Spirometry

Spirometry is a common, non-invasive pulmonary function test that measures the amount (volume) and speed (flow) of air a person can inhale and exhale, used to assess lung function and diagnose or monitor respiratory conditions such as asthma, chronic obstructive pulmonary disease (COPD), and other obstructive or restrictive lung disorders.

During the test, the patient is asked to take the deepest breath possible and then exhale as forcefully and completely as possible into a mouthpiece connected to a spirometer device. This is repeated typically three times to ensure accurate and reproducible results. The key measurements obtained include Forced Vital Capacity (FVC), which is the total volume of air exhaled after a maximum inhalation; Forced Expiratory Volume in one second (FEV1), which is the volume of air exhaled in the first second of the forced breath; and the FEV1/FVC ratio, which compares these two values and is the most critical parameter for identifying the presence and type of lung dysfunction.

Clinically, spirometry is used to detect airflow obstruction, as seen in asthma and COPD, where the FEV1/FVC ratio falls below 0.70 (70%), indicating that air is being trapped or cannot be expelled normally. It is also used to identify restrictive lung patterns, where both FVC and FEV1 are reduced but the ratio remains normal or elevated, as seen in conditions like pulmonary fibrosis or chest wall deformities. Additionally, spirometry is used to assess the severity of lung disease, monitor disease progression over time, evaluate a patient's response to bronchodilator therapy (pre and post bronchodilator testing), screen high-risk individuals such as long-term smokers, and assess fitness before surgery.

Interpretation of spirometry results is guided by predicted normal values based on the patient's age, height, sex, and ethnicity. A result is considered abnormal when key values fall below 80% of the predicted normal. An obstructive pattern shows a reduced FEV1/FVC ratio with a normal or reduced FVC, while a restrictive pattern shows a reduced FVC with a preserved or elevated ratio. A mixed pattern can also occur when both obstruction and restriction are present simultaneously.

Spirometry is widely regarded as the gold standard for diagnosing and staging obstructive lung diseases, particularly COPD, and when combined with clinical assessment and other tests such as FENO or chest imaging, it provides a comprehensive picture of a patient's respiratory health.