DLCO of 36% of predicted for 63 yr old male. Dyspnea and oxygen desaturation under exertion. Lung biopsy, CT scans and echocardiograms show no restrictive or obstructive disease. Blood tests show no anemia. No evidence of asthma. No evidence of embolisms. Shunts have been ruled out. Pulmonologists and other disease specialists baffled. Any ideas?