Overview
This lecture covers Pulmonary Function Tests (PFTs), focusing on how to differentiate between obstructive and restrictive lung diseases using key measurements and algorithms.
Pulmonary Function Tests (PFTs) Basics
- PFTs are a group of tests used to assess lung function and differentiate between obstructive and restrictive lung diseases.
- The main tests include spirometry (measuring airflow), lung volume measurements, and diffusion capacity.
- Flow volume loops and various lung volumes are essential in interpreting results.
Lung Volumes in Obstructive vs. Restrictive Disease
- Obstructive diseases have increased residual volume (RV) and functional residual capacity (FRC) due to air trapping and hyperinflation.
- Restrictive diseases show decreased RV and FRC because lungs are stiff and difficult to expand.
- Total lung capacity (TLC) is increased in obstructive disease and decreased in restrictive disease.
Flow Volume Loops
- Normal: Baseline for comparison.
- Obstructive: Loop shifts left, TLC and RV increase, Forced Vital Capacity (FVC) decreases.
- Restrictive: Loop shifts right, TLC and RV decrease, FVC is significantly reduced.
Spirometry: FEV1, FVC, and FEV1/FVC Ratio
- FEV1 (forced expiratory volume in 1 second) is significantly reduced in obstruction; mildly reduced in restriction.
- FVC is most reduced in restriction.
- FEV1/FVC ratio <70% indicates obstruction; ≥70% suggests restriction or normal.
Bronchodilator and Bronchoconstrictor Testing
- Administer a beta-2 agonist to distinguish asthma from COPD.
- FEV1 increase >12% after bronchodilator suggests reversible (asthmatic) airway disease.
- FEV1 drop ≥20% after bronchoconstrictor (methacholine) indicates inducible airway (asthma).
- Lack of reversibility suggests COPD.
Diffusion Capacity (DLCO)
- DLCO evaluates how well gases diffuse across the alveolar membrane.
- Low DLCO in obstructive diseases suggests emphysema (due to alveolar destruction).
- In restrictive diseases, low DLCO implies intrinsic disease (interstitial fibrosis), normal DLCO suggests extrinsic causes (e.g., chest wall issues).
Diagnostic Algorithm Summary
- FEV1/FVC <70% + high lung volumes = obstructive; check for reversibility and DLCO for asthma vs. COPD.
- FEV1/FVC ≥70% + low lung volumes = restrictive; DLCO distinguishes intrinsic (low DLCO) from extrinsic (normal DLCO) causes.
- Use flow volume loop shifts (left = obstructive, right = restrictive) as visual support.
Key Terms & Definitions
- PFT (Pulmonary Function Test) — group of tests evaluating lung function.
- RV (Residual Volume) — air left after maximal exhalation.
- FRC (Functional Residual Capacity) — RV + Expiratory Reserve Volume (ERV).
- TLC (Total Lung Capacity) — total air in lungs after maximal inhalation.
- FEV1 — forced expiratory volume in the first second.
- FVC — forced vital capacity, total air exhaled forcefully after full inhalation.
- FEV1/FVC Ratio — differentiates between obstructive and restrictive patterns.
- DLCO — diffusion capacity for carbon monoxide, measures gas exchange efficiency.
Action Items / Next Steps
- Review flow volume loops for obstructive and restrictive patterns.
- Practice interpreting spirometry values (FEV1, FVC, FEV1/FVC).
- Understand diagnostic steps for distinguishing asthma, COPD, and types of restrictive lung disease using PFTs.