Lung Function Tests

Lung function tests provide objective measurements of ventilatory capacity and lung volumes that are essential for diagnosing and monitoring respiratory disease. The most widely used technique is spirometry.

During spirometry, the patient performs a forced expiratory manoeuvre from total lung capacity to residual volume. The volume exhaled in the first second is the forced expiratory volume in one second (FEV1). The total volume exhaled during the entire forced manoeuvre is the forced vital capacity (FVC). The ratio FEV1/FVC is the key discriminating index. In healthy adults this ratio is approximately 0.75–0.80. A ratio below 0.70 indicates obstruction to airflow.

In obstructive lung disease (asthma, COPD), the airways are narrowed, so expiratory flow is disproportionately reduced. FEV1 falls more than FVC, lowering the FEV1/FVC ratio below 0.70. Both values may be reduced in absolute terms, but the ratio change is the diagnostic criterion. In restrictive lung disease (pulmonary fibrosis, pleural effusion, neuromuscular weakness), lung volumes are uniformly reduced. FEV1 and FVC both decrease proportionally, keeping the FEV1/FVC ratio normal or even elevated above 0.70.

Static lung volumes are defined by specific boundaries. Tidal volume (TV) is the volume of air moved in a normal quiet breath, approximately 500 mL. Inspiratory reserve volume (IRV) is the additional volume that can be inhaled above tidal volume. Expiratory reserve volume (ERV) is the additional volume that can be exhaled below tidal volume. Residual volume (RV) is the volume remaining in the lungs after maximal exhalation and cannot be measured by spirometry alone; it requires gas dilution or body plethysmography.

Capacities are sums of two or more volumes. Total lung capacity (TLC) = TV + IRV + ERV + RV. Vital capacity (VC) = TV + IRV + ERV (the maximum volume that can be exhaled after a maximal inhalation). Inspiratory capacity (IC) = TV + IRV. Functional residual capacity (FRC) = ERV + RV; this is the resting end-expiratory lung volume.

Peak expiratory flow rate (PEFR) measures the maximum flow rate achieved during a forced expiration and is a simple bedside tool for monitoring airflow obstruction, particularly in asthma. It is effort-dependent and can be measured with an inexpensive peak flow meter, making it useful for patient self-monitoring and tracking response to bronchodilator therapy. A significant improvement in FEV1 (>12% and >200 mL) after inhaled bronchodilator confirms reversible airflow obstruction consistent with asthma.