Lung Volumes, Capacities, and Pulmonary Function Measurements

Why Lung Volumes and Capacities Matter

Spirometry is one of the most common tests a respiratory therapist performs and interprets. Understanding what the numbers mean—and when they fall outside normal—is a core clinical skill tested on the NBRC credentialing exams and required for state licensure. A single spirometry session gives you objective data about how much air a patient can move and how fast they can move it, allowing you to detect obstructive diseases (such as asthma or COPD) and restrictive diseases (such as pulmonary fibrosis or neuromuscular weakness).

Key Lung Volumes

Lung volumes are the building blocks of pulmonary function measurement.

• Tidal Volume (TV or VT): The volume of air moved in or out during a normal, relaxed breath. In a healthy adult at rest, this is roughly 500 mL, though individual values vary.
• Inspiratory Reserve Volume (IRV): The extra air you can inhale above a normal tidal breath.
• Expiratory Reserve Volume (ERV): The extra air you can push out beyond a normal tidal exhalation.
• Residual Volume (RV): Air that stays in the lungs after a maximal exhalation. You cannot measure RV with a simple spirometer; it requires gas dilution or body plethysmography.

Key Lung Capacities

A capacity is the sum of two or more volumes.

• Vital Capacity (VC): IRV + TV + ERV. The total volume you can exhale after a maximal inhalation. It reflects the usable range of lung movement.
• Inspiratory Capacity (IC): TV + IRV.
• Functional Residual Capacity (FRC): ERV + RV. The volume left in the lungs at the end of a normal exhalation. Like RV, FRC cannot be measured by spirometry alone.
• Total Lung Capacity (TLC): All four volumes combined. Also requires techniques beyond basic spirometry.

Forced Expiratory Measurements

Forced maneuvers reveal airway resistance and dynamic airflow, which resting volumes do not show.

• Forced Vital Capacity (FVC): The total volume exhaled as fast and completely as possible after a maximal inhalation.
• FEV₁ (Forced Expiratory Volume in 1 second): The volume exhaled in the first second of an FVC maneuver. This is the single most clinically important spirometry value.
• FEV₁/FVC ratio: Expresses FEV₁ as a fraction of FVC. A low ratio (below the lower limit of normal, roughly below 0.70 in many adults) points toward an obstructive pattern. A normal or elevated ratio with reduced absolute volumes points toward a restrictive pattern.
• FEF 25–75% (Forced Expiratory Flow, mid-expiratory): Average flow over the middle half of the FVC maneuver; sensitive to small-airway disease.

Obstructive vs. Restrictive Patterns

Obstructive pattern: Airways are narrowed, slowing expiratory flow. FEV₁ is reduced more than FVC, so the FEV₁/FVC ratio falls below the lower limit of normal. Examples: asthma, COPD, bronchiectasis.

Restrictive pattern: The lungs or chest wall cannot expand fully, reducing all volumes. FVC and TLC are reduced, but FEV₁/FVC ratio is normal or elevated because both volumes shrink proportionally. Examples: pulmonary fibrosis, obesity, neuromuscular disease, pleural effusion.

Percent-Predicted Values

Absolute numbers mean little without context. A vital capacity of 3.2 L may be normal for a small woman but severely reduced for a tall man. Reference equations (such as those from the Global Lung Function Initiative or published ATS/ERS guidelines) use age, sex, height, and race/ethnicity to calculate the predicted value for that individual. You then express the measured value as a percentage of predicted:

Percent predicted = (Measured value ÷ Predicted value) × 100

Values at or above 80% of predicted are generally considered within the normal range for FVC and FEV₁, though the lower limit of normal from reference equations is the preferred threshold.

Performing an Acceptable Spirometry Maneuver

For results to be valid, technique must meet acceptability and repeatability criteria.

1. Explain the test fully before starting.

2. Have the patient sit upright or stand (document position).

3. Apply the nose clip and seal lips around the disposable mouthpiece.

4. Instruct a maximal inhalation, then a blast exhalation—fast, hard, and sustained for at least 6 seconds (or until a plateau is reached).

5. Obtain at least three acceptable maneuvers; the two largest FVC values and two largest FEV₁ values should be within 150 mL of each other (repeatability criterion per ATS/ERS).

Safety and Infection Control

Patient safety and infection control are non-negotiable.

• Use a new disposable mouthpiece and nose clip for every patient. Never reuse between patients.
• In-line bacterial/viral filters should be used per your facility policy.
• Spirometry is contraindicated or must be used cautiously in patients with recent myocardial infarction, unstable angina, recent eye or abdominal surgery, aortic aneurysm, or severe hypertension—always verify the order and patient status before proceeding.
• Forced maneuvers significantly increase intrathoracic and intra-abdominal pressure; coach patients but stop immediately if they appear distressed.
• Calibrate the spirometer with a 3-liter calibration syringe before each session. Document calibration results.
• Wash hands before and after the procedure.