Respiratory Sounds

How to Cite This Chapter: Bhalla A, Hambly N, Szczeklik W, Jankowski M. Respiratory Sounds. McMaster Textbook of Internal Medicine. Kraków: Medycyna Praktyczna. Accessed June 25, 2024.
Last Updated: February 13, 2022
Last Reviewed: February 13, 2022
Chapter Information

Definition and PathogenesisTop

1. Normal respiratory sounds:

1) Normal lung or “vesicular” sounds are soft, nonmusical, and audible over almost entire peripheral lung zones during inspiration and early expiration. They are produced by turbulent air flow through lobar and segmental bronchi. In disease states, there may be diminished intensity due to decreased generation of sound energy, impaired sound transmission, or both. Decrease in sound generation may be due to impaired respiratory drive or impaired flow of air to the peripheral airways (foreign body or obstructive airway diseases). Impaired transmission of sounds may be due to the presence of fluid or air in the pleural space, consolidated lung, large bullae in patients with emphysema, or by chest wall deformities or obesity.

2) Normal tracheal sounds are hollow nonmusical sounds with a wide spectrum of frequencies that are clearly heard at the suprasternal notch or the lateral neck in both respiratory cycles. Pathologic tracheal or “bronchial” sounds are audible over peripheral lung areas and may suggest lung consolidation (due to inflammation, infection, hemorrhage, protein, or malignancy). In patients with upper airway obstruction, tracheal sounds may become musical and can present as either stridor or localized wheeze.

2. Abnormal respiratory sounds:

1) Crackles are nonmusical, short (<0.25 seconds), explosive respiratory sounds heard mostly during inspiration, caused by the sudden equalization of gas pressures between two areas of the lung. They occur during the opening of previously closed small airways. Crackles may be transiently apparent in healthy people but disappear after a few deep inspirations.

a) Fine crackles, formerly termed crepitations or “velcro rales,” are heard mid-to-late inspiration mostly in dependent lung regions, uninfluenced by cough or body position, and not transmitted to the mouth. These high-pitched sounds may be due to pulmonary fibrosis, congestive heart failure, or pneumonia. Of note, fine crackles are minimal or absent in sarcoidosis, as the disease affects mostly central lung zones.

b) Coarse crackles are heard early in inspiration and throughout expiration, can be transmitted to the mouth, and can change with cough, but they are not influenced by changes in body position. These low-pitched sounds are commonly observed in the setting of bronchiectasis and other conditions characterized by secretions in the airways.

2) Wheezes and rhonchi are musical continuous breath sounds (>0.25 seconds), which may be high-pitched (wheezes) or low-pitched (rhonchi) and are generally audible during expiration. Wheezes (hissing, whistling sounds) are produced by the turbulent flow of air through narrowed airways, while rhonchi are mainly caused by secretions present in the airways. Expiratory wheeze is mostly caused by narrowing of the airways within the chest, which can occur in the setting of asthma, chronic obstructive pulmonary disease, aspiration of gastric contents, or heart failure. Of note, localized wheeze may be due to a focal process, including a tumor, foreign body, or mucous plug.

3) Stridor is a particularly loud, high-pitched, continuous sound, more clearly heard on inspiration over the upper airways or sometimes even without a stethoscope. This sound is caused by large airway narrowing and may indicate obstruction of the larynx or trachea. Stridor may be heard in patients with vocal cord dysfunction, epiglottitis, airway edema, anaphylaxis, laryngotracheitis, extrinsic compression of the trachea, or a foreign body.

4) Squawk, also known as “squeak,” is a mixed sound consisting of short wheezes accompanied by crackles that are heard in the middle to the end of inspiration. Squawks are most frequently present in patients with hypersensitivity pneumonitis and less often in patients with other interstitial lung diseases, bronchiectasis, or pneumonia.

5) Pleural friction rub is caused by the rubbing of the parietal and visceral layers of the pleura due to the deposition of fibrin in the course of an inflammatory or neoplastic process. This is generally biphasic in nature and heard best in basal and axillary regions.


History and physical examination (differential diagnosis of respiratory diseases: Table 1). Diagnostic studies include mainly chest radiography, which may be supplemented with computed tomography (CT) of the thorax and pulmonary function tests (spirometry and others). In patients with dyspnea it is useful to perform pulse oximetry and, if abnormal, arterial blood gas measurements.


Table 1.34-1. Differential diagnosis of respiratory diseases based on physical findings


Chest movements


Vocal fremitusa

Breath sounds

Displacement of the mediastinumb


Asymmetric, motion impaired on the side of infiltrate



Bronchial breathing, crackles



Asymmetric, motion markedly impaired on the side of atelectasis (if large)


– Reduced (atelectasis caused by airway obstruction)

– Increased (atelectasis caused by airway compression)

– Reduced lung sounds

– Occasional crackles

– Bronchial breathing may be audible

Towards the side of atelectasis in setting of lobar collapse

Fibrosis (bilateral)

Slightly impaired symmetrically

Slightly dull

Slightly reduced

– Reduced lung sounds

– Crackles


Pleural effusion

Asymmetric, motion impaired on the side of effusion



Absent breath sounds; in the setting of small pleural effusions pleural friction may be audible

Away from the side of effusion (if large)


Asymmetric, motion impaired on the side of pneumothorax



Absent breath sounds

With tension pneumothorax, away from the side of the lesion

Airway obstruction

– Symmetrically increased

– Work of accessory respiratory muscles usually seen

Usually normal

Unchanged or reduced

– Wheezes and rhonchi

– Prolonged expiratory phase

– Normal lung sounds, may be reduced with occasional crackles


a Transmission of spoken words while listening with a stethoscope (eg, “blue balloons,” “toy boat”).

b Displacement of the trachea may sometimes be observed on physical examination of the neck.

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