Spirometry: What the Test Measures and How to Understand the Results

Spirometry is a fundamental pulmonary function test that measures the volume and flow of air that can be inhaled and exhaled. It is a central tool in the objective assessment of lung health, helping physicians evaluate conditions such as asthma and COPD.

How the Test is Performed

Spirometry requires active patient participation and maximal effort to yield reliable results.

1. A clip may be placed on the nose to ensure breathing occurs only through the mouth.
2. The patient breathes into a mouthpiece connected to the spirometer.
3. The core maneuver involves taking the deepest breath possible, followed by a forceful, rapid, and complete exhalation until the lungs are as empty as possible.
4. This maneuver is repeated several times to ensure the measurements are consistent and meet quality standards ^[2].

Bronchodilator Testing

Spirometry is frequently performed before and after the administration of an inhaled bronchodilator. This helps assess whether any observed airflow limitation is responsive to medication, which provides important clinical context.

Understanding the Key Measurements

The spirometer records several indices. Physicians interpret these values by comparing them to appropriate validated reference equations based on factors such as the patient's age, height, and sex.

Modern interpretation strategies, such as those from the ERS/ATS ^[1], emphasize comparing patient results against the Lower Limit of Normal (LLN) and using z-scores to quantify how far a result deviates from the expected average, rather than relying solely on fixed percentages.

The primary measurements include:

1. FVC (Forced Vital Capacity)

The total volume of air exhaled forcefully and completely after a maximal inhalation.

• Values below the LLN (a negative z-score beyond the normal range) may indicate a reduction in usable lung volume.
• A reduced FVC can occur in restrictive lung diseases, but it can also be seen in severe obstructive diseases due to air trapping.

2. FEV1 (Forced Expiratory Volume in 1 second)

The volume of air exhaled during the first second of the FVC maneuver.

• FEV1 is influenced by airway caliber, lung volumes, and expiratory effort. While a value below the LLN suggests impaired expiratory flow, a reduced FEV1 is nonspecific and must always be interpreted in conjunction with the FVC and the FEV1/FVC ratio.
• In patients with confirmed airflow obstruction, the FEV1 z-score or percentage of predicted is often used to grade the severity of the obstruction.

3. FEV1/FVC Ratio

The ratio of FEV1 to FVC. This is a critical metric for identifying airflow obstruction.

• A ratio falling below the LLN indicates that a disproportionately small amount of air is exhaled in the first second relative to the total volume, which is the physiological hallmark of an obstructive ventilatory defect.

Interpreting the Physiological Patterns

Based on these measurements, spirometry can suggest specific physiological patterns:

Obstructive Pattern

Identified by an FEV1/FVC ratio below the LLN. This pattern indicates airway narrowing or dynamic collapse during exhalation.

• It is commonly seen in asthma, COPD, and bronchiectasis.
• If spirometry is repeated after a bronchodilator and the FEV1 or FVC improves significantly, this indicates bronchodilator responsiveness. While significant responsiveness is a feature of asthma, its presence or absence on a single test does not definitively confirm or exclude either asthma or COPD.

Suggestion of a Restrictive Pattern

Characterized by an FVC below the LLN in the presence of a normal or elevated FEV1/FVC ratio.

• This pattern suggests a reduction in total lung volume, which may be associated with interstitial lung diseases, chest wall deformities, or neuromuscular weakness.
• Important Note: Spirometry can only suggest a restrictive defect. A definitive physiological diagnosis of restriction requires the measurement of Total Lung Capacity (TLC) using more advanced tests like plethysmography or gas dilution techniques.

Limitations of Spirometry

Spirometry is an effort-dependent test. Suboptimal technique, coughing, or an incomplete breath can affect the accuracy of the results. Furthermore, spirometry represents lung function at a single point in time. In conditions with variable airflow limitation, such as asthma, spirometry may appear entirely within normal limits between symptomatic episodes.