Evaluation of bronchial wall thickness using high resolution CT in healthy and asthmatic cats

Abstract

Feline asthma syndrome (FAS) is a respiratory disease with clinical symptoms such as wheezing and coughing due to airway hyper-responsiveness. The airway hyper-responsiveness is generally caused by allergic reactions. FAS is very similar to asthma syndrome in human. Cat has been used as animal model of asthma for human medicine.Asthma in human can be diagnosed with pulmonary function tests or methacholine challenge test. However, voluntary pulmonary function tests are impossible for animals such as cats. The goldstandard for FAS iscurrently unavailable. In veterinary medicine,the diagnosis of FAS can be through excluding other diseases such as infectious disease or cardiac disease.In human medicine, bronchial walls can be measured and evaluated through high-resolution computed tomography (HRCT) to assess the severity of asthma or therapeutic response of patient with asthma. However, HRCT has not been used to measure bronchial walls of cats. Therefore, the objective of this study was to use HRCT to determine the thickness of bronchial walls of clinically diagnosed asthmatic cats compared to that of clinically healthy cats. Effects of different ventilation techniques under general anesthesia on the bronchial lumen and bronchial wall were evaluated. Under general anesthesia, bronchial lumen to the adjacent artery ratio (BA ratio), the ratio of bronchial wall thickness to whole bronchial diameter(TD ratio) and ratio of bronchial wall thickness to adjacent pulmonary artery (TA ratio) were measured in positive pressure inspiration and in end expiration. To evaluate the usefulness of HRCT, computed tomographic scans were performed under both high resolution and conventional conditions using the same experimental animals. Images and measured indices obtained from conventional CT(CCT) and HRCT were compared and analyzed. To compare HRCT and CCT, TD ratio, percentage of bronchial wall area to whole bronchial area(WA%) and TA ratio were measured. Coefficient of variation was performed in values from HRCT and CCT to compare size of variation between two groups. To compare reproducibility in two observers, intraclass correlation coefficient (ICC) was performed in TD ratio, WA% and TA ratio in CCT and HRCT separately.To determine the differences of clinically healthy cats and asthmatic cats, TD ratio, WA%, and TA ratio were measured. Images of 64 bronchi from 16 healthy cats and 16 bronchi from 4 clinically asthmatic cats were obtained. In positive pressure induced inspiration, mean BA ratio was 0.87 ± 0.12. In end-expiration, mean BA ratio was 0.74 ± 0.11. BA ratio between the two different ventilation techniques were significantly (p<0.01) different.The mean TD ratio during positive pressure inspiration and end expiration were 0.18 ± 0.02 and 0.21 ± 0.03and were significantly different (p<0.01). The mean TA ratio during positive pressure inspiration and end expiration were 0.25 ± 0.05 and 0.26 ± 0.06 and were not significantly different. The coefficient of variation from indices of HRCT was lower than that of CCT. The ICC of HRCT in TD ratio, WA% and TA ratio between two observers was higher than ICC of CCT. In addition, the thickness of bronchial wall measured from the axial image at same level of both HRCT and CCT was manually measured to compare image qualities between CCT and HRCT. Due to delineation of images, bronchial wall thickness measured with HRCT was significantly thinner than that with CCT. The bronchial walls and pulmonary arteries were measured in clinically healthy 16 cats and clinically asthmatic 4 cats under general anesthesia with positive pressure inspiration using HRCT. As a result, the mean TD ratio, WA%, and TA ratio in healthy cats were0.18 ± 0.02, 62 % ± 6.27 %, and 0.25 ± 0.05, respectively. Under the same condition, the mean TD ratio, WA%, and TA ratio in asthmatic cats were 0.22 ± 0.24, 68.98 % ± 5.57, and 0.37 ± 0.06, respectively. The three indices in clinically diagnosed asthmatic catswere significantly higher than those in healthy cats(p<0.01). However, no significant difference was found in gender or age between the two groups of cats. These results revealed that HRCT could be a useful method to assessbronchial wall thickness of cats with FAS. And among the four indices, TA ratio showed higher reproducibility, sensitivity and specificity. Although the limitation of this study was that asthma group of cats was clinically diagnosed with asthmatic syndrome, the evaluation of bronchial wall thickness by HRCT could be very useful for the diagnosis of FAS.