How do I interpret a pulmonary function test flow‑volume loop and volume‑time curve to differentiate obstructive, restrictive, and upper‑airway obstruction patterns?

Interpreting Pulmonary Function Test Graphs

To differentiate obstructive, restrictive, and upper airway obstruction patterns, examine both the flow-volume loop shape and volume-time curve alongside the FEV₁/VC ratio, with obstruction showing a "scooped out" concave expiratory curve and FEV₁/VC below the 5th percentile, restriction showing a convex pattern with preserved or elevated FEV₁/VC, and upper airway obstruction displaying characteristic plateau patterns on inspiratory or expiratory limbs. 1, 2

Essential First Steps in Graph Interpretation

Examine Both Display Types Together

• Always review both the flow-volume loop and volume-time curve together, as the flow-volume display provides superior detail for the initial portion (first 1 second) of the maneuver, while the volume-time graph shows more detail for the latter part. 1
• The flow-volume loop is particularly useful for assessing effort magnitude during initial portions and detecting submaximal efforts through overlay of multiple curves. 1
• Ensure the volume-time display includes at least 0.25 seconds (preferably 1 second) before exhalation starts to properly evaluate back-extrapolated volume and initial effort. 1

Verify Maneuver Quality and Repeatability

• The shapes of the maneuvers must be repeatable across at least three acceptable tests for any interpretation to be valid, especially when identifying plateau effects in upper airway obstruction, which can be mimicked by variable poor effort. 1
• Overlay multiple flow-volume curves registered at the point of maximal inhalation to evaluate repeatability, though interpretation becomes difficult if maximal inhalation varies between attempts. 1

Identifying Obstructive Patterns

Flow-Volume Loop Characteristics

• Look for a characteristic "scooped out" or concave appearance of the expiratory curve, with flows less than expected over the entire volume range. 1, 2
• The concave pattern becomes more pronounced with increasing severity of obstruction. 1
• In moderate airflow limitation (as in asthma), the concavity is present but less dramatic than in severe COPD. 1

Key Numeric Criteria

• FEV₁/VC ratio below the 5th percentile of predicted confirms obstruction, and this is the primary diagnostic criterion. 1, 2, 3
• Use VC rather than FVC when possible, as it more accurately identifies obstructive patterns. 2
• Peak expiratory flow (PEF) is typically reduced but is less specific than the FEV₁/VC ratio. 1

Volume-Time Curve Features

• The volume-time curve in obstruction shows a prolonged time to reach plateau, often requiring >6 seconds to complete exhalation. 1
• The curve demonstrates a gradual, slow rise rather than the rapid initial rise seen in normal subjects. 1

Critical Pitfall: The "Pseudo-Restrictive" Pattern

• When FEV₁ and FVC are both reduced but FEV₁/VC is normal or near-normal, this most frequently reflects submaximal effort or patchy small airway collapse, NOT restriction. 1, 2, 3
• This pattern may also occur when flow is so slow that the subject cannot exhale long enough to empty the lungs to residual volume. 1
• The flow-volume curve should appear concave toward the end of the maneuver if this represents true obstruction with incomplete emptying. 1
• Always measure TLC to distinguish true restriction from this pseudo-restrictive pattern—TLC will be normal or elevated in obstruction with air-trapping. 1, 2, 3

Identifying Restrictive Patterns

Flow-Volume Loop Characteristics

• The flow-volume curve shows a convex pattern with flow higher than expected at a given lung volume, appearing "tall and narrow". 1, 2
• The entire loop is shifted to the left (toward lower volumes) but maintains its shape. 1
• The expiratory curve lacks the concavity seen in obstruction. 1, 2

Key Numeric Criteria

• FEV₁/VC ratio is normal (>5th percentile) or even elevated (>85-90%), which distinguishes restriction from obstruction. 1, 2, 4
• Both FEV₁ and VC are proportionally reduced. 1, 4
• TLC below the 5th percentile of predicted is mandatory to confirm true restrictive defect—spirometry alone cannot prove restriction. 1, 2, 4

Volume-Time Curve Features

• The volume-time curve shows a rapid initial rise but reaches a lower total volume than predicted. 1
• Time to plateau is normal or even shortened compared to normal subjects. 1

Essential Diagnostic Rule

• A reduced VC with normal or elevated FEV₁/VC only suggests restriction—you must measure TLC by body plethysmography to confirm the diagnosis. 1, 2, 4
• A reduced VC is associated with a low TLC only about half the time. 1
• Never diagnose restriction based on single-breath TLC estimates (such as VA from DLCO testing), as these systematically underestimate TLC by up to 3 liters in severe obstruction. 1, 3

Identifying Upper Airway Obstruction Patterns

Fixed Upper Airway Obstruction

• Both inspiratory and expiratory limbs show plateau (flattening) patterns, creating a "box-like" appearance. 1
• The plateau must be reproducible across multiple maneuvers to distinguish it from poor effort. 1
• Maximum flows are limited equally during inspiration and expiration. 1

Variable Extrathoracic Obstruction

• The inspiratory limb shows marked flattening or plateau, while the expiratory limb is relatively preserved. 1
• This occurs because negative intraluminal pressure during inspiration worsens the obstruction, while positive pressure during expiration tends to open the airway. 1
• Common causes include vocal cord paralysis and tracheomalacia above the thoracic inlet. 1, 5

Variable Intrathoracic Obstruction

• The expiratory limb shows flattening or plateau, while the inspiratory limb is relatively preserved. 1
• During expiration, positive pleural pressure compresses the intrathoracic airway, worsening the obstruction. 1
• During inspiration, negative pleural pressure opposes the negative intraluminal pressure, limiting the effect of the obstruction. 1

Specific Diagnostic Criteria for Upper Airway Obstruction

• Forced inspiratory flow at 50% of vital capacity (FIF₅₀) ≤100 L/min suggests upper airway obstruction. 6
• Ratio of FEF₅₀/FIF₅₀ ≥1.0 indicates disproportionate limitation of inspiratory flow. 6
• FEV₁/PEFR ratio ≥10 mL/L/min is another useful indicator. 6

Critical Evaluation Steps

• If any inspiratory curve shows abnormality, review all inspiratory curves from that testing session—if more than one is abnormal, pursue both anatomical and functional evaluation. 5
• An abnormal inspiratory curve with otherwise normal spirometry warrants evaluation for upper airway pathology. 5
• Confirm suspected upper airway obstruction with imaging (CT) and/or endoscopic visualization—flow-volume loops alert you to the possibility but cannot definitively diagnose the anatomic lesion. 1

Additional Patterns

• Flow oscillations (saw-tooth pattern) on either inspiratory or expiratory phase suggest mechanical instability of the airway wall. 1
• Unilateral main bronchus obstruction may show higher flow at the beginning of forced inspiration than toward the end, reflecting delayed gas filling. 1

Mixed Obstructive-Restrictive Pattern

• Both FEV₁/VC ratio is reduced (below 5th percentile) AND TLC is reduced (below 5th percentile). 1
• The flow-volume loop shows features of both patterns: concave expiratory curve with overall reduced volumes. 1
• This pattern requires measurement of lung volumes for definitive diagnosis. 1

Systematic Interpretation Algorithm

1. First, assess maneuver quality and repeatability—without reproducible efforts, no interpretation is valid. 1

2. Second, calculate FEV₁/VC ratio:

   • If <5th percentile → obstruction present 1, 2, 3
   • If ≥5th percentile → obstruction ruled out 1, 2

3. Third, examine the expiratory flow-volume curve shape:

   • Concave ("scooped") → confirms obstruction 1, 2
   • Convex → suggests restriction 1, 2
   • Plateau on expiration → consider variable intrathoracic upper airway obstruction 1

4. Fourth, examine the inspiratory flow-volume curve:

   • Plateau or flattening → evaluate for upper airway obstruction 1, 5
   • Normal → upper airway obstruction unlikely 1, 5

5. Fifth, if VC is reduced with normal FEV₁/VC:

   • Measure TLC to distinguish true restriction from pseudo-restriction 1, 2, 4, 3
   • TLC low → true restriction 1, 2, 4
   • TLC normal/high → air-trapping or poor effort 1, 2, 3

6. Sixth, assess for hyperinflation in obstruction:

   • Measure RV and RV/TLC ratio—elevation above upper limit of normal suggests emphysema or severe asthma 1, 3
   • TLC normal or elevated confirms pure obstructive pattern 1, 3

Common Pitfalls to Avoid

• Do not diagnose restriction without measuring TLC—reduced FVC alone proves nothing. 1, 2, 4, 3
• Do not ignore poor effort as the most common cause of proportionally reduced FEV₁ and FVC with normal ratio. 1, 2
• Do not overlook the inspiratory curve—4.6% of patients have abnormal inspiratory curves, but evaluation is undertaken in only 17% of cases. 5
• Do not assume normal PFTs exclude clinically important tracheobronchomalacia—21% of patients with moderate to severe TBM have normal spirometry and 17% have no distinctive flow-volume loop abnormality. 7
• Do not use single-breath VA measurements to confirm restriction—they underestimate TLC by up to 3 liters in severe obstruction. 1, 3