16 The extent of emphysema in the lung can be quantitatively assessed from the HRCT by counting the number of voxels with an attenuation value less than a specified threshold. Emphysema can be detected by HRCT as areas of low attenuation. The chronic inflammation associated with COPD also leads to destruction of the alveoli walls, causing the enlargement of the air spaces within the lung that is the defining feature of emphysema. The normalization is not perfect, however, as percentage wall area nevertheless has the tendency to increase with airway generation. It is calculated by expressing the total airway wall area as a percentage of the whole area enclosed by the external perimeter of the airway wall (airway wall area plus luminal area). 11 Percentage wall area has been introduced as a way of normalizing the wall thickness measurement for different airway generations. 14Īirway wall thickness decreases with increasing airway generation due to the normal anatomy of the airways. 12 Thickening of the walls of the central airways correlates weakly with spirometric measures of airways obstruction, 13 but the strength of the association increases as the geometric measurements are taken from more distal airways at the fifth and sixth generations. 10 Increased wall thickening in COPD is associated with worsening air flow limitation and increased inflammation 11 and, as measured by quantitative computed tomography analysis, predicts increased symptoms and worse health outcomes. Although the technique is limited by resolution in its assessment of the very small airways, it is reliable down to the sixth airway generation. Quantitative analysis of high-resolution computed tomography (HRCT) of the lungs has been demonstrated to be a useful method of characterizing and measuring the anatomy and geometry of the bronchial tree. COPD is a chronic inflammatory disease of the lungs that results in progressive narrowing of the airways, 9 and it was therefore hypothesized that the characteristics of crackles would be altered in the presence of COPD. The pressure required to cause sudden reopening of closed airways has been shown to be related to the airway diameter 7, 8 therefore, it can be hypothesized that the characteristics of crackles as described above may be related to airway geometry. 3 The number of closed airways that reopen directly affects the crackle number, as each reopening event causes one or more crackles. 1, 3, 6 Crackle duration is related to airway size, with smaller airways thought to give rise to crackles of shorter duration. 1), which is determined as the time from the beginning of a crackle to the point where the crackle has completed 2 cycles 5 and quantity (such as the number of crackles per breathing cycle ). 3, 4 Crackles can be described in terms of duration (such as the crackle 2-cycle duration see Fig. The mechanism to generate crackles is believed to be associated with the sudden reopening of closed airways, which result from secretion obstruction, bronchoconstriction, chronic inflammation, and loss of airway cartilaginous support. Crackles have been reported to be the most frequent of adventious lung sounds that are indicative of pulmonary diseases. Recent technological advances mean that lung sounds and their characteristics can now be recorded and objectively quantified, providing useful information on lung health. Use of standard auscultation to listen to lung sounds is an accepted component of routine clinical assessment, but it is a subjective process.
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