Inspiratory Plateau on the Flow-Volume Loop in a Young Woman
A 24-year-old woman is referred with 3 months of exertional breathlessness and an audible inspiratory "wheeze" that worsens with exercise and singing and resolves on its own. Chest radiograph and blood count are normal; she has never smoked. Spirometry: FEV1 3.30 L (94% predicted), FVC 3.85 L (98% predicted), FEV1/FVC 0.86, FEF50 3.9 L/s (82% predicted), FIF50 1.6 L/s (38% predicted). The inspiratory limb of the flow-volume loop shows a flat plateau across the middle half of the breath, reproducible on two efforts; the expiratory limb is smooth and not concave. The third effort shows a normal inspiratory limb but only 60% of the best FVC. Bronchodilator response is +110 mL (3%).
1. Characterize the pattern and build a differential.
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The pattern is a variable extrathoracic obstruction.
- Expiratory flow is preserved (FEF50 82% predicted), so there is no intrathoracic/airway-disease signature and the FEV1/FVC is normal.
- Inspiratory flow is cut off with a plateau (FIF50 38% predicted). The mid-flow ratio FEF50/FIF50 = 3.9/1.6 = about 2.4, well above 1.
- The mechanism is transmural pressure across the extrathoracic airway: during inspiration the lumen is exposed to negative intraluminal pressure and tends to collapse, so a lesion above the thoracic inlet produces inspiratory-limited flow. During expiration the same segment is splinted open.
Differential: inducible laryngeal obstruction (vocal cord dysfunction), laryngeal or vocal cord structural lesion, tonsillar or base-of-tongue enlargement, thyroid or other cervical mass, extrathoracic tracheal stenosis, and obesity. Poor or inconsistent inspiratory effort is the competing explanation that must always be excluded first.
Note what the loop does not show: this is not asthma, and a flat inspiratory limb does not localize the level.
2. The third effort had a normal inspiratory limb. How does that change your interpretation, and what do you do next?
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It weakens the finding substantially and must not be waved away.
- The third maneuver only reached 60% of the best FVC, so it did not begin at full inflation. An inspiratory limb can only be judged from a full exhalation to RV, and a truncated breath cannot fairly be compared with the full ones.
- Even so, non-reproducibility across efforts is the classic signature of a technique or effort problem rather than fixed obstruction. A structural extrathoracic lesion should plateau on every full breath.
- Therefore you cannot report an obstruction pattern as established. Before anything else: repeat the session with a nose clip, a firm mouth seal, vigorous coaching for a maximal inspiratory effort, and require at least three efforts that are both acceptable and reproducible.
If the plateau persists on reproducible full breaths, the next step is provocation, not more resting spirometry: exercise or voluntary hyperventilation challenge with loops or spirometry performed while symptomatic, plus laryngoscopy during an episode. A resting loop alone cannot diagnose inducible laryngeal obstruction, and a normal loop between episodes is expected.
3. Write how you would report this study and what you tell the referring clinician.
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Report: descriptive and hedged. State the measured values, the flow-volume morphology (reproducible inspiratory plateau on two of three efforts, with one non-reproducible effort excluded because inspiratory vital capacity was 60% of the best FVC), the normal expiratory limb, the absent bronchodilator response, and the technical limitations. Do not enter "vocal cord dysfunction" or "upper airway obstruction" as a diagnosis; say the pattern is suggestive of variable extrathoracic obstruction and requires confirmation.
To the clinician:
- This is not an asthma pattern and does not explain her symptoms by itself; if she is on escalating inhaled therapy, that can be reviewed.
- The single most useful next test is an evaluation during symptoms - exercise or hyperventilation provocation with post-challenge flow-volume loops, and laryngoscopy while symptomatic.
- Refer to otolaryngology and to speech-language pathology for laryngeal control therapy if inducible laryngeal obstruction is confirmed.
- Warn the patient and the chart that a normal loop between attacks does not exclude the diagnosis, so she is not dismissed when asymptomatic.