How do you interpret a lung volume loop in pulmonary function tests?

How to Interpret a Lung Volume Loop (Flow-Volume Loop)

Begin by examining the flow-volume loop shape and pattern alongside key spirometric values (FEV1, VC, FEV1/VC ratio) and total lung capacity (TLC) to systematically classify the ventilatory defect as obstructive, restrictive, mixed, or normal. 1

Step 1: Assess the FEV1/VC Ratio First

• Check if FEV1/VC is below the 5th percentile of predicted to identify obstruction 1
• Use VC (the largest vital capacity from any maneuver) rather than just FVC, as VC more accurately identifies obstructive patterns since FVC is more dependent on flow and volume histories 1
• If FEV1/VC is normal or increased (>85-90%), proceed to evaluate for restriction 1

Step 2: Examine the Flow-Volume Loop Shape

Obstructive Pattern

• Look for flows that are less than expected over the entire volume range with a characteristic "scooped out" or concave appearance of the expiratory curve 1
• The loop may show reduced peak expiratory flow and progressive flattening toward the end of expiration 1
• Beware: When both FEV1 and FVC are concomitantly decreased with a normal FEV1/FVC ratio, this most frequently reflects poor effort or failure to inhale/exhale completely, NOT obstruction 1, 2

Restrictive Pattern

• The flow-volume curve shows a convex pattern with flow higher than expected at a given lung volume 1
• The loop appears narrow and tall, with reduced total volume but preserved or increased flow rates relative to volume 1
• Critical caveat: A reduced VC with normal FEV1/VC does NOT prove restriction—it is only suggestive and requires TLC measurement for confirmation 1, 2

Mixed Pattern

• Shows features of both obstruction (concave expiratory curve) and restriction (reduced total volume) 1
• Requires both FEV1/VC and TLC below the 5th percentile for definitive diagnosis 1

Step 3: Evaluate the Inspiratory Curve

• Examine the inspiratory portion of the loop for truncation, flattening, or abnormal shape to detect upper airway obstruction 3, 4
• Fixed upper airway obstruction shows flattening of both inspiratory and expiratory curves 3
• Variable extrathoracic obstruction shows flattening primarily of the inspiratory curve 3, 4
• Variable intrathoracic obstruction shows flattening primarily of the expiratory curve 3
• If the inspiratory curve is abnormal on more than one loop, pursue anatomical and functional evaluation for upper airway pathology 4

Step 4: Confirm with Lung Volume Measurements

For Suspected Obstruction

• Measure TLC, RV, and RV/TLC ratio to assess for hyperinflation and confirm obstruction 1
• Increased TLC, RV, or RV/TLC above upper limits suggests emphysema, asthma, or other obstructive diseases 1
• Normal TLC with reduced FEV1/FVC may indicate patchy peripheral airway collapse rather than classic obstruction 1

For Suspected Restriction

• TLC measurement by body plethysmography is mandatory to confirm true restriction (TLC <5th percentile) 1, 2
• A reduced VC is associated with low TLC only about half the time 1
• Never use single-breath TLC estimates (like VA from DLCO test) to diagnose restriction, as these systematically underestimate TLC, especially in obstruction where underestimation can reach 3 liters 1, 2

Step 5: Integrate Clinical Context

• When FEV1/VC is low and VC is normal, restriction can be ruled out 1
• When FEV1/VC is low and VC is reduced without TLC measurement, state that VC is reduced (likely from hyperinflation) but superimposed restriction cannot be ruled out 1
• If values are near the lower limit of normal, consider additional testing: bronchodilator response, DLCO, gas exchange evaluation, or exercise testing 1

Common Pitfalls to Avoid

• Do not diagnose restriction based on spirometry alone—always confirm with TLC measurement 1, 2
• Do not overlook poor effort as the most common cause of proportionally reduced FEV1 and FVC with normal ratio 1, 2
• Do not ignore the inspiratory curve—abnormalities may indicate upper airway obstruction that requires specific evaluation 4
• In patients with severe COPD or asthma refractory to treatment, maintain high suspicion for upper airway masses that can mimic distal airway obstruction on the flow-volume loop 5
• Review all flow-volume loops from the testing session if one shows an abnormality, as patterns may vary between efforts 4