Tidal Volume (TV): The volume of air moved into or out of the lungs during a single normal, resting breath. It represents the baseline ventilatory effort.
Inspiratory Reserve Volume (IRV): The additional volume of air that can be forcibly inhaled after a normal tidal inspiration. It measures the lung's ability to expand further.
Expiratory Reserve Volume (ERV): The extra volume of air that can be forcibly exhaled after a normal tidal expiration. This indicates the functional reserve of the lungs.
Residual Volume (RV): The volume of air remaining in the lungs after a maximal forced expiration. Crucially, RV cannot be measured directly by a spirometer because this air never leaves the lungs.
Lung capacities are calculated by summing specific lung volumes to provide a broader picture of respiratory health. The most clinically significant is the Vital Capacity (VC).
Vital Capacity (VC) is the maximum volume of air that can be exhaled after a maximum inhalation. It is calculated as .
Total Lung Capacity (TLC) is the sum of all lung volumes, including the residual volume (). Because RV cannot be measured by spirometry, TLC must be estimated using other techniques like helium dilution.
Spirometry is the primary tool for distinguishing between different types of lung disease based on flow and volume characteristics.
| Feature | Obstructive (e.g., Asthma, COPD) | Restrictive (e.g., Fibrosis) |
|---|---|---|
| Primary Issue | Difficulty exhaling air quickly | Difficulty fully expanding lungs |
| Vital Capacity | Often normal or slightly reduced | Significantly reduced |
| Flow Rate (FEV1) | Significantly reduced | Reduced proportionally to volume |
| FEV1/FVC Ratio | Decreased (typically < 0.7) | Normal or increased |
In obstructive diseases, the airways are narrowed, making it hard to push air out fast. In restrictive diseases, the lung tissue is stiff or the chest wall is limited, reducing the total amount of air the lungs can hold.
The Residual Volume Trap: Always remember that a spirometer cannot measure Residual Volume (RV) or any capacity that includes it (like FRC or TLC). If an exam asks how to measure RV using only a spirometer, the answer is that it is impossible.
Calculating Breathing Rate: To find the breathing rate from a trace, count the number of peaks (breaths) over a specific time interval (e.g., 60 seconds). Ensure you check the units on the x-axis (seconds vs. minutes).
Oxygen Uptake Calculation: On a closed-circuit trace, draw a line connecting the bottom of the tidal troughs. The gradient (slope) of this line represents the rate of oxygen consumption in per unit of time.
Verification: When calculating Vital Capacity, ensure the sum of matches the total vertical distance between the maximum peak and the minimum trough on the graph.
