Diagnosis: Variable Extrathoracic Airway Obstruction (VEAO)
A flattened inspiratory loop on spirometry with normal FEV1 and FVC indicates variable extrathoracic upper airway obstruction, most commonly caused by vocal cord dysfunction, but requires direct visualization to confirm the specific etiology.
Understanding the Flow-Volume Loop Pattern
The inspiratory portion of the flow-volume loop provides critical diagnostic information about upper airway pathology that standard spirometric values (FEV1, FVC) cannot detect 1.
A flattened or truncated inspiratory curve indicates extrathoracic airway obstruction where the negative intrathoracic pressure during inspiration causes dynamic collapse of a weakened or dysfunctional upper airway 1.
Normal FEV1 and FVC values exclude lower airway obstruction (such as asthma or COPD), which would manifest as a reduced FEV1/FVC ratio 2, 3.
The mid-flow ratio (FEF50/FIF50) becomes elevated in these cases, often exceeding 2.0, which helps quantify the degree of inspiratory limitation 1.
Diagnostic Approach
Confirm the Pattern is Reproducible
Review all flow-volume loops from the testing session, not just one curve, as transient artifacts can mimic pathology 1.
If more than one inspiratory curve shows flattening, the abnormality is likely real and warrants further investigation 1.
Ensure technical quality by verifying the patient performed maximal inspiratory efforts and maintained an adequate seal without tongue obstruction 4.
Calculate the Mid-Flow Ratio
Use the loop with the best combined inspiratory and expiratory curves to calculate FEF50/FIF50 1.
A ratio >2.0 strongly suggests variable extrathoracic obstruction, though this should be interpreted alongside the visual appearance of the loop 1.
Most Common Etiologies
Vocal Cord Dysfunction (VCD)
VCD is the most frequently identified cause of isolated inspiratory flow limitation in patients with otherwise normal spirometry 1.
Patients often present with dyspnea, stridor, or throat tightness that may be misdiagnosed as asthma, particularly in younger patients and athletes 1.
Other Structural Causes
Laryngeal pathology including masses, stenosis, or laryngomalacia can produce this pattern 1.
Tracheal lesions at the thoracic inlet may also cause variable extrathoracic obstruction 1.
Management Algorithm
Step 1: Direct Laryngoscopy
All patients with reproducible inspiratory loop flattening require laryngoscopy to visualize the vocal cords and upper airway during both rest and provocation 1.
Laryngoscopy should ideally be performed during symptomatic periods or with exercise provocation if VCD is suspected 1.
Step 2: Specialized Testing if Initial Evaluation is Negative
Consider bronchoscopy if laryngoscopy is normal but clinical suspicion remains high for tracheal pathology 1.
CT imaging of the neck and chest may identify structural lesions not apparent on endoscopy 1.
Step 3: Treatment Based on Etiology
For confirmed VCD: Speech therapy with breathing retraining exercises is first-line treatment 1.
For structural lesions: Surgical intervention may be required depending on the specific pathology identified 1.
Critical Clinical Pitfall
The most important pitfall is failing to investigate the inspiratory abnormality at all. In one study, only 17% of patients with abnormal inspiratory curves underwent any evaluation, and only 30% of those with consistently abnormal curves were investigated 1. This represents a significant missed opportunity, as a specific treatable etiology was identified in 52% of evaluated patients 1.
Do not dismiss the finding simply because FEV1 and FVC are normal—these values reflect lower airway function and will not detect isolated upper airway pathology 1, 2.