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. SUB JECTS AND METHODS. Eight patients with idiopathic pulmonary fibrosis w ere examined with inspiratory and expiratory CT scans. Planar and tomo graphic (single-photon emission computed tomography, SPECT) scintigrap hy, using inhalation of krypton-81m gas (ventilation) and IV injection of Tc-99m-albumin macroaggregates (perfusion), also was performed. Th e 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 expir atory CT scans, the cross-sectional area of the cystic spaces diminish ed significantly (unlike emphysematous spaces). Sixty-seven percent of lung quadrants, corresponding to those with marked or moderate involv ement with cystic spaces, showed a mismatched V/Q pattern on scintigra ms (absent perfusion, normal ventilation); 27% of quadrants had matche d V/Q defects, and 6% did not show defects. Two patients had, in addit ion, large cystic spaces typical of emphysema, but the coexistent fibr osis prevented the gross air trapping seen in bullous emphysema. CONCL USION. The cystic air spaces that are often seen on CT scans of patien ts with idiopathic pulmonary fibrosis are unperfused (probably due to vascular obliteration) but are usually normally ventilated. This V/Q m ismatch 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 mis taken for emphysema unless V/Q scintigraphy is done.