Sibilant rhonchus (older term)
In one sentenceA continuous, musical sound made up of one dominant tone.
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?
A single, uninterrupted musical tone with a clear pitch. It is mostly heard in expiration. In adults, a single wheeze often lasts half a second to a second and may not fill the whole of expiration. The pitch often falls slightly. The phase may vary.
Where is it heard?
If focal and fixed, it suggests local obstruction. In widespread disease it may also be heard at random sites.
What causes it?
As air flows fast through a narrowed, flexible airway, the wall begins to vibrate. The vibration sustains itself and produces a musical tone at a single pitch. During expiration, intrathoracic pressure compresses the airways, so wheezes are mostly expiratory. Pitch does not tell you the airway diameter. Flow, wall stiffness and pressure determine it together.
Quantitative criteria
- CORSA: dominant frequency above about 100 Hz and duration longer than about 100 ms. These are operational criteria, not natural boundaries.
- On the spectrogram, a single tone line appears in expiration, with a much fainter harmonic above it (the second harmonic is about 16 dB weaker). In recordings from 41 adult patients, a single wheeze lasted a median of 0.55 s; the pitch fell in 58%, stayed flat in 26%, and rose in 15%. The pitch does not tremble regularly; it drifts slowly and irregularly by a small amount. In this site's example, the tone lasts about 1 s, the pitch falls from about 390 Hz to 335 Hz, and it is the same in every breath. This is the pattern expected with a fixed narrowing.
Clinical meaning
A focal monophonic wheeze is a warning sign when it recurs at the same site, does not change with coughing and comes with diminished breath sounds over the same area. It suggests a mucus plug, foreign body, endobronchial tumour or stenosis. It needs imaging and, when necessary, bronchoscopy. Do not let this finding get lost within a general diagnosis of "asthma/COPD".
Sounds it can be confused with
Stridor is much louder over the neck and is mostly inspiratory.
In a polyphonic wheeze, several tones are heard at the same time.
The boundary between a low-pitched wheeze and a rhonchus is blurred.
Note the point where you hear the wheeze and listen again after a cough. If it is focal and persistent, it stays in the same place.
See the reference list for sources.