By its fundamental. A wheeze may contain several harmonically related frequencies, but its pitch is always
determined by its lowest (or fundamental) frequency.
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If a polyphonic CAL may have more than one frequency, how does one determine its pitch?
By its fundamental. A wheeze may contain several harmonically related frequencies, but its pitch is always determined by its lowest (or fundamental) frequency.
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What is the clinical significance of expiratory crackles?
They are an important predictor of disease severity. In patients with interstitial lung disease, for example, the number of expiratory crackles has been shown to correlate directly with a reduction in diffusing capacity. In fact, because usually fewer than inspiratory crackles (and thus easier to count), expiratory crackles may be even more valuable for the assessment of disease severity. Can crackles occur in exhalation?
Crackles are primarily inspiratory, and yet 10% do occur in exhalation—usually at mid- or late-expiration in patients with either obstructive or restrictive disease. In obstructive processes (such as bronchitis or bronchiectasis), they tend to be coarse, early expiratory, gravity independent, and profuse. They decrease in number with coughing. In restrictive processes (such as pulmonary fibrosis or connective tissue disease), they are fine, mid- or late expiratory, gravity dependent, and scanty. They do not resolve with coughing. What is the mechanism of production of late-expiratory
crackles? There are two schools of thought: (1) they are produced by the closure (not the reopening) of stiff and fibrotic small airways, and (2) they are produced by the reopening of small airways, very much like lateinspiratory crackles. Using Forgacs’ model, this would occur as follows: High interstitial pressure (from interstitial fibrosis, for example) would collapse a small airway. The inspiratory traction would snap the airway open, thus creating a “pop” in late inspiration. The airway would then recoil in early exhalation, closing its lumen once again. This, in turn, would set up a new reopening, which this time occurs in late exhalation, when the intraluminal air exceeds pressure in the adjacent airways. T he reopening produces a late-expiratory crackle. Is there a correlation between late-inspiratory crackles and severity of IPF?
Yes. In addition to fewer crackles, milder forms of the disease have crackles that are only late inspiratory and gravity dependent (i.e., limited to the bases in upright patients). As the disease progresses, crackles become paninspiratory (even though still predominant in endinspiration). They may also persist despite postural changes, eventually extending to higher lung regions. Finally, they may become associated with late inspiratory squeaks Are crackles common in patients with idiopathic pulmonary fibrosis (IPF)?
Yes. In fact, so common (up to 100%) that their absence argues strongly against the diagnosis. Where are asbestosis crackles localized?
Usually at the bases, first centrally (along the midaxillary lines) and then posterolaterally. How common are crackles in asbestosis?
Very common. In large population studies, they are present in 15% of asbestos workers versus 3% of normal people. Crackles also are an early sign of disease, increasing in frequency and number with increased duration of exposure. By the time asbestosis is clinically evident, late-inspiratory, fine, and high pitched crackles are heard in more than half of all patients. They are a good marker of disease severity, reflecting more on the duration of asbestos exposure than the vital capacity itself. Hence, crackles can be a valuable tool for the monitoring of exposed workers. Is there a correlation between the number of crackles and disease severity?
Yes. In asbestosis, for example, the number of crackles correlates directly with severity of the underlying disease. This rule applies also to other interstitial processes. Automatic crackle detection and counting have indeed been developed for both diagnosis and follow-up. Why are crackles so rare in sarcoidosis but so common in other fibrotic lung diseases?
Because of the different distribution in lung scarring, as clearly seen on high-resolution computed tomography (CT). Idiopathic pulmonary fibrosis tends to be associated with lower lobe location and subpleural scarring, which are strong radiologic predictors of the presence of crackles. Sarcoidosis is instead associated with upper lobe location and peribronchial fibrosis. |