Objective: Regional ventilation and perfusion were studied in patients with idiopathic pulmonary fibrosis (cryptogenic fibrosing alveolitis) to seek an explanation for the mismatched ventilation/perfusion (V/Q) seen on scintigrams, which may suggest pulmonary embolic disease.
Subjects and methods: Eight patients with idiopathic pulmonary fibrosis were examined with inspiratory and expiratory CT scans. Planar and tomographic (single-photon emission computed tomography, SPECT) scintigraphy, using inhalation of krypton-81m gas (ventilation) and IV injection of 99mTc-albumin macroaggregates (perfusion), also was performed. The lungs were divided into quadrants (cranial, caudal, right, left) for analysis.
Results: Cystic air spaces with a "honeycomb" appearance occupied more than 33% of the cross-sectional area in 75% of all lung quadrants (n = 16), more than 66% of the area in 44% of quadrants, and less than 33% of the area in the remaining 25% of quadrants. On expiratory CT scans, the cross-sectional area of the cystic spaces diminished significantly (unlike emphysematous spaces). Sixty-seven percent of lung quadrants, corresponding to those with marked or moderate involvement with cystic spaces, showed a mismatched V/Q pattern on scintigrams (absent perfusion, normal ventilation); 27% of quadrants had matched V/Q defects, and 6% did not show defects. Two patients had, in addition, large cystic spaces typical of emphysema, but the coexistent fibrosis prevented the gross air trapping seen in bullous emphysema.
Conclusion: The cystic air spaces that are often seen on CT scans of patients with idiopathic pulmonary fibrosis are unperfused (probably due to vascular obliteration) but are usually normally ventilated. This V/Q mismatch on scintigrams explains the large physiologic dead space seen at rest and on exercise and could suggest pulmonary embolism unless a CT scan is obtained. Conversely, the larger cystic spaces might be mistaken for emphysema unless V/Q scintigraphy is done.