Clearance rates of soluble radioaerosols of sodium pertechnetate (99mTc04; mol wt 163) and diethylenetriaminepenta-acetate (99mTc-DTPA; mol wt 492) were determined in seven normal subjects and ten patients with systemic sclerosis affecting the lungs. Twenty millicuries (mCi) each of 99mTc04 and 99mTc-DTPA in 5 ml saline were aerosolised and inhaled using a disposable "Blount" nebuliser on two different days. Two regions of interest over each posterior lung field were monitored with a scintillation camera, and data were stored on magnetic tape using a Hewlett Packard Data Analyser. Decreasing levels of radioactivity were plotted semilogarithmically and half-time (TI) removal rates were calculated. The Ti values in normal subjects did not differ significantly from Ti values of the patients with TcO4. However, the removal rates of the higher molecular weight solute were significantly faster from lower lung zones in patients with systemic sclerosis than in the normal subjects. The faster absorption of DTPA from lower lung zones of the patients could be due to regional abnormalities of alveolar epithelium at the lung bases, presumably as a result of greater retractive forces secondary to fibrosis.

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