A 72-year-old smoker with interstitial lung disease.

The patient is a 72-year-old white female ex-smoker (40-50 pack-years) with a medical history notable for coronary artery disease, status post coronary artery bypass grafting in 1988. She was referred for evaluation of progressive dyspnea on exertion over a two year period. She would have to stop after walking approximately half a block, more from leg claudication than dyspnea. She was able to climb one flight of stairs. There were no significant complaints of cough, fevers, night sweats, or weight loss. She did not keep pets. On examination she was in no apparent distress. Her oxygen saturation on room air was 98%. Her lung examination revealed crackles over the lower one third of the lung fields bilaterally. Early clubbing was noted. A chest roentgenogram showed bilateral increased interstitial markings with lower lobe predominance. There were bullous changes in the right upper lobe. A recent dobutamine echocardiogram showed no ischemia and good left ventricular systolic function. Table 1 shows the pulmonary function test results. What is your diagnosis?