Respiratory sounds in 5 minutes
Where does the sound you hear over the chest come from, and why does it change?
Short answerBreath sound arises from air flowing fast through the trachea and large bronchi. It passes through the lung to reach the chest wall. When a sound changes, its source, the airflow or the path it travels has changed.
Where does the sound arise?
Breath sound arises in the trachea and large bronchi. In these airways air flows fast and irregularly, and this irregular flow produces a rustling noise.
In the small airways, air flows very slowly and makes almost no sound. You hear the sound most clearly over the neck, above the trachea. There the stethoscope sits right over the place where the sound arises.
Why is the sound softer over the chest wall?
Before it reaches the chest wall, the sound passes through air-filled lung. Air-filled lung filters out the high-pitched part of the sound. So a lower-pitched, softer rustle reaches the chest wall. We call this the vesicular breath sound.
The vesicular breath sound is louder during inspiration, that is, while breathing in. During expiration, while breathing out, it is short and faint.
Why does a sound change?
In disease, sound changes for three reasons. To find the reason, ask the same three questions in every patient.
- Has the source changed? A new sound arises. For example, a narrowed bronchus that vibrates produces a wheeze. A closed small airway that suddenly opens produces a crackle.
- Has the flow changed? The sound becomes weaker or stronger. Sound decreases if the patient breathes shallowly, if the airways are severely narrowed (obstruction) or if mucus plugs a bronchus.
- Has the path changed? The sound reaches the chest differently. Solid lung (consolidation) carries sound better. Fluid (pleural effusion) or air (pneumothorax) between the lung and the chest wall weakens the sound. Hyperinflated lung and a thick chest wall also weaken the sound.
Consolidated lung does not produce a new sound. It carries the sound from the trachea to the chest wall without filtering it. So over a consolidation you hear a loud, high-pitched sound similar to the one over the trachea. This is called bronchial breathing.
Diminished breath sounds do not always mean disease. Sound also decreases when the patient breathes shallowly, when the chest wall is thick or when the stethoscope does not sit well on the skin. So ask the patient to take slightly deeper breaths through an open mouth, and compare with the other side.
Each question rests on one of the three rules below. In later lessons we will return to these rules by their numbers.
Sound comes from eddies in the air that flows fast through the trachea and large bronchi. If the patient breathes shallowly, or if flow falls sharply, the sound falls too.
Normal air-filled lung filters out the high-pitched part of the sound. Solid (consolidated) lung carries sound better. Fluid or air between the lung and the chest wall weakens the sound.
If a narrowed airway vibrates, you hear a wheeze or stridor. If a closed small airway opens suddenly, you hear a crackle. If roughened pleural layers rub against each other, you hear a pleural rub.
Details: Where does sound come from? · How does sound reach the chest wall? · How do adventitious sounds arise?
Which adventitious sounds are there?
New sounds heard on top of the normal breath sound are called adventitious (added) sounds. They fall into two large families, according to how they sound to the ear. Two sounds do not fit either family fully.
| Family | What does it sound like? | Examples |
|---|---|---|
| Continuous | Long, musical | Wheeze, Rhonchus, Stridor |
| Discontinuous | Short, explosive crackling | Fine crackles, Coarse crackles |
| Other | Fits neither family fully | Squawk, Pleural friction rub |
A rhonchus is a low-pitched continuous sound and belongs to the same family as a low-pitched wheeze. It is linked to secretions or narrowing in a large airway and may change with coughing. Telling the two apart by ear is not always easy.
How do you describe a sound?
In every lesson we will ask the same four questions to describe a sound.
During inspiration or expiration? At the start or at the end of inspiration?
Musical or explosive? Fine or coarse? Low-pitched or high-pitched? Few or many?
In one area or widespread? On one side or on both sides?
Does it change with coughing, deep breathing, position or treatment?
The answers to the four questions will not always give you a diagnosis. But they lead you to the right question: Has the source changed? Has the flow changed? Has the path changed?
Which mistakes should you avoid at the start?
The three myths below can lead you to misinterpret what you hear.
If there is no wheeze, there is no obstruction.
In severe obstruction, flow falls so much that the airway cannot vibrate. So the wheeze may disappear. This is called a "silent chest", and it is a danger sign.
If you hear crackles, the patient has pneumonia.
Crackles are also heard in fibrosis, heart failure and bronchiectasis. The timing and distribution of the crackles, and whether they change with coughing, show you which disease to consider.
Crackles are never heard in a healthy person.
In a healthy person who has been breathing shallowly for a while, soft crackles may be heard over the lower parts of the lungs, the bases. These crackles disappear after a few deep breaths (The limits of normal).
- Sound arises in the trachea and large bronchi. Air-filled lung filters out the high-pitched part, and you hear the vesicular breath sound over the chest wall.
- When a sound changes, ask three questions: Has the source changed? Has the flow changed? Has the path changed?
- Adventitious sounds form two families: long, musical continuous sounds and short, explosive discontinuous sounds.
- Describe every sound with four questions: When? What does it sound like? Where? What changes it?
To continue: Classification and terms · Sound atlas · Clinical cases
Check yourself
3 short questions. The explanation opens when you choose an answer.