Pleural rub · friction rub
In one sentenceA superficial, local, non-musical sound. It creaks, or resembles "creaking leather".
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?
It resembles dry leather or footsteps in snow. It recurs in inspiration and expiration, often at similar points (mirror image).
Where is it heard?
Local. It may change clearly within a few centimetres. Often over the lateral and lower chest.
What causes it?
The inflamed pleural layers become rough. As they slide over each other during breathing, they catch, tension builds and they suddenly slip. This stick–slip movement produces a series of brief impulses. The sound forms outside the airway, so it does not change with coughing.
Quantitative criteria
- In phonopneumographic studies, pleural rubs showed more heterogeneity in initial deflection polarity than pulmonary crackles. They also varied more with small changes in recording position.
- No numerical thresholds have been defined.
Clinical meaning
It is mainly a localising finding. Think of pleural surface involvement, rather than an event inside the airways, when a creaking sound recurs over a small area, persists after coughing and comes with pleuritic pain. Causes include infection, pulmonary infarction, autoimmune pleuritis and malignancy. Its absence is weak negative evidence. Recognition varies from observer to observer.
Sounds it can be confused with
Coarse crackles may change with coughing and sound deeper. A rub is superficial and does not change with coughing.
Fine crackles are typically inspiratory and bibasal. A rub is local and superficial.
Press the stethoscope firmly over the painful area and ask the patient to hold their breath. If the sound stops, its source is respiratory movement (a pericardial rub continues with the heartbeat).
See the reference list for sources.