What are rales (rales) and crackles in the lungs?

What Are Rales/Crackles?

Rales (also called crackles) are short, interrupted, discontinuous breath sounds heard during lung auscultation that are generated when abnormally closed airways suddenly open during inspiration or close at the end of expiration. 1

Acoustic Characteristics and Mechanism

Crackles are produced through a specific mechanical process:

• The sounds occur when collapsed or fluid-filled airways suddenly snap open during inspiration or close during expiration, creating brief explosive acoustic events 1, 2
• Inspiratory crackles are approximately twice as numerous as expiratory crackles (3,308 vs 1,841 in one analysis) and have predominantly negative acoustic polarity (76% vs 31%) 2
• The pitch of crackles progressively increases during inspiration, rising approximately 40 Hz from early to mid-inspiration and another 40 Hz from mid to late-inspiration, reflecting sequential recruitment of progressively smaller airways 3

Clinical Classification

Crackles are categorized by their acoustic quality and timing:

Fine Crackles (Most Clinically Significant)

• "Dry" or "Velcro-type" quality, occurring predominantly during end-inspiration, most commonly heard initially at lung bases 4
• Detected in more than 80% of patients with idiopathic pulmonary fibrosis (IPF), representing the most sensitive clinical finding for interstitial lung disease (ILD) 5, 4, 6
• Often appear before other symptoms or pulmonary function abnormalities 6

Coarse Crackles

• More commonly heard in bronchiectasis and may indicate secretions in larger airways, in contrast to the fine crackles of interstitial lung disease 4

Key Clinical Associations

Interstitial Lung Disease

• Fine crackles extend progressively from lung bases toward upper zones as disease advances 5
• Bilateral late or pan-inspiratory crackles at posterior lung bases are recognized diagnostic criteria for asbestosis 5, 6

Acute Pulmonary Edema

• Fine crackles are the predominant finding, typically beginning at lung bases and progressing upward as congestion worsens 4, 6

Pneumonia

• The likelihood of radiographic pneumonia increases in the presence of crackles on auscultation 4
• Persistent late inspiratory crackles induced in dependent lungs when placed in lateral decubitus positions suggest pneumonia 4, 7

Important Clinical Caveats

Normal Variant

• Crackles can occur in normal subjects without lung disease when inhaling from residual volume (RV), particularly over the anterior chest 8
• In one study, 26 of 52 normal nonsmoking subjects had profuse, loud crackles over the anterior chest when inhaling from RV, though no crackles occurred during inhalation from functional residual capacity 8

Diagnostic Limitations

• Lung auscultation has overall pooled sensitivity of only 37% with specificity of 89%, and the low sensitivity in different clinical settings hampers clinical utility 4
• The presence of crackles is a highly nonspecific diagnostic sign and must be interpreted in clinical context 5

Recommended Diagnostic Approach When Crackles Are Detected

When fine crackles are detected, obtain HRCT imaging, as chest radiography lacks sensitivity for early ILD 6:

• For suspected ILD: Perform HRCT in prone position at lung bases to detect interstitial fibrosis and consider referral to an ILD specialist 6
• For suspected heart failure: Obtain BNP/NT-proBNP levels, echocardiography, and chest radiography for pulmonary venous congestion 6
• For asbestos exposure history: Document exposure duration and intensity, and use HRCT to increase specificity of radiographic findings 6
• Assess clinical context including occupational exposures, cardiac history, systemic autoimmune diseases, and medication history 6
• Perform pulmonary function testing including spirometry and diffusing capacity, though crackles often precede functional abnormalities 6