What is the difference between simple and complex restrictive patterns on spirometry?

Distinguishing Simple from Complex Restrictive Patterns on Spirometry

A "simple" restrictive pattern refers to spirometry showing reduced vital capacity with a normal or elevated FEV₁/VC ratio (≥5th percentile), while a "complex" restrictive pattern indicates confirmed true restriction (TLC <5th percentile) measured by body plethysmography—the critical distinction is that simple patterns on spirometry alone are unreliable and require lung volume measurement to confirm actual restriction. 1, 2

Understanding the Simple Restrictive Pattern

Simple restriction is a spirometric suggestion, not a diagnosis:

• Reduced FVC with normal or elevated FEV₁/VC ratio (>85–90%) creates a spirometric pattern suggestive of restriction, but this is frequently misleading. 1
• Only 41–58% of patients with reduced FVC actually have true restriction when TLC is measured—the majority have other explanations such as poor effort, gas trapping, or submaximal exhalation. 3, 4
• The flow-volume loop appears "tall and narrow" with a convex shape, showing higher flows than expected for a given lung volume, with the entire loop shifted leftward. 2
• The volume-time curve shows a rapid initial rise but reaches a lower total volume with normal or shortened time to plateau. 2

Critical pitfall: A reduced VC does not prove restriction—it may be suggestive when FEV₁/VC is normal or increased, but spirometry alone cannot establish a restrictive defect. 1, 2

Defining the Complex Restrictive Pattern

Complex restriction requires objective confirmation beyond spirometry:

• TLC <5th percentile of predicted measured by body plethysmography is the mandatory criterion to confirm true restrictive lung disease. 1, 2
• This measurement distinguishes true parenchymal or chest wall restriction from pseudo-restrictive patterns caused by obstruction with gas trapping. 2, 5
• Single-breath TLC estimates (VA from DLCO) systematically underestimate TLC by up to 3 liters in severe obstruction and must never be used to diagnose restriction. 1, 2

When TLC is measured and confirms restriction, you can then characterize:

• Pure restriction: TLC <5th percentile with FEV₁/VC ≥5th percentile. 1
• Mixed obstruction-restriction: Both TLC <5th percentile AND FEV₁/VC <5th percentile, indicating coexistence of both defects. 1, 5

Algorithmic Approach to Interpretation

Step 1: Assess the FEV₁/VC ratio

• FEV₁/VC <5th percentile → obstruction is present; proceed to measure lung volumes to assess for coexisting restriction. 1, 2
• FEV₁/VC ≥5th percentile → obstruction is ruled out; if VC is reduced, consider measuring TLC. 1, 2

Step 2: Evaluate vital capacity

• Normal VC (≥80% predicted) → restriction is extremely unlikely (<3% probability); lung volume measurement can be avoided unless clinical suspicion is high. 3, 6
• Reduced VC with normal FEV₁/VC → measure TLC to differentiate true restriction from pseudo-restriction or poor effort. 1, 2

Step 3: Measure TLC when indicated

• TLC <5th percentile → confirms true restriction (complex pattern). 1, 2
• TLC normal or elevated → excludes restriction; the reduced VC likely reflects gas trapping, hyperinflation, or submaximal effort. 2, 5, 3

Step 4: Assess for mixed defects in obstructive patients

• When FEV₁/VC is low and VC is also reduced, a superimposed restriction cannot be ruled out without TLC measurement. 1
• Conversely, when FEV₁/VC is low but VC is normal, superimposed restriction can be confidently excluded. 1

Key Clinical Nuances

The predictive value of spirometry varies by severity:

• Positive predictive value is poor: Even with a classic "restrictive pattern" (low FVC, elevated FEV₁/FVC), only 58% have confirmed restriction on lung volumes. 3, 4
• Negative predictive value is excellent: Normal VC makes restriction highly unlikely (negative predictive value >97%). 3, 6
• Severity matters: Spirometry can reliably predict restriction only when FVC is severely reduced (<55% predicted in males, <40% in females); mild reductions are unreliable. 6

Common causes of pseudo-restrictive patterns:

• Poor effort or submaximal exhalation is the most common cause of proportionally reduced FEV₁ and FVC with a normal ratio. 1, 2
• Gas trapping in severe obstruction prevents complete exhalation, artificially lowering FVC while TLC remains normal or elevated. 5, 3
• Early airway closure in obstructive disease can mimic restriction on spirometry. 5

Special consideration for borderline TLC values:

• When TLC is at the lower limit of normal and coexists with a disease expected to cause restriction (e.g., lung resection, interstitial lung disease), the restrictive defect may be difficult to prove statistically even though it is clinically present. 1
• In these cases, additional testing (DLCO, gas exchange, exercise testing) should be performed to fully characterize the functional impairment. 1