Cystic fibrosis (CF) is an autosomal recessive disorder caused by a mutation (eg, ∆F508) in the gene encoding the CF transmembrane conductance regulator.  A defect in this chloride channel results in the build-up of thick, dehydrated mucus, which causes progressive damage to various organ systems, such as the respiratory and gastrointestinal tracts.

The lungs are the predominant organ affected in CF due to the accumulation of viscous secretions in the airways.  Bacteria colonize the airways and cause recurrent infection and chronic inflammation that eventually leads to scarring of the bronchial walls (ie, bronchiectasis).  Progressive bronchiectasis (ie, weakened, dilated bronchioles that collapse easily) and mucus-plugging result in obstructive lung disease.

An obstructive pattern on spirometry is characterized by decreased forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC).  Because the decrease in FEV1 is more profound in obstructive disease, FEV1/FVC ratio is also reduced.  The airway obstruction prevents full expiration (ie, causes air trapping), leading to increased residual volume (RV) and hyperinflated lungs with increased total lung capacity (TLC).  This progressive increase is clinically apparent by a barrel-shaped chest on examination and lung hyperexpansion on x-ray.

(Choices A and C)  As RV increases or decreases, TLC follows concordantly; therefore, these results are unlikely to represent a true clinical situation.

(Choice B)  Restrictive lung disease (eg, pulmonary fibrosis) is characterized by a normal-to-increased FEV1/FVC ratio because FVC decreases to a greater extent than FEV1.  RV and TLC are both decreased.

(Choice E)  Normal RV with increased TLC can be seen in highly trained athletes due to increased FVC.

Educational objective:
Pulmonary function testing in cystic fibrosis reveals an obstructive pattern, characterized by a decreased FEV1/FVC ratio and an increased total lung capacity and residual volume.