Restrictive Spirometry, Low DLCO, and Bird Exposure
A 54-year-old never-smoker presents with 8 months of progressive exertional dyspnea and dry cough. Spirometry before bronchodilator: FEV1 1.50 L (50% predicted), FVC 1.90 L (55% predicted), FEV1/FVC 0.79. After 400 mcg albuterol, FEV1 is 1.55 L and FVC 1.95 L. Lung volumes by plethysmography: TLC 4.0 L (66% predicted), RV 1.4 L (70% predicted). DLCO is 52% predicted, corrected for hemoglobin. Room-air arterial blood gas: pH 7.42, PaCO2 35 mm Hg, PaO2 58 mm Hg, HCO3 22 mEq/L. Chest radiograph shows bilateral reticular opacities and reduced lung volumes. She has no occupational exposure history, but keeps two birds indoors. The clinician asks whether this is asthma, COPD, or interstitial lung disease and how to assess oxygen needs.
1. Interpret the spirometry, bronchodilator response, lung volumes, and DLCO. Is the pattern obstructive, restrictive, or mixed?
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The FEV1/FVC ratio of 0.79 is preserved, which argues against obstruction. Both FVC and TLC are reduced, so this is a restrictive ventilatory defect. The post-bronchodilator change is FEV1 +50 mL (about 3.3%) and FVC +50 mL; this does not meet the usual significant bronchodilator response criteria of at least 12% and 200 mL improvement. The DLCO is low at 52% predicted, indicating impaired alveolar-capillary diffusion. Taken together, the findings fit a restrictive lung disease with reduced diffusion rather than asthma or COPD. Asthma would typically show reversible airflow obstruction with a reduced FEV1/FVC ratio, and COPD would show persistent obstruction, often with hyperinflation rather than reduced TLC. The low TLC and low DLCO are important clues to interstitial or alveolar disease.
2. What is the most likely diagnosis given her exposure history, and what diagnostic step would you recommend next?
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The combination of restrictive physiology, reduced DLCO, reticular opacities, and indoor bird exposure suggests hypersensitivity pneumonitis, specifically bird fancier's lung. The next diagnostic step is a high-resolution CT scan of the chest, ideally with inspiratory and expiratory views, to look for ground-glass opacity, centrilobular nodules, air trapping, and fibrosis. Serum IgG precipitins to bird antigens can support the exposure but are not diagnostic because exposed asymptomatic people may also be positive. Pulmonary function alone cannot confirm the cause. A multidisciplinary assessment, antigen avoidance, and sometimes bronchoalveolar lavage or surgical lung biopsy may be needed if the diagnosis remains uncertain. This should not be labeled as COPD or asthma simply because she is dyspneic; the physiology and exposure point elsewhere.
3. How would you assess and treat her hypoxemia? Include targets and a test.
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Her room-air PaO2 of 58 mm Hg indicates resting hypoxemia. Prescribe supplemental oxygen to maintain SpO2 approximately 88–92% at rest, and reassess with a 6-minute walk test using continuous pulse oximetry. If SpO2 falls below 88% during exertion, or if she desaturates by at least 4% to below 88%, ambulatory oxygen is reasonable. The 6-minute walk also provides information about exertional desaturation, distance, and oxygen titration that a resting ABG cannot. Recheck arterial blood gases if there is worsening dyspnea or suspected CO2 retention. Address the underlying hypersensitivity pneumonitis with antigen avoidance and specialist management, because oxygen alone does not treat the parenchymal disease.