The presence of a delayed response to cat antigen in allergic patients has been postulated but can not be demonstrated by conventional lung function tests. The purpose of this study was to determine if indeed there is a delayed response in the small airways using a novel quantitative CT technique.

Ten subjects with known cat-induced airway hyperreactivity were studied over 3 days. On day 1 baseline (BL) pulmonary function tests (PFT) and HRCT were performed following which patients were exposed to cat room challenge (CRC). HRCT and PFT were repeated 6 and 22 hours post-CRC. All HRCT studies were performed at residual volume (RV) under spirometric control. The BL and 22hr CT studies were acquired pre- and post-methacholine challenge test (MCT). All scans were acquired on the same scanner using a 10 mm spiral reconstruction every 1mm at 3cm above and below the carina. Automated segmentation of lung fields provided 12 regions of interest (ROI) from which lung attenuation curves (LAC), including median and 10th percentile attenuation, were derived. Changes in small airways reactivity were assessed by comparing the leftward shift (decreased attenuation) of LAC between pre- and post-MCT for BL and 22 hr post-CRC using random model effect.

All subjects displayed immediate decline in FEV1 (mean decrease of 30 ± 11%). Six hours post-CRC, despite only moderate decreases in PFT (mean decrease 10 ± 14%), a significant shift was observed in median attenuation (mean shift -54.09Hu ± 6.80, p<.001) and its10th percentile (mean shift -45.67Hu ± 5.12, p<.001) compared to BL. At 22 hr post-CRC, no significant FEV1 drop was demonstrated but median and 10th percentile attenuation remained significantly left-shifted (increased air trapping) with mean change of -15.88Hu ± 5.20 (p=.002) and -10.42Hu ± 4.26 (p=.014), respectively. MCT at 22 hr induced further LAC shift of the median (-16.85Hu ± 5.40, p=.002) and 10th percentile (-12.81Hu ± 4.46, p=.004) compared to BL LAC shift.

Quantitative CT analysis characterized air trapping and sensitization to methacholine up to 22 hours post cat challenge despite return to normal spirometry.