Lung Sounds Indicating Fluid in the Lungs
The lung sound that indicates fluid in the lungs is called "crackles" (also known as rales), which are short, interrupted, explosive sounds heard during inspiration when abnormally closed airways suddenly open. 1, 2
Mechanism and Characteristics of Crackles
Crackles are generated by sudden airway opening during inspiration when fluid, exudate, or edema causes airways to stick together and then explosively separate as air rushes through during breathing. 2 The mechanism works as follows:
- Inspiratory crackles occur when collapsed or fluid-filled airways suddenly snap open, creating a negative polarity sound (76% of inspiratory crackles have negative polarity). 2
- Expiratory crackles are less common and occur when airways suddenly close, but these are far less energetic than inspiratory crackles. 2
- Inspiratory crackles are almost twice as numerous as expiratory crackles in patients with fluid-related lung pathology. 2
Timing and Pitch Patterns
The timing and pitch of crackles provide important diagnostic information about the underlying pathology:
- Crackle pitch progressively increases during inspiration in 79% of patients with pneumonia, heart failure, or interstitial fibrosis, rising approximately 40 Hz from early to mid-inspiration and another 40 Hz from mid to late-inspiration. 3
- This progressive pitch increase reflects sequential recruitment of smaller diameter airways as lung volume increases during inspiration. 3
- In pneumonia specifically, crackles are typically coarse and mid-inspiratory early in the disease course, then shift toward late inspiration as the patient improves. 4
Clinical Context for Different Fluid-Related Conditions
The characteristics of crackles vary depending on the underlying cause of fluid accumulation:
Pneumonia
- Crackles are coarse and predominantly mid-inspiratory in early pneumonia. 4
- As pneumonia resolves, crackle timing shifts later in inspiration and individual crackle duration becomes shorter. 4
- The diseased lung shows significantly different frequency characteristics compared to the healthy lung in unilateral pneumonia. 4
Heart Failure (Pulmonary Edema)
- Crackles indicate increased extravascular lung water from cardiogenic pulmonary edema. 5
- The number of crackles correlates with severity of pulmonary congestion. 5
- Crackles should decrease or disappear with adequate diuretic treatment. 5
Interstitial Fibrosis
- Crackles follow similar pitch progression patterns as other fluid-related conditions. 3
- These patients demonstrate the characteristic inspiratory pitch increase reflecting progressive airway recruitment. 3
Important Clinical Caveats
- Crackles must be interpreted in clinical context as they can occur in multiple cardiopulmonary disorders with different pathophysiology. 1
- The timing (early vs. late inspiratory), pitch, and waveform characteristics differ between pulmonary disorders and reflect different underlying mechanisms. 1
- Auscultation alone has limitations; when fluid in the lungs is suspected clinically, ultrasound should be used to confirm the presence of pleural effusion or pulmonary edema, as it is more sensitive than chest radiography. 6, 7