Flattened Inspiratory Loop on Pulmonary Function Test Interpretation

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, according to the American Academy of Allergy, Asthma, and Immunology 1
Other causes include laryngeal abnormalities such as exercise-induced laryngeal prolapse, exercise-induced laryngomalacia, and arytenoid collapse, as reported by the American Academy of Allergy, Asthma, and Immunology 2
Structural causes include enlarged lymph nodes, tumors, tracheal stenosis, or other forms of upper airway obstruction, as noted by the American Academy of Allergy, Asthma, and Immunology and the European Respiratory Society 1, 3

The flattening specifically occurs on the inspiratory portion of the flow-volume loop, creating a characteristic truncated or plateau appearance, according to the American Academy of Allergy, Asthma, and Immunology 2
This pattern contrasts with obstructive patterns that primarily affect the expiratory loop, as reported by the European Respiratory Society 3
The inspiratory flattening reflects decreased airflow during inspiration due to inappropriate adduction of the vocal cords or other extrathoracic narrowing, as noted by the American Academy of Allergy, Asthma, and Immunology 2

Patients with vocal cord dysfunction typically do not respond to standard asthma or COPD therapy, according to the American Academy of Allergy, Asthma, and Immunology 1

Direct visualization of the vocal cords via laryngoscopy during symptoms is the gold standard for confirming VCD, as reported by the American Academy of Allergy, Asthma, and Immunology 2
Consider performing laryngoscopy during exercise for exercise-induced laryngeal dysfunction (EILD), according to the American Academy of Allergy, Asthma, and Immunology 2

VCD can coexist with asthma but typically doesn't respond to asthma medications, as noted by the American Academy of Allergy, Asthma, and Immunology 1
Unlike exercise-induced bronchoconstriction, VCD symptoms typically occur during exercise with inspiratory stridor, according to the American Academy of Allergy, Asthma, and Immunology 2
Symptoms of VCD/EILD typically resolve within approximately 5 minutes of discontinuing exercise, as reported by the American Academy of Allergy, Asthma, and Immunology 2

VCD is more common in young adult female athletes and middle school to high school-aged athletes, according to the American Academy of Allergy, Asthma, and Immunology 2

Recognition of this pattern should prompt referral for direct visualization of the upper airway, as recommended by the European Respiratory Society 3
Patients with VCD typically do not respond to bronchodilators or other asthma medications, as noted by the American Academy of Allergy, Asthma, and Immunology 1, 2
Appropriate diagnosis can prevent unnecessary treatment with asthma medications and guide specific therapy for the underlying condition, according to the American Academy of Allergy, Asthma, and Immunology 1