What are the different types of lung sounds in patients with respiratory symptoms?

Types of Lung Sounds in Respiratory Disease

Lung sounds are classified into normal breath sounds and adventitious (abnormal) sounds, with adventitious sounds further divided into continuous sounds (wheezes and rhonchi) and discontinuous sounds (crackles), each having distinct acoustic characteristics and clinical significance. 1, 2

Normal Breath Sounds

Normal breath sounds occur during respiration when no respiratory pathology exists and represent unobstructed airflow through the airways. 2 These sounds should be present bilaterally and symmetrically across all lung fields during routine auscultation.

Adventitious (Abnormal) Lung Sounds

Discontinuous Sounds: Crackles (Rales)

Crackles are discontinuous, explosive sounds typically heard during inspiration and represent one of the most clinically significant adventitious findings. 1

• Crackles are detected in more than 80% of patients with pulmonary fibrosis, making them a key diagnostic finding. 3
• Dry, end-inspiratory "Velcro" crackles in patients beyond 50 years of age are characteristic of Idiopathic Pulmonary Fibrosis (IPF). 3
• In pneumonia, inspiratory crackles occur in 81% of patients versus 28% of controls, while expiratory crackles occur in 65% versus 9% of controls. 4
• Bilateral crackles suggest pulmonary congestion from heart failure, often accompanied by interstitial edema and pleural effusions on chest X-ray. 3
• Crackles are typically heard during inspiration in conditions like bronchiolitis and pneumonia in pediatric populations. 1

Continuous Sounds: Wheezes

Wheezes are high-pitched continuous sounds with a dominant frequency of 400 Hz or more, representing a raspy whistling sound noted on expiration or inspiration associated with lower airway obstruction. 1, 5

• Wheezes are commonly found in asthma and COPD and are produced by fluttering of airway walls induced by critical airflow velocity through narrowed airways. 1, 5
• The presence of wheezing during normal breathing or prolonged phase of forced exhalation is a key finding in asthma, though absence does not rule out asthma. 1
• The proportion of the respiratory cycle occupied by wheeze (tw/ttot) correlates with the degree of bronchial obstruction, though the relationship is too scattered to predict FEV1 from wheeze duration alone. 5
• Wheeze intensity or pitch does not correlate with pulmonary function severity. 5

Continuous Sounds: Rhonchi

Rhonchi are low-pitched, continuous rumbling sounds with a dominant frequency of approximately 200 Hz or less, typically heard during forced expiration and often clearing with coughing. 1, 5

• Rhonchi occur in 19% of pneumonia patients versus 0% of controls. 4
• These sounds are associated with mucous hypersecretion and airflow limitation in COPD. 3
• The term "rhonchi" is frequently misused by clinicians to describe both continuous and discontinuous adventitious sounds, reflecting poor standardization in lung sound terminology. 6

Other Adventitious Sounds

Stridor is a high-pitched sound reflecting upper airway obstruction, most commonly seen in infants and representing the most common indication for flexible bronchoscopy in this population. 1 Laryngomalacia is the most common congenital laryngeal anomaly causing persistent stridor in children. 1

Pleural friction rub is recognized by the majority of physicians but less reliably than other adventitious sounds. 6

Critical Clinical Pitfalls

Physical examination findings alone, including the presence of rhonchi, wheezes, or crackles, are insufficient for diagnosis and must be confirmed with objective testing such as spirometry. 1 This represents a fundamental principle emphasized across multiple respiratory society guidelines.

• The degree of airways obstruction cannot be predicted from symptoms or signs alone, including breath sounds. 1
• Treatment decisions should be based on spirometry results rather than the presence or characteristics of breath sounds alone. 1
• In immunocompromised patients, lung ultrasound or CT imaging is preferable as standard physical examination findings may be absent despite radiographic pneumonia. 7
• When tachypnea is combined with abnormal breath sounds, the negative predictive value reaches 97% for pneumonia—meaning their combined absence is highly reassuring. 7

Diagnostic Accuracy Considerations

Deep learning-based automated classification detects abnormal sounds with 86.5% accuracy and classifies them into crackles, wheezes, or rhonchi with 85.7% overall accuracy, compared to human clinicians whose accuracy ranges from 53.4% (interns) to 80.1% (fellows). 8 This highlights the inherent difficulty and subjectivity in lung sound interpretation, even among experienced clinicians.