Guide — Pulmonary Function Testing
Flow-Volume Loops
Reading the spirometry loop at a glance — the normal shape, the scooped expiratory limb of obstruction, the tall narrow loop of restriction, and the flattened plateaus that flag fixed and variable upper-airway obstruction. The loop’s shape is a fast pattern-recognition tool that the raw numbers alone can miss.
8 min read · Pulmonary Function Testing
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
The flow-volume loop plots airflow against exhaled (and inhaled) volume in a single picture. Its shapeis a fast, powerful pattern-recognition tool — obstruction, restriction, and upper-airway lesions each distort the loop in a characteristic way. Learn the shapes once and you can read most loops at a glance before you ever look at the ratio.
Key Concepts — The Shapes
- The normal loop. A rapid rise to peak expiratory flow, then a straight or gently convex decline to residual volume (the expiratory limb above the axis), and a rounded inspiratory limb below the axis.
- Obstruction.A “scooped” or concave (coved) expiratory limb — flow drops off quickly as the small airways collapse. Severe obstruction (emphysema) shows a steeple or dog-leg pattern, with a reduced peak flow and prolonged expiration.
- Restriction.A tall, narrow loop — the shape is preserved (even peaked) but everything is scaled down in volume because the FVC is small (a witch’s-hat appearance).
- Upper-airway obstruction shows plateaus. Fixed obstruction (for example tracheal stenosis or a large goiter) flattens BOTH the inspiratory and expiratory limbs. Variable extrathoracic obstruction (for example vocal cord dysfunction or an extrathoracic mass) flattens the INSPIRATORY limb. Variable intrathoracic obstruction (for example an intrathoracic tracheal tumor or tracheomalacia) flattens the EXPIRATORY limb.
Assessment & Findings
| Pattern | Affected Limb | What You See |
|---|---|---|
| Normal | Both limbs preserved | Rapid rise to peak expiratory flow, then a straight or gently convex decline; rounded inspiratory limb below the axis |
| Obstruction | Expiratory limb | Scooped / concave (coved) expiratory limb; severe emphysema shows a steeple or dog-leg with reduced peak flow and prolonged expiration |
| Restriction | Whole loop scaled down | Tall, narrow loop - shape preserved (even peaked) but small FVC gives a witch's-hat appearance |
| Fixed upper-airway | Both limbs | Flattening of BOTH inspiratory and expiratory limbs (e.g. tracheal stenosis, large goiter) |
| Variable extrathoracic | Inspiratory limb | Flattening of the INSPIRATORY limb (e.g. vocal cord dysfunction, extrathoracic mass) |
| Variable intrathoracic | Expiratory limb | Flattening of the EXPIRATORY limb (e.g. intrathoracic tracheal tumor, tracheomalacia) |
- Read the expiratory limb first: concave or scooped means obstruction; tall and narrow means restriction.
- A plateau on one or both limbs means an upper-airway obstruction, and which limb flattens localizes it.
- A poor-effort loop (no sharp peak, ragged) is uninterpretable — repeat it.
RT Priorities & Interventions
- Coach the effort first. Get a clean, reproducible loop before interpreting the shape — a sharp peak and consistent tracings are the prerequisite for everything below.
- Recognize upper-airway plateaus. They can be missed by the numbers alone and may signal a stridor-causing lesion that needs urgent attention.
- Pair shape with the numbers. Read the loop alongside the FEV₁/FVC ratio, the lung volumes, and DLCO for the full picture — the shape narrows it down, the numbers confirm it.
Common Pitfalls
- Interpreting a submaximal or ragged loop as disease.
- Missing an upper-airway plateau because the FEV₁/FVC can look near-normal in a fixed obstruction.
- Confusing the scooped obstructive limb with restriction — restriction is narrow but keeps its shape.
Board Exam Pearls
- A concave (scooped) expiratory limb means obstruction; a tall, narrow loop means restriction.
- Fixed upper-airway obstruction flattens both limbs.
- Variable extrathoracic obstruction flattens the inspiratory limb.
- Variable intrathoracic obstruction flattens the expiratory limb.
- Mnemonic: extrathoracic hits inspiration, intrathoracic hits expiration.
FAQ
How do I tell obstruction from restriction on the loop?
Look at the expiratory limb. Obstruction scoops it into a concave (coved) shape as flow falls off. Restriction keeps the loop's shape but shrinks it into a tall, narrow form because the vital capacity is small.
What does a flat plateau on the loop mean?
An upper-airway (large-airway) obstruction. If both the inspiratory and expiratory limbs are flattened, it is a fixed obstruction; if only one flattens, it is variable - inspiratory flattening points extrathoracic, expiratory flattening points intrathoracic.
Why does extrathoracic obstruction worsen on inspiration?
During inspiration, negative airway pressure tends to collapse an extrathoracic (outside the chest) variable lesion, flattening the inspiratory limb. An intrathoracic lesion is instead compressed by rising pleural pressure during a forced expiration, flattening the expiratory limb.
Can the loop be normal yet the patient have disease?
Early or mild small-airway disease can show only subtle scooping, and a poor effort can hide or mimic findings. The loop is always read alongside the numbers, lung volumes, and DLCO.
Put it to work
When the loop shows obstruction, the next question is whether it reverses. Run the bronchodilator-response numbers.
Open the Bronchodilator Response calculator →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Pulmonary function testing and flow-volume loop chapters.
- Pellegrino R, Viegi G, Brusasco V, et al. Interpretative strategies for lung function tests. Eur Respir J. 2005;26(5):948-968.