Relationship between hypostasis effect in the lung parenchyma on CT and small airway resistance: a preliminary study

Objective To investigate the correlation between hypostasis effect in the lung parenchyma on CT (HELC) and small airway resistance. Methods Forty-two consecutive patients and 6 volunteers without parenchyma pathologic change on CT or history of chest operation were enrolled in the study, which was approved by the hospital institution, and informed consent was obtained. CT scans of 3 slices with thickness 5 mm were performed at the levels of aorta arch, sub-bronchial carina and diaphragmatic dome in both supine and prone positions with smooth breathing and reconstruction of 1.25 mm. CT and pulmonary function test (PFT) studies were performed within 2 days. Patients were divided into two groups according to the ratio of the forced expiratory volume in 1 s and the forced vital capacity of predict% (FEV1.0/FVC%) as gold standard: group A (normal small airway resistance, SAR) with FEV1.0/FVC% ≥ 80%, and group B: (abnormal SAR) with FEV1.0/FVC% < 80%. Two senior radiologists blinded to PFT results reviewed CT images from PACS with consensus to identify and document hypostasis effect, defined as crescent-shaped ground glass in the dorsal subpleural lung parenchyma in supine and disappeared in prone position, shown in CT images. The number and frequency of cases with hypostasis effect on CT between the two groups were compared statistically with Chi square test. Result Thirty-three cases were in group A with FEV1/FVC of 92.0%±7.5% (80.4% to 105.2%), and 24 cases of them (66.7%) showed hypostasis effect on CT scans; 15 cases were in group B with FEV1.0/FVC% of 63.4%±10.0% (46.6% to 78.0%), and only 2 cases (13.3%) demonstrated hypostasis effect (χ2=14.653, P<0.01). Conclusions The rate of HELC appearance decreases with increasing of small airway resistance. Lack of hypostasis effect in lung CT imaging with smooth breathing during CT scan may reflect increased intra pulmonary pressure caused by airflow limitation, and may be used as a new indicator of COPD. Key words: Airway resistance; Pulmonary ventilation; Tomography, X-ray computed