flowvolume

Flowvolume, or the flow-volume loop, is a graphical representation of airway airflow as a function of lung volume during forced breathing. It is a component of pulmonary function testing used to assess both inspiratory and expiratory flow and to help characterize airway patency and other respiratory conditions. The loop is typically derived from a spirometry maneuver in which the patient performs a maximal inspiration to total lung capacity followed by a forceful, rapid expiration to residual volume, sometimes repeated for accuracy. The expiratory limb plots flow against volume during expiration, and the inspiratory limb plots flow during the subsequent inspiration.

Interpreting the flow-volume loop involves recognizing typical patterns. In obstructive diseases, the expiratory limb often shows a scooped or concave shape due to reduced mid-to-late expiratory flow, while the inspiratory limb is usually relatively preserved. Upper airway obstructions can produce characteristic changes: fixed upper airway obstruction yields flattening of both limbs, whereas variable obstructions produce limb-specific flattening. Specifically, variable extrathoracic obstruction tends to flatten the inspiratory limb more than the expiratory limb, and variable intrathoracic obstruction tends to flatten the expiratory limb more than the inspiratory limb.

Clinical use and limitations: The flow-volume loop helps differentiate obstructive from restrictive patterns, identify upper airway obstructions, and aid in monitoring disease progression or response to therapy. It complements standard spirometry indices. However, interpretation depends on patient effort and quality of the maneuver, normative reference values, and clinician expertise. It is not a stand-alone diagnostic test for all airway diseases but a useful tool when integrated with other pulmonary function data.