Pulmonary Function Testing and Interpretation in Practice

What Is Spirometry and Why Does It Matter?

Spirometry is the most commonly performed pulmonary function test in clinical practice. It measures how much air a patient can exhale and how fast they can exhale it. Respiratory therapists use spirometry to detect obstructive and restrictive lung diseases, monitor disease progression, evaluate treatment response, and clear patients for surgery. Because the test is effort-dependent, your coaching skill directly affects the quality of results and, ultimately, patient care decisions.

Key Spirometry Measurements

The two most important values from a forced vital capacity (FVC) maneuver are:

• FVC (Forced Vital Capacity): The total volume of air exhaled forcefully from full inhalation to complete exhalation. Reduced FVC can indicate restriction, air trapping, or poor effort.
• FEV1 (Forced Expiratory Volume in 1 second): The volume exhaled in the first second of the FVC maneuver. It reflects the speed of airflow and is sensitive to airway obstruction.
• FEV1/FVC ratio: Dividing FEV1 by FVC tells you whether obstruction is present. A ratio below the lower limit of normal (LLN)—often approximated as less than 0.70 in many guidelines, though the LLN from reference equations is preferred—suggests obstruction.

Results are compared to percent-predicted values derived from reference equations that account for the patient's age, sex, height, and race/ethnicity. A value at or above the LLN (generally ≥ 80% predicted using older fixed thresholds, though LLN from reference equations is the standard) is considered normal.

Device Calibration

Before every testing session, calibrate the spirometer using a 3-liter calibration syringe. Inject the syringe volume three times at different flow rates (slow, medium, fast). The spirometer must read within ±3.5% of 3.00 liters on each stroke. Document the calibration result. Never test patients on an uncalibrated device—inaccurate results can lead to misdiagnosis and incorrect treatment. If the device fails calibration, remove it from service and troubleshoot before proceeding.

Pre-Test Contraindication Screening

Always screen the patient before testing. The FVC maneuver generates significant intrathoracic pressure. Contraindications include (but are not limited to):

• Recent myocardial infarction (typically within 1 month)
• Recent thoracic, abdominal, or eye surgery
• Pneumothorax (current or recent)
• Hemoptysis of unknown origin
• Unstable cardiovascular status
• Thoracic or abdominal aneurysm

If any contraindication is present, withhold the test and notify the supervising clinician or ordering provider. Scope of practice: as a respiratory therapy student and later as a credentialed RT, you do not independently authorize testing over a contraindication—escalate to the physician.

ATS/ERS Acceptability Criteria

The American Thoracic Society and European Respiratory Society (ATS/ERS) publish the internationally recognized standards for spirometry. For a maneuver to be acceptable it must meet both start-of-test and end-of-test criteria.

Start-of-test (good effort launch):

• Extrapolated volume (back-extrapolated volume) must be less than 5% of FVC or less than 0.150 L, whichever is greater.
• The flow-volume curve must show a sharp, near-vertical rise to peak expiratory flow.

End-of-test:

• The patient must exhale for at least 6 seconds (3 seconds for children under age 10).
• A volume-time plateau must be present (volume change < 0.025 L in the final second), OR the patient cannot or should not continue due to symptoms.

For repeatability, the two largest FVC values and the two largest FEV1 values among acceptable maneuvers must each be within 0.150 L of each other (or within 8% if FVC < 1.0 L, per current guidance). Aim to collect at least three acceptable maneuvers and no more than eight total.

Coaching the Patient

Effective coaching is the RT's most important contribution to test quality. Use a clear, enthusiastic script:

1. Explain the procedure in plain language. Ask the patient to breathe normally into the mouthpiece, inhale as deep as possible, then "blast the air out as hard and as fast as you can and keep going until I say stop."

2. Fit the nose clip snugly before each maneuver.

3. Seal the lips firmly around the mouthpiece—no air leaks.

4. During exhalation, use active cues: "Keep going, keep going, push, push, almost there…" until the plateau is achieved.

5. After each maneuver, provide specific feedback: "Great effort—now let's try to keep blowing even longer on the next one."

Interpreting the Pattern

Use this stepwise approach:

1. Check FEV1/FVC ratio. If below the LLN → obstruction is present.

2. Check FVC. If FVC is below the LLN with a normal or elevated ratio → possible restriction (confirm with full lung volumes if available).

3. If both FEV1/FVC and FVC are reduced → mixed (combined obstructive-restrictive) pattern.

4. If both are normal → normal spirometry.

5. Grade severity by percent-predicted FEV1 (obstructive) or FVC (restrictive) using the ATS/ERS severity classification (mild ≥ 70% predicted, moderate 60–69%, moderately severe 50–59%, severe 35–49%, very severe < 35% — values approximate; always use the reference adopted by your institution).

Common obstructive diagnoses: asthma, COPD, bronchiectasis.

Common restrictive diagnoses: pulmonary fibrosis, obesity, neuromuscular disease, pleural effusion.

Safety Reminders

• Always use a fresh disposable mouthpiece and filter for each patient.
• Stop the test immediately if the patient experiences chest pain, dizziness, syncope, or significant oxygen desaturation.
• Have supplemental oxygen and emergency equipment accessible.
• Disinfect or replace reusable spirometer components per manufacturer and infection control guidelines.