, air collects at the front. Listening only from behind can mislead you.
- Loss of sound alone does not separate an effusion from a pneumothorax.
Next step
Lung ultrasound or chest X-ray. If you suspect a tension pneumothorax, do not wait for imaging before you intervene.
Try this pattern in a case
Going deeperresidents · specialists · numbers and sources
Frequency-dependent loss
Experimental transmission measurements and computer models showed a marked, frequency-dependent reduction in the chest wall signal in pneumothorax. The model of Royston et al. showed how pleural air reduces the surface response. The main predictions of a more recent finite element model were compared with measurements in three healthy men. In these men, sound at 50–600 Hz was introduced at the mouth. The pathologies were studied by simulation, not with patient data. The model predicted reduced transmission in pneumothorax.
Ultrasound
In acute respiratory failure, lung ultrasound can show a pneumothorax with high accuracy. In critically ill patients, it may be better than auscultation for some structural abnormalities.
Reading asymmetry
A low-energy region may result from pleural air or fluid, severe regional obstruction, poor contact or shallow ventilation. Healthy lungs are not perfectly symmetrical either. At matched airflow, the left posterior base has been measured as about 32% louder than the corresponding point on the right. An asymmetry becomes meaningful when it is new, persistent and consistent across neighbouring points.
See the reference list for sources.