Respiratory function testing (RFT) - elements of interpretation

Respiratory function tests (or explorations) study respiratory function, essentially through the study of respiratory volumes and flows and gas exchanges at rest and during exercise.

Although dependent on the patient's cooperation, respiratory function tests are one of the main examinations contributing to the development and follow-up of chronic dyspnea. The aim of this article is not to be exhaustive, but to provide all doctors, whatever their specialty, with the information they need.

Lung volumes

Lung volumes are measured by spirometry (measurement of mobilizable volumes) and inert gas dilution or plethysmography (measurement of inert volumes). These measurements are highly sensitive, and can be used to differentiate :

• Obstructive pathologies :
  • Increased CPT and RV
  • Increased RV / CPT ratio
• Restrictive pathologies:
  • CPT decreased to < 80% of predicted value
  • Increased RV / CPT ratio
  • RV variable according to pathology

Airflow resistances

Airway resistance is determined by measuring forced expiratory and inspiratory volumes as a function of time using a spirometer. The results are usually presented as two curves:

Measurement of the Tiffeneau ratio (FEV1/FVC) and the morphology of the flow-volume curves can be used to differentiate between :

• Obstructive pathologies :
  • Decreased FEV1 with Tiffeneau < 0.7 (according to GOLD criteria - does not recognize very distal obstructive disease).
  • An abnormally concave expiratory flow-volume curve (reduced expiratory flow at low volumes). In the most severe cases of obstruction, PEF will also be lower than predicted.
  • In geriatric patients, a curve characterizing mild obstruction should be considered physiological.
  • Note that in cases of obstruction of the upper airways (larynx, trachea) or sometimes of the bronchial tubes, a "plateau" curve may be observed.

• Restrictive pathologies
  • Decreased FEV1 with Tiffeneau > 0.7 (proportional decrease in FVC). Tiffeneau > 1.1 suggests pulmonary fibrosis.
  • The flow-volume curve is a "reduced model" of the normal curve: all predicted values are decreased in a relatively harmonious way, so the slopes are preserved (decrease in expiratory and inspiratory flows for all volumes).

Reversibility test in an obstructive syndrome

An important step in the development of an obstructive syndrome is to determine whether it is (partially) reversible to the administration of a bronchodilator. Reversibility is defined as an improvement in FEV1 or FVC > 12% and 200 ml ten minutes after administration. In this case, if Tiffeneau remains < 0.7, partial reversibility is assumed.

If the test is negative, it is useful to repeat it at follow-up EFR (susceptibility to bronchodilators varies over time → many false negatives). Administration of long-acting bronchodilators in the hours preceding the test is a frequent cause of false negatives.

Respiratory syndromes

Restrictive syndromes

EFR characteristics :

• CPT and CV decreased with CPT < 80%.
• FEV1 and FVC reduced with Tiffeneau > 0.7

Etiologies :

• Chest wall involvement :
  • Bone involvement :
    • Scoliosis, ankylosing spondylitis, funnel chest, abdominal obesity, pleural damage, cardiomegaly, abundant ascites, diaphragmatic hernias
  • Neuromuscular disorders:
    • diaphragmatic disorders, spinal lesions, hemiplegia, poliomyelitis, muscular dystrophies, myopathies, amyotrophic lateral sclerosis, Guillain-Barré, myasthenia, Lambert-Eaton, botulism
• Parenchymal involvement :
  • Lobar or pulmonary excision, atelectasis, pneumonia, pulmonary edema, interstitial lung disease, pulmonary fibrosis, pneumoconiosis, alveolar proteinosis, tuberculosis sequelae