TLDR;
This video by Ninja Nerd provides a comprehensive overview of Pulmonary Function Tests (PFTs) and their role in diagnosing obstructive versus restrictive lung diseases. It explains how lung volumes are measured using plethysmography, details the process of forced spirometry, and discusses bronchodilator and bronchoconstriction tests. The video also covers DLCO testing and presents a diagnostic approach to interpreting PFT results.
- PFTs help differentiate between obstructive and restrictive lung diseases.
- Key measurements include lung volumes, forced spirometry, and DLCO.
- Bronchodilator tests help determine the reversibility of airway obstruction.
Pulmonary Function Tests (PFTs) Introduction [0:07]
Pulmonary Function Tests (PFTs) are a collection of tests used to determine whether a patient has an obstructive or restrictive lung disease. PFTs can further classify the specific type of lung disease, such as asthma, COPD, or intrinsic/extrinsic restrictive lung diseases. The video will present an algorithm to synthesize this information.
Lung Volumes (Plethysmography) [0:43]
Lung volumes are typically obtained via plethysmography. In obstructive lung diseases, lungs are hyperinflated and highly compliant, leading to dynamic hyperinflation and air trapping. This results in high residual volumes and expiratory reserve volumes, which combine to create a high functional residual capacity (FRC). Patients with obstructive lung disease have a reduced inspiratory volume due to the large amount of air already in their lungs. In restrictive lung diseases, lungs are fibrotic and stiff, with low compliance and increased elasticity. This leads to low reserve volumes and expiratory reserve volumes, resulting in a reduced functional residual capacity (FRC). Patients with restrictive lung disease also have difficulty taking deep breaths, leading to reduced tidal volumes and inspiratory volumes. Total lung capacity (TLC), which is the sum of inspiratory reserve volume, tidal volume, expiratory reserve volume, and residual volume, is large in obstructive diseases and small in restrictive diseases.
Forced Spirometry [7:54]
Forced spirometry involves measuring how much air a person can forcefully exhale after taking a deep breath. The apparatus measures the expiration at one second (FEV1) and throughout the entire expiration (FVC). In obstructive lung disease, the FEV1 is significantly reduced, indicating difficulty in getting air out quickly. In restrictive lung disease, the forced vital capacity (FVC) is significantly low. The FEV1/FVC ratio is used to differentiate between obstructive and restrictive diseases. A ratio less than 70% suggests obstructive lung disease, while a ratio greater than or equal to 70% suggests restrictive lung disease.
Bronchodilator / Bronchoconstriction Tests [12:10]
For patients with obstructive lung disease, bronchodilator tests help determine if the condition is COPD or asthma. A short-acting beta-2 agonist is administered to relax bronchial smooth muscles. If the FEV1 increases by greater than or equal to 12%, it suggests asthma, indicating a reversible airway disease. If the increase is less than 12%, it suggests COPD, an irreversible process. To further confirm asthma, a bronchoconstriction test using methacholine can be performed. If the FEV1 drops by greater than or equal to 20% after methacholine administration, it supports the diagnosis of asthma, indicating an inducible airway.
DLCO Testing [14:49]
DLCO (Diffusion Capacity of the Lungs for Carbon Monoxide) testing involves measuring how much carbon monoxide diffuses across the alveoli into the blood. In obstructive lung diseases like emphysema, the destruction of alveolar septa decreases the surface area available for gas exchange, which reduces the DLCO. In restrictive lung diseases with fibrosis, the thickening of the respiratory membrane also decreases the DLCO. If a patient has obstructive lung disease and the bronchodilator test suggests COPD, a low DLCO indicates emphysema, while a normal DLCO suggests chronic bronchitis. For restrictive lung diseases, a low DLCO indicates intrinsic lung diseases (interstitial lung disease), while a normal DLCO suggests extrinsic lung diseases.
Diagnostic Approach [17:35]
The diagnostic approach to PFTs involves first checking the FEV1/FVC ratio. If it is less than 70%, it suggests obstructive lung disease. Check the total lung capacity, residual volume, and functional residual capacity, which should be increased. Administer a bronchodilator test to differentiate between asthma and COPD. A greater than 12% increase in FEV1 suggests asthma, while a less than 12% increase suggests COPD. If the FEV1/FVC ratio is greater than 70%, it could be restrictive or normal. Check the TLC, residual volume, and FRC, which would be low in restrictive lung disease. Then, check the DLCO to differentiate between extrinsic and intrinsic restrictive lung diseases. A low DLCO indicates intrinsic lung disease, while a normal DLCO suggests extrinsic lung disease.
