Cause of regional ventilation-perfusion mismatching in patients with idiopathic pulmonary fibrosis: a combined CT and scintigraphic study.

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.

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.

CONCLUSIONS

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.