During pulmonary auscultation as part of a routine check-up on a 62-year-old man, you hear a few fine, late inspiratory crackles. The patient had no history of heart disease, and findings from the remainder of his examination were normal.
Crackles are short, discontinuous popping sounds that usually are associated with cardiac or pulmonary disorders. Early studies with sound recording and band-pass filters revealed that fine crackles were associated with high frequencies, and coarse crackles were lower-frequency sounds. Later studies suggested the size of the affected airways was responsible for the character of the sounds produced.
Initial theories on what causes crackles focused on the belief that the sounds were caused by air bubbling through fluid in the smaller airways of the lungs. Forgacs noted the sounds were usually persistent, even after coughing, and were consistent over time . Both of these features suggested that fluid was not responsible for the sounds. He proposed that a gradient of gas pressure develops across abnormal airways that collapse during expiration. When air rushes in during inspiration, the popping sound is generated as the airway reopens. Recording studies and mathematical theories based on stress-relaxation points have supported this theory .
Crackles are generated in lung tissue that has inflammation or impaired elastic recoil. Those that occur in smaller airways have shorter, higher-pitched sounds (fine crackles), and larger airways have lower-pitched sounds of relatively longer duration (coarse crackles). Crackles that occur early in the inspiratory cycle are more likely to be from the proximal airways. Late inspiratory sounds are likely to be from more distal sources, as the air movement affects these airways later in time.
Crackling can be heard at times in patients with no lung disease, but the sounds are more variable over time. In the case of normal lung tissue, it has been suggested that alveolar fluid, distal to small airway collapse (atelectasis), causes the sound as the air pressure equalizes on inspiration. In addition, ageing causes a reduction in the elastic recoil pressure of the lungs, increasing the incidence of crackles in older patients.
Crackles are also variously referred to as rales or crepitations. The American Thoracic Society has suggested the term crackles, and specifies coarse crackles as loud, low-pitched sounds of at least 10 ms in duration. They specify fine crackles as "less loud", higher-pitched sounds of less than 5 ms in duration.
Crackles that develop as a result of heart failure are typically dependent on gravity. When cardiac failure is present, crackles will be heard at the lung bases if the patient is sitting up. As heart failure worsens, crackles will be heard proportionately farther up in the pulmonary fields. If the patient is lying on his or her side, the crackles will be heard in the dependent lung. Crackles from pulmonary disease are not dependent on gravity.
If the patient breathes deeply and rapidly, the smaller airways may remain opened and crackles will be artificially suppressed.
It is important to note that crackles may not be heard if other loud lung sounds are present, the patient is guarding against taking a deep breath because of pain, or fluid is in the pleural space.
Examples of causes of crackles include congestive heart failure, bronchiectasis, chronic obstructive pulmonary disease (COPD), pneumonia, and sarcoidosis. Essentially, any illness or disorder that disrupts normal elastic recoil in the airways causing inflammation, or results in pulmonary vascular congestion, can produce crackles.
Pneumonia can produce late coarse crackles because of local airway inflammation and the presence of alveolar fluid.
Restrictive pulmonary diseases, such as pulmonary fibrosis, cause fine crackles late in the inspiratory cycle that can continue into the expiratory cycle. These crackles have a higher frequency than the crackles of bronchiectasis, heart failure, and COPD. Sarcoidosis and asbestosis cause similar fine, late inspiratory crackles.
Bronchiectasis causes coarse mid-to-late inspiratory crackles. COPD also causes coarse crackles, but they tend to be early in the inspiratory cycle and usually resolve before the late inspiratory crackles heard with bronchiectasis. Breathing quickly during the examination may cause temporary resolution of the sounds in patients with COPD.
In congestive heart failure, the crackles are coarse and may be present throughout both the inspiratory and expiratory cycles. A distinguishing feature, as noted before, is that the sounds occur in the dependent part of the lung field.
The ability of a trained examiner to note the presence or absence of crackles is good. Accuracy begins to drop to a level as low as 70% when trying to determine such factors as when in the respiratory cycle the crackles occur and whether they are fine or coarse. This variability is probably related to the variable hearing sensitivity of the examiner, as well as variations in stethoscopes and technique.
The presence of crackles is itself a nonspecific finding. However, when crackles are heard, other physical findings can be useful in directing the examiner toward a working diagnosis. For example, a patient who has had clear lung fields on prior examinations but who now has a fever, cough, and crackles is likely to have pneumonia. Supportive testing can be done based on that assessment. In addition, other pulmonary examination findings, such as fremitus or dullness to percussion, can help in determining a more specific diagnosis.
Conversely, the absence of crackles does not preclude disease. If airflow is very low, such as with bronchospasm, the sounds will not be heard.
As they are currently defined by the American Thoracic Society, crackles are distinct in that they are discontinuous. Continuous sounds, such as the musical sounds of wheezing and the lower, gurgling sounds of rhonchi, are usually easy to distinguish from crackles.
Short, discontinuous sounds can mimic crackles. Pleural friction rubs are usually coarse and low pitched but on occasion can cause abrupt, short sounds. Short musical sounds, sometimes called "squawks", can occur in fibrotic pulmonary disorders. These are essentially short wheezes and might be confused with crackles.
The first step would be to go through his medical history to determine whether he has had any prior pulmonary abnormalities or abnormal examination results. A history of asbestos exposure or other potential causes of fibrotic lung diseases should be sought. The focus of the examination and history also should address possible cardiac disease and previously undiagnosed pulmonary disorders.
A chest radiograph is an inexpensive, readily available examination that often can identify pulmonary abnormalities or signs of congestive heart failure. If needed, computed tomography, echocardiography, and pulmonary function testing can be used to help distinguish between restrictive, obstructive, and cardiac causes of the findings.
|1.||Forgacs P. The functional basis of pulmonary sounds. Chest 1978;73:399-405.
|2.||Fredberg JJ, Holford SK. Discrete lung sounds: crackles (rales) as stress-relaxation quadrupoles. J Acoust Soc Am 1983;73:1036-46.|