What is the role of pulmonary function testing in the workup of cystic fibrosis (CF)?

Updated: Sep 28, 2020
  • Author: Girish D Sharma, MD, FCCP, FAAP; Chief Editor: Kenan Haver, MD  more...
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Typically, peripheral airway involvement resulting from CF manifests as an obstructive defect with airtrapping and hyperinflation; oxyhemoglobin desaturation may occur because of a ventilation-perfusion mismatch. In the early stages, forced expiratory volume in 1 second (FEV1) may be normal, and forced expiratory flow (FEF) after 25-75% of vital capacity has been expelled (FEF 25-75) is reduced, suggesting small airway involvement.

Progression of disease has been correlated with a change in FEV1. A 2012 Danish study, using a longitudinal modeling technique specifically aimed at analyzing long sequences of repeated measurements of FEV1 measurements in CF patients reported that on average a change in FEV1 of greater than 13% (ie, twice the error SD to give a 95% confidence range) is likely to represent a true within-patient variation over time, whereas a lesser change may be due to transient (recoverable) fluctuation. [48]

The associated air trapping results in an elevated ratio of residual volume to total lung capacity (RV/TLC). With hyperinflation, TLC is also increased. In patients with advanced disease, extensive lung changes with fibrosis are reflected as restrictive changes characterized by declining TLC and vital capacity.

Lung clearance index (LCI) calculated from multiple breath inert gas (sulfur hexafluoride-SF6/helium gas mixture) washout has been used to demonstrate ventilation inhomogeneity, an early marker of lung disease in young children with CF. [49, 50, 51] LCI is a sensitive early marker of CF in young children, comparable with high-resolution CT scanning (HRCT), and is gaining wider acceptance by clinicians and researchers. [52]

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