A spirometer is a simple medical device that is used to measure the volume of air inhaled and exhaled by the lungs. It is one of the most common and important tests conducted to evaluate lung function. The data obtained from spirometry helps diagnose obstructive lung diseases like asthma, COPD or restrictive lung conditions like fibrosis.

How does a Spirometer work?

A basic spirometer consists of a mouthpiece attached to a tube that is connected to either a mechanical or electronic sensor. When a person breathes into the mouthpiece, the sensor records the amount and speed of air inhaled and exhaled. Modern spirometers use lightweight and compact electronic sensors to obtain readings.

During a Spirometry test, the person is asked to take deep breaths and exhale as much as possible through the mouthpiece. Three to eight readings are typically taken and the highest values are recorded. The readings obtained are compared to results from a healthy individual of same age, height, sex and ethnicity.

Key Measurements from Spirometry

There are a few key measurements that are analyzed to evaluate lung function:

- Forced Vital Capacity (FVC): Total amount of air exhaled with maximal force after taking the deepest breath possible.

- Forced Expiratory Volume in 1 second (FEV1): Amount of air that can be exhaled in the first second of forced exhalation.

- FEV1/FVC ratio: Ratio of FEV1 to FVC expressed as a percentage. This gives an indication of obstructive or restrictive lung diseases.

- Peak Expiratory Flow Rate (PEFR): Maximum flow achieved during expiration done with maximum force.

- Inspiratory capacity (IC): Maximum volume of air inhaled after a normal expiration.

- Total Lung Capacity (TLC): Total amount of air present in lungs after maximum inhalation.

The above values are compared to reference values based on multiple factors and deviations help diagnose lung conditions.

Interpreting Spirometry Results

Decreased FEV1 and FEV1/FVC ratio are markers of obstructive lung diseases like asthma and COPD where air cannot be expelled easily from the lungs. Restrictive lung conditions result in reduced FVC and TLC indicating stiffness of lung tissue.

Some key patterns observed on spirometry:

- Obstructive Pattern: Reduced FEV1/FVC ratio (<70-75% of predicted value) with low FEV1. Seen in asthma, COPD.

- Restrictive Pattern: Reduced FVC and TLC (<80% of predicted value) with normal or increased FEV1/FVC. Seen in fibrosis, chest wall disorders.

- Mixed Pattern: Combination of features of both obstructive and restrictive patterns. Seen in severe COPD, pulmonary fibrosis with air trapping.

- Normal Spirometry: All values are >80% of predicted and no significant abnormalities seen. Rules out obstructive or restrictive lung disease.

Role of Spirometry in Diagnosing and Managing Lung Diseases

Spirometry is invaluable in early detection of lung diseases when symptoms may not be prominent yet. It plays a key role in:

- Diagnosing obstructive and restrictive lung diseases.

- Confirming clinical diagnosis in borderline or unclear cases.

- Monitoring progression or response to treatment in asthma, COPD over time with serial tests.

- Screening individuals at high risk for early signs of lung damage from occupational exposure.

- Assessing impact of surgical procedures like lung volume reduction surgery in advanced COPD.

- Ruling out other pulmonary or non-pulmonary conditions mimicking lung diseases.

When combined with symptoms, radiology and bronchoprovocation tests, spirometry forms the cornerstone of establishing a definitive diagnosis for most lung diseases. Its repeated use is vital in effective long-term management as well.

Spirometry is a simple yet powerful diagnostic tool that provides objective parameters to quantitatively evaluate lung function. It plays a vital role in detecting underlying pulmonary pathologies, guiding treatment decisions and monitoring therapy response over time. With its widespread use in clinics and availability of portable devices, spirometry has become an indispensable test for respiratory medicine specialists.