What does a pulmonary function test showing normal expiratory findings, including normal Forced Expiratory Volume in 1 second (FEV1) and Forced Vital Capacity (FVC), and a flattened inspiratory flow loop indicate in a patient with shortness of breath?

Flattened Inspiratory Flow Loop with Normal Expiratory Findings

A flattened inspiratory flow loop with normal FEV1, FVC, and expiratory findings indicates variable extrathoracic upper airway obstruction, and direct laryngoscopy is the confirmatory test for diagnosing the cause of shortness of breath in this clinical scenario. 1

Understanding the Flow-Volume Loop Pattern

The flow-volume loop provides critical diagnostic information about the location and nature of airway obstruction:

• A repeatable plateau of forced inspiratory flow with preserved expiratory flow suggests variable extrathoracic central or upper airway obstruction 1
• This pattern occurs because during inspiration, the negative intraluminal pressure cannot be opposed by atmospheric pressure surrounding the extrathoracic airways (pharynx, larynx, extrathoracic trachea), causing the obstruction to worsen 1
• During expiration, positive intraluminal pressure keeps the airway open, explaining why expiratory parameters (FEV1, FVC) remain normal 1

Diagnostic Approach

Key Spirometric Clues

The FEV1/PEF ratio is particularly useful for detecting upper airway obstruction:

• An FEV1/PEF ratio >8 (or >0.8 when using consistent units) suggests central or upper airway obstruction may be present 1
• In your patient, normal FEV1 with likely reduced peak inspiratory flow would elevate this ratio 1

Specific Diagnostic Criteria

Classic findings for variable extrathoracic obstruction include: 2

• Forced inspiratory flow at 50% vital capacity (FIF50%) ≤100 L/min 2
• FEV1/PEFR ratio ≥10 mL/L/min 2
• Preserved expiratory flows with isolated inspiratory flow limitation 2

Confirmatory Testing

Direct laryngoscopy is the definitive diagnostic test for evaluating suspected upper airway obstruction causing dyspnea in this clinical context. This allows direct visualization of:

• Vocal cord dysfunction (paradoxical vocal cord motion during inspiration)
• Laryngeal pathology (masses, edema, stenosis)
• Dynamic airway collapse during respiration
• Structural abnormalities of the larynx or extrathoracic trachea

1, 2

Common Differential Diagnoses in Adolescents

In a 17-year-old with this pattern, consider:

• Vocal cord dysfunction (most common in this age group with exercise-induced symptoms) 1
• Laryngeal pathology (webs, cysts, or inflammation)
• Functional laryngeal obstruction
• Subglottic stenosis

1, 2

Critical Pitfalls to Avoid

• Do not assume asthma based on dyspnea alone - the normal expiratory findings and FEV1/FVC ratio effectively rule out lower airway obstruction 1
• Ensure maximal patient effort - submaximal inspiratory effort can create a pseudo-plateau, so technician confirmation of effort quality is essential 1
• Do not rely on PEF alone for diagnosis, as it can be severely affected by upper airway obstruction even when FEV1 is normal 3
• Bronchodilator testing is not indicated here, as the problem is anatomic/functional upper airway obstruction, not bronchospasm 1