What does a flattened expiratory flow loop on a Pulmonary Function Test (PFT) indicate?

Flattened Expiratory Flow Loop on PFT

A flattened expiratory flow loop indicates fixed upper airway obstruction when both inspiratory and expiratory limbs are flattened, or variable intrathoracic obstruction when only the expiratory limb shows flattening, though this pattern can also represent severe distal airway obstruction from COPD or asthma. 1

Primary Diagnostic Patterns

Fixed Upper Airway Obstruction

• When both inspiratory AND expiratory limbs show plateau/flattening, this indicates fixed upper airway obstruction such as tracheal stenosis, tumors, or enlarged lymph nodes compressing the trachea 1
• The flattening must be reproducible across three or more maneuvers to distinguish from poor effort, which typically varies between attempts 1
• This pattern requires direct visualization of the upper airway via bronchoscopy or CT imaging to identify the structural lesion 1

Variable Intrathoracic Obstruction

• Isolated expiratory flattening (with normal inspiratory limb) suggests variable intrathoracic obstruction, where the airway collapses during forced expiration 1
• Causes include tracheobronchomalacia, large endobronchial tumors, or dynamic airway collapse 2, 3
• In tracheobronchomalacia, 81.6% of patients show low maximum forced expiratory flow with potential biphasic morphology (19.7%) or notched expiratory loop (9.2%) 3

Severe Distal Airway Obstruction

• A critical pitfall: smooth flattening of the expiratory curve can mimic distal airway obstruction from severe COPD or asthma rather than upper airway pathology 2
• This pattern shows relatively normal peak expiratory flow followed by smooth flattening, representing small airway collapse and air trapping 2, 4
• Late expiratory flattening with an AB-BC angle <149.7° on flow-volume loop analysis correlates with small airway disease on CT imaging in 47.3% of cases 4

Algorithmic Diagnostic Approach

Step 1: Assess Loop Reproducibility

• Review all three acceptable maneuvers - variable flattening suggests poor effort rather than true obstruction 1
• Fixed obstruction produces consistent plateau patterns across all attempts 1

Step 2: Determine Which Limb is Affected

• Both limbs flattened → Fixed upper airway obstruction → Proceed to imaging (CT chest/neck) or bronchoscopy 1
• Only expiratory limb flattened → Distinguish between variable intrathoracic obstruction vs. severe distal obstruction 1, 2

Step 3: Evaluate Peak Flow and FEV1/FVC

• If peak expiratory flow is markedly reduced with plateau from onset → Variable intrathoracic obstruction 1
• If peak flow relatively preserved with late flattening and FEV1/FVC <0.70 → Severe COPD/asthma pattern 2, 4

Step 4: Assess Response to Bronchodilator

• No response to bronchodilator with persistent flattening strongly suggests structural upper airway lesion rather than reversible airway disease 2
• Patients with large endotracheal masses fail standard COPD therapy despite appearing to have severe obstructive disease 2

Critical Clinical Pitfalls

Delayed Diagnosis of Upper Airway Masses

• High suspicion is essential when patients with "severe COPD" fail to improve with standard therapy - this may represent misdiagnosed upper airway obstruction 2
• Large endotracheal tumors can produce atypical flow-volume loops mimicking distal airway disease, leading to significant diagnostic delays 2
• The presence of emphysema and tobacco use does not exclude concurrent upper airway pathology 2

Tracheobronchomalacia Underrecognition

• PFTs are normal in 21.1% of patients with moderate to severe tracheobronchomalacia, so normal spirometry does not exclude clinically important disease 3
• 17.1% of symptomatic TBM patients have no distinctive flow-volume loop abnormality 3
• PFTs and flow-volume loops should not be used to decide whether TBM is present or clinically important - bronchoscopy during forced expiration remains the gold standard 3

Small Airway Disease Detection

• Late expiratory flattening may be the earliest manifestation of obstructive lung disease before FEV1/FVC becomes abnormal 4
• Visual assessment of flattening is highly variable between interpreters (Fleiss' kappa 0.320), but improves to 0.522 when using objective angle measurements 4
• Measuring the AB-BC angle provides objective quantification: values <149.7° indicate significant late expiratory flattening consistent with small airway dysfunction 4

When to Pursue Further Evaluation

Immediate bronchoscopy or CT imaging indicated when:

• Reproducible plateau pattern on both inspiratory and expiratory limbs 1
• Isolated expiratory flattening with markedly reduced peak flow and no bronchodilator response 2
• Clinical symptoms disproportionate to spirometric severity 2, 3
• "Severe COPD" pattern refractory to standard therapy 2

Consider dynamic bronchoscopy when:

• Expiratory flattening with symptoms of airway collapse (cough, dyspnea) but normal static imaging 3
• Suspected tracheobronchomalacia based on clinical presentation 3