What is spirometry?

Spirometry: A Fundamental Lung Function Test

Spirometry is a physiological test that measures the volume and flow of air during inhalation and exhalation as a function of time, serving as the most useful and commonly available pulmonary function test for diagnosing, monitoring, and assessing respiratory conditions. 1, 2

Key Parameters Measured

Spirometry measures several important lung function parameters:

• Forced Vital Capacity (FVC): The maximum volume of air exhaled with maximally forced effort from a maximal inspiration, expressed in liters 1
• Forced Expiratory Volume in 1 second (FEV1): The maximal volume of air exhaled in the first second of forced expiration from full inspiration, expressed in liters 1
• FEV1/FVC ratio: The proportion of the vital capacity exhaled in the first second, used to detect airflow obstruction 3
• Peak Expiratory Flow (PEF): The highest flow achieved from a maximum forced expiratory manoeuvre 1
• Inspiratory Capacity (IC): Volume change when taking a slow full inspiration from passive end-tidal expiration to maximum inspiration 1

Equipment Requirements

Spirometry requires standardized equipment that must:

• Accumulate volume for at least 15 seconds 1
• Measure volumes of ≥8 L with accuracy of at least ±3% of reading or ±0.050 L 1
• Handle flows between 0-14 L/s with resistance to airflow <1.5 cmH₂O/L/s 1
• Display both flow-volume and volume-time curves for quality control 1

Testing Procedure

The procedure involves:

1. Patient seated upright wearing a nose clip 1
2. Creating an airtight seal around the mouthpiece 1
3. Taking a maximal inspiration followed by a forceful, complete expiration 1
4. Continuing exhalation until no more air can be expelled (plateau reached) 1
5. Performing multiple maneuvers (typically 3-8) until reproducibility criteria are met 4

Quality Control and Acceptability

For valid results, spirometry must meet strict quality standards:

• Back-extrapolated volume <5% of FVC or <0.150 L 3
• No hesitation or abnormal start to the maneuver 1
• No coughing or glottis closure during the test 1
• Complete exhalation with appropriate plateau 1
• Two largest FEV1 and FVC values within 0.150 L of each other 3

Clinical Applications

Spirometry is essential for:

• Diagnosis: Identifying obstructive and restrictive ventilatory defects 3
• Monitoring: Tracking disease progression and treatment response 5
• Screening: Evaluating at-risk individuals (smokers, occupational exposures) 6
• Prognosis: Predicting outcomes in respiratory diseases 5
• Bronchodilator testing: Assessing airway reversibility (positive response: increase of ≥12% and ≥200 mL in FEV1 or FVC) 4

Interpretation Principles

• Obstruction is defined as FEV1/VC below the 5th percentile of predicted value 3
• Restriction requires confirmation with Total Lung Capacity (TLC) measurement 3
• The European Respiratory Society recommends using the 5th percentile as the lower limit of normal rather than fixed ratios 3
• Severity of obstruction is graded based on FEV1 percentage of predicted:
  • Mild: ≥80%
  • Moderate: 50-79%
  • Severe: 30-49%
  • Very Severe: <30% 3

Common Pitfalls

• Inadequate patient effort leading to falsely low values 1
• Improper technique or poor seal around mouthpiece 1
• Using fixed FEV1/FVC ratio of 0.70 can overestimate obstruction in older individuals 3
• Relying solely on FEV1 may miss abnormalities in other parameters 3
• Failure to use appropriate reference values based on age, sex, height, and ethnicity 3

Spirometry remains the cornerstone of pulmonary function testing, providing objective, reproducible measurements that are essential for respiratory care and management.