Methacholine-Induced Temporal Changes in Airway Geometry and Lung Density by CT: Discussion

Methacholine-Induced Temporal Changes in Airway Geometry and Lung Density by CT: DiscussionSeveral investigators- have shown HRCT to be a useful tool in assessing airway reactivity. Those studies, along with other imaging methods,” have utilized single time points to assess airway response, and no previous study has directly assessed density changes associated with induced bronchoconstricmaximal response to a bolus of methacholine to pigs as detected by CT occurred 30 s following the challenge. This maximal response could be measured in terms of changes in airway CSA (mean decrease of  31 ± 3%) and lung density. At 2 min, the response was always smaller than at 30 s and had essentially returned to baseline. The time course of lung density changes paralleled the airway CSA response.
Previous studies have demonstrated that reconstructed radiograph CT attenuation coefficients can be used as an accurate index of regional lung density. The current comparison of lung density assessed with both sharp and normal reconstruction algorithms shows that the sharp algorithm used in HRCT does not alter the basic lung density information content. Thus, the same reconstructed CT section can be used for simultaneous assessment of central airways CSA and peripheral lung density. Our current study supports the concept that HRCT-based tion. The speed of the EBCT scanner in the present study has allowed us to verify the time course of the bronchoconstrictive response to bolus methacholine and to relate this response to peripheral lung density temporal variations. Airway CSA for all size ranges was found to be least (31% decrease from baseline) at the 30 s time point and to return to baseline within 2 to 4 min. Overall, the density of the lung decreased during bronchoconstriction. Fungal Infection
Decreased lung density in response to methacho-line injection was found to be greater in the dependent lung regions. Given that the density changes were maximal at 30 s and resolved within 2 to 4 min, it is unlikely for changes in extravascular lung water or lung tissue to account for these findings. More likely explanations for the observed changes in lung density in the present study are an increase in regional lung air content and/or a decrease in regional intravascular blood volume.

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