Flattening of the Inspiratory Limb of the Flow–Volume Loop
Flattening (or truncation) of the inspiratory limb of the flow–volume loop indicates a variable extrathoracic central or upper airway obstruction, with flow limitation occurring during inspiration. [1]
This pattern is evaluated as a functional or intermittent narrowing of the larynx or extrathoracic trachea rather than fixed intrathoracic obstruction. [1]
Physiologic Interpretation
A repeatable plateau of forced inspiratory flow with relatively preserved forced expiratory flow suggests variable extrathoracic obstruction. [1]
Failure to visualize a classic inspiratory plateau does not exclude central or upper airway pathology, and further evaluation is recommended when clinical suspicion exists. [1]
Recommended Evaluation
Repeatable inspiratory and expiratory flow–volume curves should be obtained with near-maximal patient inspiratory and expiratory effort. [1]
Suspected variable extrathoracic obstruction should prompt referral for endoscopic airway visualization, because spirometry pattern recognition only alerts to the possibility of obstruction. [1]
Flexible laryngoscopy should be performed when symptoms are occurring to document abnormal vocal cord/laryngeal closure or inducible laryngeal obstruction physiology. [2]
Provocation testing may be performed during pulmonary function testing (exercise challenge or inhaled methacholine/histamine) to reproduce symptoms, followed by breathing test confirmation and/or laryngoscopy when symptoms develop. [2]
Differential Diagnosis Targets
The inspiratory-loop flattening phenotype is used to target conditions that cause inspiratory flow limitation at the level of the larynx/vocal folds or extrathoracic trachea, including vocal cord dysfunction and inducible laryngeal obstruction variants. [1]
Clinical correlation is required because inadequate test effort, inability to perform the maneuver, or other conditions can also contribute to nonclassic inspiratory-flow contours. [3]
Management Strategy
Disease-specific medication is not the main treatment for vocal cord dysfunction/inducible laryngeal obstruction physiology. [2]
The main treatment is learning laryngeal control techniques that reduce inappropriate vocal fold or laryngeal closure during inspiration, typically delivered by a speech therapist or psychologist experienced in treatment of vocal cord dysfunction/inducible laryngeal obstruction. [2]
Stress-management interventions should be incorporated when emotional triggers are present. [2]
If comorbid asthma is present, control of asthma should be optimized to reduce overlapping dyspnea triggers. [2]
If symptoms are triggered by post-nasal drip or acid reflux (GERD), management of these triggers should be addressed with the treating clinician. [2]
Common Pitfalls to Avoid
Bronchodilator response patterns in spirometry do not rule out central or upper airway obstruction. [1]
Normal baseline pulmonary function testing can occur when symptoms are not active during testing, which can delay correct diagnosis without symptom-timed laryngoscopy. [2]
Failure to obtain repeatable forced inspiratory and forced expiratory loops with near-maximal effort decreases diagnostic value and can lead to under-recognition of inspiratory flow limitation patterns. [1]
Treatment Goals
The therapeutic goal is restoration of coordinated inspiratory airflow by reducing inappropriate laryngeal/vocal fold closure during symptomatic episodes using structured laryngeal control and trigger management. [2]