Functional classification of common impairments assessed by conventional pulmonary function tests and their pathophysiological determinants
| Obstructive ventilatory impairments# | Narrowing of the airways in the lung by physical obstruction or by dynamic airway collapsing. More proximal airway properties determine airflow resistance at large lung volumes and drive the FEV1/FVC measurement; more distal airway properties determine airflow resistance at small lung volumes and drive flow measurements later in a maximal exhalation. Because airway obstruction impairs lung emptying, it is often accompanied by air trapping and hyperinflation that may reduce the FVC but is more directly assessed by the RV measurement. |
| Restrictive ventilatory impairments# | Reduction in the size of the lung. This may reflect lung parenchymal or an inability to fully inhale due to extrapulmonary factors (e.g. weakness, chest wall abnormalities and obesity). Lung restriction reduces FEV1, FVC (but not the FEV1/FVC ratio) and TLC. |
| Gas transfer impairments | Reduction in transport of gas (carbon monoxide transfer as a surrogate for oxygen) between the alveolar spaces and alveolar capillary blood. This may be due to a reduction in alveolar surface area, abnormal alveolar–capillary membrane properties or reduced pulmonary capillary blood (haemoglobin) volume. Impaired gas transfer is generally assessed by analysis of carbon monoxide uptake during a breath-hold (DLCO). Some conditions can lead to an increase in gas transfer. |
FEV1: forced expiratory volume in 1 s; FVC: forced vital capacity; RV: residual volume; TLC: total lung capacity; DLCO: diffusing capacity of the lung for carbon monoxide. #: many authorities also use the term “ventilatory impairments” to group obstructive and restrictive impairments.