PFT
Pulmonary Function Tests (PFTs) and Clinical Utility
I. Introduction
Pulmonary Function Tests are a group of non-invasive diagnostic tests that assess the functional status of the respiratory system by measuring airflow, lung volumes, and gas exchange.
II. Components of PFTs
1. Spirometry (The Gold Standard)
Measures the volume of air an individual can inhale or exhale as a function of time.
- Key Parameters:
- FVC (Forced Vital Capacity): Total volume of air exhaled forcibly after a full inspiration.
- FEV1 (Forced Expiratory Volume in 1 second): Volume exhaled in the first second.
- FEV1/FVC Ratio: The fraction of air exhaled in the first second (Normal > 80-85% in children).
- FEF 25β75%: Forced expiratory flow at mid-expiration (marker of small airway disease).
2. Flow-Volume Loops
A graphical representation of airflow (y-axis) vs. volume (x-axis).
- Normal: Rapid peak flow followed by a linear decline.
- Obstructive: "Scooped out" or concave expiratory limb.
- Restrictive: Tall, narrow ("Witchβs hat") shape.
- Fixed Upper Airway Obstruction: Flattening of both inspiratory and expiratory loops (Box pattern).
3. Static Lung Volumes (Body Plethysmography)
Measures volumes that cannot be measured by spirometry alone.
- TLC (Total Lung Capacity): Volume in lungs at maximal inflation.
- RV (Residual Volume): Volume remaining after maximal exhalation.
- FRC (Functional Residual Capacity): Volume remaining after normal tidal exhalation.
4. Diffusion Capacity (DLCO)
- Measures the ability of the lungs to transfer gas (Carbon Monoxide) from alveoli to red blood cells.
- Decreased in: Interstitial Lung Disease (fibrosis), Emphysema, Anemia.
- Increased in: Pulmonary hemorrhage, Polycythemia, Asthma (mild).
5. Pediatric Specific: Impulse Oscillometry (IOS)
- Used in children < 6 years who cannot perform forced maneuvers.
- Uses sound waves to measure respiratory resistance and reactance during normal tidal breathing.
III. Interpretation of Patterns
| Parameter | Obstructive Pattern (e.g., Asthma, CF) | Restrictive Pattern (e.g., Fibrosis, Scoliosis) |
|---|---|---|
| FEV1 | Decreased (<80%) | Decreased or Normal |
| FVC | Normal (or decreased in severe air trapping) | Decreased (<80%) |
| FEV1/FVC Ratio | Decreased (<0.8) | Normal or Increased |
| TLC | Normal or Increased (Hyperinflation) | Decreased |
| RV | Increased (Air trapping) | Decreased |
 |
||
| Bronchodilator Reversibility Test: |
- Significant reversibility (Asthma) is defined as an increase in FEV1 by β₯12% AND β₯200 ml after salbutamol inhalation.
IV. Clinical Utility
1. Diagnostic Utility
- Diagnosis of Disease: Confirms airflow limitation in Asthma and COPD.
- Differentiating Etiology: Distinguishes between Obstructive (Asthma/CF) and Restrictive (ILD/Chest wall deformity) causes of dyspnea.
- Upper Airway Obstruction: Flow-volume loops can detect tracheal stenosis or vocal cord dysfunction (variable extrathoracic obstruction).
2. Monitoring and Prognostic Utility
- Disease Progression: Monitoring decline in lung function in Cystic Fibrosis (FEV1 is a key predictor of mortality) and Duchenne Muscular Dystrophy.
- Response to Treatment: assessing efficacy of steroids or bronchodilators in asthma.
- Drug Toxicity: Monitoring for drug-induced lung injury (e.g., Bleomycin, Methotrexate).
3. Pre-operative Assessment
- Assesses risk for postoperative pulmonary complications, especially in thoracic or upper abdominal surgeries (e.g., Scoliosis correction).
4. Disability Evaluation
- Quantifies impairment for medical-legal or insurance purposes.