Pulmonary Function Tests (PFTs) are simple, non-invasive tests that measure how well the lungs are working. They help assess how much air the lungs can hold, how quickly air moves in and out, and how well oxygen passes from the lungs into the blood. These tests are essential for diagnosing lung diseases and monitoring treatment progress.
Types of Breathing Problems Identified
- Obstructive disorders: Air has trouble flowing out of the lungs due to narrowed airways. Examples include asthma and COPD.
- Restrictive disorders: The lungs cannot fully expand, leading to reduced lung volumes. Seen in fibrosis or chest wall abnormalities.
Why PFTs Are Done
- To check for lung disease in patients with cough, breathlessness, wheezing, cyanosis, etc.
- To monitor progression of known lung disorders
- To check response to medicines or treatment
- To evaluate lung function before surgery
- To monitor drug toxicity (e.g., amiodarone)
Main Types of Pulmonary Function Tests
- Spirometry
- Body plethysmography & lung volume measurement
- Diffusion capacity tests
- Airway reactivity tests
- Six-minute walk test
1) Spirometry
Spirometry is the most common PFT. It measures how much air a person can inhale and exhale, and how fast they can exhale. It helps diagnose diseases like asthma, COPD, and other airway disorders.
Indications
- Symptoms like cough, wheeze, or breathlessness
- Monitoring known conditions such as COPD, asthma, fibrosis
- Detecting respiratory complications in systemic diseases
- Pre-operative evaluation
- Assessing exposure to toxins or occupational hazards
Calibration of Spirometer
- Device must be calibrated or checked at the start of each session
- Calibration methods vary based on device manufacturer
- Weekly accuracy checks using a healthy volunteer are recommended
Procedure
The patient inhales deeply, then blows out as hard and fast as possible until no air is left. Encouragement is important to ensure maximum effort.
Key Spirometry Measurements
1. Vital Capacity (VC)
- FVC: Forced Vital Capacity — total air exhaled forcefully after deep inhalation.
- SVC: Slow Vital Capacity — total air exhaled slowly after deep inhalation.
2. Forced Expiratory Volume (FEV)
Measures how much air is exhaled during the first 0.5, 1, 3, or 6 seconds. FEV₁ is most clinically important.
FEV₁/FVC ratio helps identify obstruction:
- <70% = Obstruction
- FEV₁ >80% = Mild
- 50–80% = Moderate
- 30–50% = Severe
- <30% = Very severe
3. Forced Expiratory Flow (FEF)
Measures airflow in small airways. Useful in detecting early or mild airway obstruction.
Peak Expiratory Flow Rate (PEFR)
- Maximum speed of exhalation
- Measured using peak flow meters
- Used in asthma monitoring
2) Body Plethysmography & Lung Volumes
This test measures different lung volumes using a sealed chamber (“a glass box”). It helps determine how much air remains in the lungs after normal or forced breathing.
Measures include:
- Tidal Volume (TV)
- Inspiratory Reserve Volume (IRV)
- Expiratory Reserve Volume (ERV)
- Residual Volume (RV)
- Inspiratory Capacity (IC)
- Functional Residual Capacity (FRC)
- Total Lung Capacity (TLC)
- Vital Capacity (VC)
3) Diffusion Capacity Tests
These tests check how well gases (mainly carbon monoxide, CO) pass from air sacs into the blood. They are useful for diagnosing interstitial lung diseases.
Single Breath Test
- Patient inhales a mix containing a small amount of CO and holds breath for 10 seconds
- Exhaled gas is tested to measure CO uptake
- Normal DLCO = 25–30 ml/min/mmHg
Steady State Test
- Patient breathes CO continuously for several minutes
- Expired gas is analyzed
- Lower values suggest reduced gas transfer
4) Airway Reactivity Tests
A) Bronchodilator Study
- Spirometry done before and after a bronchodilator
- Positive response: FEV₁ ↑ by 12% or FVC ↑ by 200 ml
B) Bronchoprovocation Test
- Airways exposed to agents like methacholine, histamine, adenosine or allergens
- Used when asthma diagnosis is unclear
- Helps measure airway hyperreactivity
C) Exercise Challenge Test
- Used to detect exercise-induced bronchospasm
- Patient exercises for 6–8 minutes
- FEV₁ measured repeatedly after exercise
- 10–15% drop = abnormal
5) Six-Minute Walk Test
Simple test measuring how far a patient can walk in six minutes. It is useful in assessing patients with COPD, CHF and pulmonary hypertension.
- Normal adults walk 500–630 meters
- Children walk ~470 meters
- Test stops if chest pain, dizziness or very low oxygen saturation occurs
- Oxygen saturation ≤ 88% suggests need for supplemental oxygen
Detailed Notes:
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