What does a flattened inspiratory loop on a pulmonary function test (PFT) indicate?

Flattened Inspiratory Loop on Pulmonary Function Test Interpretation

A flattened inspiratory loop on a pulmonary function test most strongly indicates vocal cord dysfunction or other forms of variable extrathoracic airway obstruction.

Primary Causes of Flattened Inspiratory Loop

• Variable extrathoracic airway obstruction is the most common cause of a flattened inspiratory loop, with vocal cord dysfunction (VCD) being the predominant diagnosis 1
• Other causes include laryngeal abnormalities such as exercise-induced laryngeal prolapse, exercise-induced laryngomalacia, and arytenoid collapse 1
• Structural causes include enlarged lymph nodes, tumors, tracheal stenosis, or other forms of upper airway obstruction 1

Diagnostic Pattern Recognition

• The flattening specifically occurs on the inspiratory portion of the flow-volume loop, creating a characteristic truncated or plateau appearance 1, 2
• This pattern contrasts with obstructive patterns that primarily affect the expiratory loop 1
• The inspiratory flattening reflects decreased airflow during inspiration due to inappropriate adduction of the vocal cords or other extrathoracic narrowing 1

Clinical Significance

• When present with normal spirometric values (normal FEV1/FVC ratio), a flattened inspiratory loop should prompt further evaluation for extrathoracic airway obstruction 2
• This finding is particularly significant when patients present with respiratory symptoms that are unresponsive to standard asthma or COPD therapy 1, 3
• The pattern may be intermittent, so multiple flow-volume loops should be examined to confirm the abnormality 2

Diagnostic Approach

• If one inspiratory curve shows an abnormality, all inspiratory curves from that PFT session should be reviewed 2
• If more than one inspiratory curve is abnormal, both anatomical and functional evaluation should be undertaken 2
• Direct visualization of the vocal cords via laryngoscopy during symptoms is the gold standard for confirming VCD 1
• Consider performing laryngoscopy during exercise for exercise-induced laryngeal dysfunction (EILD) 1

Differential Considerations

• VCD can coexist with asthma but typically doesn't respond to asthma medications 1
• Unlike exercise-induced bronchoconstriction (which affects expiration and peaks 5-20 minutes after exercise), VCD symptoms typically occur during exercise with inspiratory stridor 1
• Symptoms of VCD/EILD typically resolve within approximately 5 minutes of discontinuing exercise 1

Important Clinical Pearls

• A flattened inspiratory loop with normal spirometry values is often missed or underdiagnosed 2
• Only about 17% of patients with abnormal inspiratory curves receive appropriate evaluation 2
• The mid-flow ratio (ratio of forced expiratory flow at 50% to forced inspiratory flow at 50%) is typically increased in extrathoracic obstruction 2
• VCD is more common in young adult female athletes and middle school to high school-aged athletes 1

Management Implications

• Recognition of this pattern should prompt referral for direct visualization of the upper airway 1
• Patients with VCD typically do not respond to bronchodilators or other asthma medications 1
• Appropriate diagnosis can prevent unnecessary treatment with asthma medications and guide specific therapy for the underlying condition 1