HOW A SPIROMETRY TEST CHECKS YOUR LUNG HEALTH

Lung Volumes and Capacities

Lung volumes refer to the specific amounts of air in the lungs during different phases of the respiratory cycle. These are measured directly or indirectly using spirometry or other pulmonary function tests. Lung capacities are combinations of two or more lung volumes that provide a broader understanding of lung function.

Lung Volumes

1. Tidal Volume (TV):
   • The amount of air inhaled or exhaled during a normal breath.
   • Normal value: Approximately 300-500 mL in adults.

2. Inspiratory Reserve Volume (IRV):
   • The additional volume of air that can be forcibly inhaled after a normal tidal inspiration.
   • Normal value: 1900-3300 mL.

3. Expiratory Reserve Volume (ERV):
   • The additional volume of air that can be forcibly exhaled after a normal tidal expiration.
   • Normal value: 700-1200 mL.

4. Residual Volume (RV):
   • The volume of air remaining in the lungs after maximal exhalation, which cannot be expelled.
   • Normal value: Approximately 1200 mL.

Lung Capacities

1. Inspiratory Capacity (IC):
   • The maximum volume of air that can be inhaled following a normal exhalation.
   • Formula: IC = TV + IRV.
   • Normal value: About 2400-3800 mL.

2. Functional Residual Capacity (FRC):
   • The volume of air remaining in the lungs at the end of a normal expiration.
   • Formula: FRC = ERV + RV.
   • Normal value: About 1800-2200 mL.

3. Vital Capacity (VC):
   • The total amount of air that can be exhaled after a maximal inhalation.
   • Formula: VC = TV + IRV + ERV.
   • Normal value: About 4800 mL.

4. Total Lung Capacity (TLC):
   • The maximum volume of air the lungs can hold after maximal inspiration.
   • Formula: TLC = TV + IRV + ERV + RV.
   • Normal value: About 6000 mL.

These measurements are typically displayed on a spirogram, where they can be visually analyzed for abnormalities.

Forced Expiratory Volume and Maximum Breathing Capacity Test

Forced Expiratory Volume (FEV)

The Forced Expiratory Volume test measures how much air a person can forcefully exhale within specific time intervals during a forced vital capacity maneuver.

• Procedure:
  1. The patient inhales maximally to reach Total Lung Capacity (TLC).
  2. They then exhale as forcefully and completely as possible into the spirometer until no more air can be expelled.
• Key Measurements:
  1. FEV₁: Air expelled in the first second.
     • A healthy individual should expel at least 70%-80% of their Forced Vital Capacity (FVC) within this time frame.
     • Reduced FEV₁ is indicative of airway obstruction.
  2. FEV₁/FVC Ratio: Expresses FEV₁ as a percentage of FVC.
     • Normal ratio: ≥70%.
     • A reduced ratio suggests obstructive lung disease.

Maximum Breathing Capacity

Also referred to as Maximal Voluntary Ventilation (MVV), this test evaluates the maximum amount of air an individual can inhale and exhale over one minute.

• Procedure:
  1. The patient breathes rapidly and deeply for about 12 seconds while connected to a spirometer.
• Key Measurement: MVV is extrapolated to represent one minute by multiplying the result by five.

Pulmonary Function Tests for Diagnosing Restrictive and Obstructive Disorders

Pulmonary function tests, including spirometry, are critical tools for distinguishing between restrictive and obstructive lung diseases based on characteristic patterns in lung volumes, capacities, and flow rates.

Obstructive Disorders

Obstructive disorders involve difficulty with airflow due to narrowed or blocked airways, often caused by conditions like asthma, chronic obstructive pulmonary disease (COPD), or bronchitis.

• Characteristic Findings:
  1. Decreased FEV₁ (<80% predicted).
  2. Reduced FEV₁/FVC ratio (<70%).
• Mechanism: Airway obstruction limits expiratory flow rates, leading to prolonged expiration times and increased residual volume due to trapped air.

Restrictive Disorders

Restrictive disorders involve reduced lung expansion due to stiffened lung tissue or abnormalities in chest wall mechanics, such as pulmonary fibrosis or scoliosis.

• Characteristic Findings:
  1. Decreased TLC (<80% predicted). This reduction confirms restriction when measured via body plethysmography or gas dilution techniques since spirometry alone cannot measure TLC directly.
• Mechanism: Restricted lung expansion reduces all lung volumes proportionally but preserves the FEV₁/FVC ratio (>70%).

By analyzing these parameters together with clinical findings, physicians can accurately diagnose and manage respiratory diseases effectively.