HISTORY: A 44 y.o. male was referred for evaluation of shortness of breath (SOB) on exertion and cough. Symptoms began in Nov. 2009 when he inhaled particles from a litter box. Shortly after, patient developed fever and cough that progressed into SOB with minimal exertion. Spirometry showed restrictive pattern and flattening of inspiratory flow volume loop. Desaturation noted with moderate exertion. High-resolution CT angiogram showed small nodules but no infiltrates or interstitial lung disease. There was no evidence of myocardial ischemia by stress echo. PHYSICAL EXAMINATION: Well appearing Caucasian male. Ht: 72in; Wt: 206lb; Resting: HR: 81; BP: 110/80; SpO2: 97%; ECG: Normal; No dyspnea. DIFFERENTIAL DIAGNOSIS:Airway obstruction during exercise, Ventilatory limitation due to restriction, Exertional hypoxemia TEST AND RESULTS: PFTs: Spirometry: FVC: 61%pred; FEV1: 62%pred; FEV1/FVC: 83%; MVV: 96%pred; Inspiratory loop: Normal Lung Volumes: TLC: 66%pred; FRC PL: 66%pred; RV: 62%pred; Diffusion Capacity: DLCO: 51%pred; DLCO/VA: 87%pred EXERCISE TEST: Peak: WR: 120W; VO2: 17.5 ml/kg/min (49%pred); HR: 169; BP: 155/69; Qc: 13.1 L/min; VE: 77 L/min; VE/VCO2: 42; SpO2: 82% INTERPRETATION: Cardiovascular Response: Normal BP, HR, Qc & SV response; no ischemia; No ST segment changes; rare PVCs. Pulmonary Response: Ventilatory demand slightly increased; no expiratory flow limitations; limited inspiratory flow reserve at peak exercise; small tidal volume with increased breathing frequency driven by severe hypoxemia and restrictive lung disease. Metabolic Response: Normal O2 extraction and lactate production. FINAL/WORKING DIAGNOSIS: Impaired exercise capacity due to severe exercise induced hypoxemia (diffusion limitation) combined with restrictive lung disease. Possible development of right-to-left shunting during exercise, although unlikely. TREATMENT AND OUTCOMES: Negative right-to-left shunt by cardiac catheterization. Bronchocentric chronic interstitial pneumonitis with lymphocytes found by open lung biopsy. Patient advised to remove cats and litter box from home. He is placed on steroid therapy and antibiotic daily. He is able to increase activity, but continues to have exertional hypoxemia. Patient referred to an occupational pulmonologist for hypersensitivity pneumonitis.