🔬 Spirometry

A physiological test measuring the volume of air an individual can inhale or exhale as a function of time to assess lung function and airway patency.

Indications include the investigation of chronic respiratory symptoms such as cough, wheeze, or breathlessness to confirm a diagnosis of asthma or COPD. It is used to monitor disease progression in known respiratory conditions and to evaluate the effectiveness of interventions like bronchodilators or corticosteroids. Other indications include pre-operative risk assessment for major thoracic or upper abdominal surgery and screening for occupational lung disease.

Patients are asked to take a full inspiration to total lung capacity, then exhale as hard and fast as possible into a spirometer until the lungs are empty. This must be repeated at least three times to ensure reproducibility (within 5% or 150ml). Key parameters measured include Forced Expiratory Volume in 1 second (FEV1), Forced Vital Capacity (FVC), and the FEV1/FVC ratio. Flow-volume loops are often generated to provide visual assessment of the breath.

A normal result is defined by an FEV1/FVC ratio of =>0.7 (or above the lower limit of normal for age/sex) and both FEV1 and FVC values within the predicted range (usually >80% of predicted) based on the patient's age, height, sex, and ethnicity. The flow-volume loop should show a rapid peak expiratory flow followed by a linear or slightly convex decline back to the baseline.

Interpretation begins with confirming the validity of the effort. A ratio of FEV1 to FVC of less than 70% (0.7) indicates an obstructive defect. The severity of obstruction is then graded by the FEV1 percentage of the predicted value. If the ratio is normal but FVC is low, a restrictive defect is suspected. Reversibility is often tested by repeating the test after administering a short-acting beta-agonist; an increase in FEV1 of >12% and >200ml is typically suggestive of asthma.

Obstructive patterns are characterised by a reduced FEV1/FVC ratio (<0.7 or below the lower limit of normal). Restrictive patterns show a proportionate reduction in both FEV1 and FVC, resulting in a normal or high FEV1/FVC ratio (>0.7) but a reduced Total Lung Capacity (if measured). Specific findings such as 'scooping' of the expiratory flow-volume loop are classic for obstructive disease, while a 'witch’s hat' appearance (tall, narrow loop) suggests restriction.

Spirometry is the cornerstone for diagnosing and monitoring chronic obstructive pulmonary disease (COPD) and is a key component in the diagnostic workup for asthma. It helps differentiate between obstructive and restrictive lung pathologies, guides the escalation of inhaled therapies, and provides prognostic information regarding lung function decline. In the UK, it is often performed in primary care but requires specialist interpretation for complex cases.

Common pitfalls include insufficient exhalation time (less than 6 seconds), poor initial blast, or coughing during the procedure. Clinicians must ensure the patient has avoided bronchodilators for the appropriate duration before the test (e.g., 4-6 hours for SABA, 24-48 hours for LAMA/LABA) if checking for a baseline or reversibility. Failure to use ethnic-specific reference values can result in misdiagnosis.

Spirometry is highly effort-dependent; poor patient technique can lead to false-positive restrictive patterns. It does not measure residual volume or total lung capacity directly, so full plethysmography may be needed to confirm restriction. Results may be normal in patients with asthma when they are asymptomatic, and it cannot distinguish between different causes of restriction (e.g., pulmonary fibrosis vs. chest wall deformity).

Medical students must be able to calculate and interpret the FEV1/FVC ratio. Understanding the difference between 'obstructive' (low ratio) and 'restrictive' (normal ratio, low FVC) is a core MLA requirement. Knowledge of NICE criteria for COPD diagnosis (FEV1/FVC < 0.7) is essential.