The flow-volume loop is a plot of inspiratory and expiratory flow (on the Y-axis) against volume (on the X-axis) during the performance of maximally forced inspiratory and expiratory maneuvers.
- TLC = Left most point on X-axis
- RV = Right most point on X-axis
- Expiratory flow = Positive deflection on Y-axis
- Inspiratory flow = Negative deflection on Y-axis
- PEFR = Highest point on expiratory flow
- PIFR = Deepest point on inspiratory flow
| Condition | Mnemonic | Flow-volume loop | Features |
| Obstructive lung disease (small airways) | Shift to left Concave/Scooped out expiratory flow (↓FEV 25-75%) | ↑TLC and RV ↓FEV1 (preserved in early obstruction) | |
| Restrictive lung disease | Restrictive = Right | Shift to right | ↓TLC and RV |
| Variable intra-thoracic obstruction | What’s in is out | Flattening of expiratory curve | |
| Variable extra-thoracic obstruction | What’s out is in | Flattening of inspiratory curve | |
| Fixed large airway obstruction | Flattening of both expiratory and inspiratory curve |