Methacholine-Induced Temporal Changes in Airway Geometry and Lung Density by CT: Discussion methacholine-induced bronchoconstriction
An increase in regional lung air content in the lung during methacholine-induced bronchoconstriction would suggest air trapping. Air trapping as measured by nonimaging methods has been shown to occur in response to methacholine, presumably due to closure of peripheral airways. However, it is also possible that constriction of peripheral airways may serve to expand alveoli through interdependence phenomena or to strengthen the peripheral interstitial “skeleton” of the lung, preventing dependent alveolar compression. Gradients in regional lung expansion have previously been observed by numerous techniques., Regional CT density changes in both dogs and humans show that in the dependent lung regions where lung density is greatest, there is greater decrease in density with lung inflation. These data imply that in the nondependent lung region where the lung is least dense, the parenchyma is more inflated at baseline and therefore less able to expand further. It could be that in our current study the effect of air trapping is greatest in the dependent lung regions where the lung is more compliant. Pulmonary Infection
The possibility exists that “increased air content” reflected reduced blood volume in the lung. This could be caused either by direct effects of metha-choline, such as altered pulmonary artery BP or by secondary effects, such as air trapping with associated vascular compression and/or hypoxic vasoconstriction or reflex vascular constriction to allow for good ventilation-perfusion balance. The assumption that a vascular component attributed to the density changes is supported by Figure 4. In the crosssectional image at 30 s following methacholine injection, one can observe constriction of pulmonary arteries. Unfortunately, measurements of pulmonary arterial pressure or pulmonary vascular resistance were not obtained in the present experiments. These data could have considerably elucidated the possible mechanisms of lung density changes.
During methacholine-induced bronchoconstriction, there was also a marked increase in the degree of regional variability as reflected in the CV and the r values of the air content and lung height relationships. Moreover, although, on average, the air content of the lung increased, changes at similar lung height points were quite variable, with some regions demonstrating a large increase in air content, others demonstrating a much smaller increase, and a few demonstrating reduced air content, as shown by the example in Figure 7. CT studies of airway reactivity have led to a better appreciation of heterogeneity of the airway response to bronchoconstrictive challenges.,,, The variability of lung density changes during methacholine challenge parallels the directly measured airway CSA response.
In conclusion, HRCT imaging can be used to assess alterations in regional airway geometry and lung density. EBCT techniques defined the time frame of the response to IV bolus injection of methacholine in pigs. The maximal response observed at the 30-s time point was detected both in terms of decreased CSA of airways > 1 mm and in terms of the lung density. Overall, there was increased air content during bronchoconstriction, and this effect was significantly greater in the dependent lung regions.