FlowVolumeLoop

Flow-volume loop, also known as a flow-volume curve, is a graphical representation used in spirometry to assess airway function. The plot shows instantaneous airflow as a function of lung volume during a forced expiratory maneuver from total lung capacity to residual volume, followed by a forced inspiratory maneuver back to TLC. The result forms two limbs: expiratory and inspiratory, creating a closed loop that reflects dynamic ventilation.

In healthy individuals, the loop displays a characteristic, smooth shape with a sharp peak expiratory flow and a relatively rounded inspiratory limb. Obstructive airway diseases often reduce the peak expiratory flow and produce a scooped or concave expiratory limb, reflecting impaired expiratory airflow. Restrictive diseases typically present with reduced overall lung volumes and a smaller loop, but the shape remains comparatively more linear than in obstruction. The loop can therefore aid in differentiating obstructive from restrictive ventilatory abnormalities when assessed alongside standard spirometric values.

The flow-volume loop is also useful for evaluating specific patterns of central airway obstruction. Variable extrathoracic obstruction tends to flatten the inspiratory limb, variable intrathoracic obstruction tends to flatten the expiratory limb, and fixed central airway obstruction typically flattens both limbs. These patterns help localize the site of airway narrowing and guide further evaluation.

Obtaining a reliable loop requires patient cooperation and proper technique during spirometry. Limitations include dependence on effort, technique, and demographic factors; normative interpretation relies on reference values and quality control per established guidelines. Overall, the flow-volume loop provides a complementary view of airway mechanics beyond standard spirometric numbers.