9-02: Pulmonary Function Tests

Standard PFTs measure airflow rates, lung volumes, and the ability of the lung to transfer gas across the alveolar-capillary membrane. The 2021 American Thoracic Society/European Respiratory Society (ATS/ERS) interpretation standards for spirometry and PFTs recommended the use of the Global Lung Initiative (GLI) 2012 reference equations, which were derived from large population of healthy subjects, accounting for age, biological sex, and self-reported race; however, several studies have found that PFT interpretations based on race-specific standards may underestimate the prevalence and severity of respiratory disease in Black individuals. Therefore, in 2023, the ATS endorsed the use of more recent GLI reference standards that are race-neutral.

PFTs help in evaluating the etiology of respiratory symptoms such as dyspnea and cough, the type and extent of physiologic lung impairment, early or longitudinal evidence of lung impairment following the response to therapy, and preoperative or disability evaluations. Contraindications to PFTs include acute respiratory distress, unstable angina, pneumothorax, ongoing hemoptysis, and active tuberculosis. Test results are effort-dependent, and suboptimal effort limits validity and is a common cause of misinterpretation of results.

Spirometry and measurement of lung volumes allow assessment of the presence and severity of obstructive and restrictive lung disease (eFigure 9–1). An obstructive ventilatory impairment is marked by a reduction in airflow rates judged by a fall in the ratio of FEV₁ to FVC, and its severity is graded by the reduction in FEV₁. Causes include asthma, COPD (chronic bronchitis and emphysema), bronchiectasis, bronchiolitis, and upper airway obstruction. If airflow obstruction is evident, spirometry may be repeated 10–20 minutes after an inhaled bronchodilator is administered. In the pulmonary function laboratory, improvement in spirometry after inhaled bronchodilator in patients with asthma or COPD suggests airway reversibility with bronchodilator therapy. Its absence does not preclude a successful clinical response to bronchodilator therapy.

Restrictive ventilatory impairment is marked by a reduction in lung volumes with a normal to increased FEV₁/FVC ratio; its severity is graded by the reduction in TLC. A proportionate reduction in FEV₁ and FVC with a normal FEV₁/FVC ratio suggests pulmonary restriction but is not diagnostic. Causes for restrictive ventilatory impairment include decreased lung compliance from infiltrative disorders such as pulmonary fibrosis; reduced muscle strength from phrenic nerve injury, diaphragm dysfunction, or neuromuscular disease; pleural disease, including large pleural effusion or marked pleural thickening; prior lung resection; and obesity. The flow-volume loop combines the maximal expiratory and inspiratory flow-volume curves and is especially helpful in determining the site of airway dysfunction (see eFigure 9–1).

Measurement of DLCO reflects the ability of the lung to transfer gas across the alveolar/capillary interface. The total pulmonary diffusing capacity depends on the diffusion properties of ...