What is the mechanism of fine and coarse crepitations (crackles) in lung disease?

Mechanism of Fine and Coarse Crackles in Lung Disease

Fine and coarse crackles are produced by different physiological mechanisms: fine crackles result from sudden opening of previously closed small airways during inspiration, while coarse crackles result from air bubbling through secretions in larger airways or from sudden airway closure during expiration.

Pathophysiological Mechanisms

Fine Crackles

• Fine crackles are primarily inspiratory sounds produced by the sudden reopening of previously closed small airways, creating explosive opening events 1
• They have predominantly negative polarity (76% of inspiratory crackles), consistent with the stress-relaxation quadrupole hypothesis 1
• Fine crackles are characterized by higher frequency sounds (higher peak and maximum frequencies) compared to coarse crackles 2
• They have shorter waveform measurements, including shorter duration cycles 2
• Fine crackles are commonly associated with interstitial lung diseases, particularly pulmonary fibrosis 3

Coarse Crackles

• Coarse crackles can be both inspiratory and expiratory, often related to air bubbling through secretions in larger airways 1, 2
• Expiratory crackles are caused by sudden airway closure events that are similar in mechanism but opposite in sign and less energetic than the opening events of inspiratory crackles 1
• They have predominantly positive polarity (only 31% of expiratory crackles have negative polarity) 1
• Coarse crackles have lower frequency characteristics and longer waveform measurements 2
• They are commonly associated with conditions involving excess secretions such as chronic bronchitis 2

Acoustic Characteristics

Frequency and Waveform Differences

• Fine crackles have significantly higher peak and maximum frequencies than coarse crackles 2
• Waveform measurements (1/4 cycle duration, initial deflection width, two cycle duration, and 9/4 cycle duration) are significantly smaller for fine crackles than for coarse crackles 2
• Log peak frequency and log maximum frequency correlate better with 9/4 cycle duration and two cycle duration than with 1/4 cycle duration or initial deflection width 2

Timing and Distribution

• Fine crackles typically occur in mid to late inspiration 3
• Expiratory crackles in patients with fibrosing alveolitis occur predominantly in mid and late expiration 4
• Inspiratory crackles are almost twice as numerous as expiratory crackles in patients with various cardiopulmonary disorders 1

Clinical Significance

Diagnostic Value

• Fine crackles are present in 93% of patients with idiopathic pulmonary fibrosis (IPF) and 73% of patients with non-IPF interstitial lung disease 3
• In IPF, fine crackles are more common than cough (86%), dyspnea (80%), or abnormal pulmonary function tests 3
• The number of expiratory crackles correlates significantly with the reduction in predicted transfer factor in patients with fibrosing alveolitis, suggesting they may indicate disease severity 4

Normal vs. Pathological Crackles

• Midinspiratory fine crackles can be heard at the anterior bases in healthy individuals during slow inspirations from residual volume 5
• These non-pathological crackles occur when basilar airways, which close at the end of a forced expiration, suddenly open during inspiration 5
• Pathological crackles can often be distinguished from normal crackles by their quality, timing, and anatomic distribution 5

Pitfalls and Caveats

• There is some overlap in acoustic measurements between fine and coarse crackles for individual crackles, though average values can clearly separate the two types 2
• Observer agreement in identifying fine crackles is high (90%), but proper auscultation technique is essential 3
• The identification of crackles is not significantly affected by lung function, symptoms, emphysema, COPD, obesity, or clinician experience 3
• Early and late segments of crackles have different characteristics, likely related to the origin of the sound and the resonance of the lung respectively 2
• Crackles during normal tidal breathing are more likely to be pathological than those heard only after expiration to residual volume 5