What is the interpretation of a pulmonary function test (PFT) showing increased residual volume (RV) and RV/total lung capacity (TLC) ratio, decreased TLC and forced vital capacity (FVC), and a high forced expiratory volume in 1 second (FEV1)/FVC ratio?

Pulmonary Function Test Interpretation: Complex Restrictive Pattern

The PFT results demonstrate a complex restrictive pattern characterized by reduced lung volumes (TLC 84%, FVC 63%, VC IN 66%), air trapping (elevated RV 137%, RV/TLC 170%), and preserved to increased flow rates (FEV1/FVC 112%, elevated MEF values). 1

Physiologic Interpretation

This pattern represents a complex restrictive ventilatory defect with several key components:

• Reduced lung volumes: TLC (84%), FVC (63%), and VC IN (66%) are all below normal limits
• Air trapping: Elevated RV (137%) and markedly elevated RV/TLC ratio (170%)
• Preserved/increased flows: FEV1/FVC ratio is high (112%), with elevated flow rates (PEF 120%, MEF values 138-186%)
• Normal DLCO and normal inspiratory HRCT

This combination suggests a condition where there is difficulty fully emptying the lungs despite normal large airway function, leading to a pattern that appears restrictive on initial assessment but has features of air trapping typically seen in obstructive disorders. 1, 2

Differential Diagnosis

Most Likely Mechanisms

1. Neuromuscular weakness

   • Weakness of respiratory muscles leads to reduced inspiratory capacity and early airway closure
   • Results in air trapping despite normal intrinsic airway function
   • Consistent with preserved flow rates and normal DLCO 1, 2

2. Obesity or chest wall restriction

   • Can cause reduced lung volumes with preserved flow rates
   • Associated with early small airway closure and air trapping
   • Normal DLCO would be consistent with this mechanism 1, 2

3. Small airway dysfunction

   • Early closure of small airways during exhalation
   • Results in air trapping (elevated RV) despite normal flow in larger airways
   • High FEV1/FVC ratio and normal DLCO argue against significant large airway obstruction 1, 3

Less Likely Considerations

1. Upper airway obstruction

   • Can present with preserved FEV1/FVC ratio and increased flow rates
   • However, typically shows characteristic flow-volume loop patterns not mentioned in this case 4

2. Mixed obstructive-restrictive disease

   • Conditions like sarcoidosis, hypersensitivity pneumonitis
   • Normal HRCT makes parenchymal disease less likely 1

Clinical Significance

The pattern seen here is consistent with what has been termed "complex restriction" in the literature, which is a common but often underrecognized pattern. Studies have shown that this pattern is frequently associated with:

• Younger age and female predominance
• Higher prevalence of neuromuscular disease
• Extreme BMI (>40 kg/m² or <18.5 kg/m²)
• Diaphragmatic dysfunction
• Bronchiectasis (though normal HRCT makes this less likely here) 2

Recommended Further Evaluation

1. Clinical assessment

   • Evaluate for neuromuscular disorders (weakness, fatigue, dysphagia)
   • Assess BMI and chest wall mechanics
   • Consider symptoms of upper airway dysfunction 1

2. Additional testing

   • Maximal respiratory pressures (MIP/MEP) to assess respiratory muscle strength
   • Expiratory CT to evaluate for air trapping not visible on inspiratory images
   • Bronchodilator reversibility testing - improvement in FVC without FEV1 improvement may indicate small airway disease 1, 5

Common Pitfalls to Avoid

1. Misclassifying as simple restriction

   • Relying solely on FVC to diagnose restriction without TLC measurement can be misleading
   • Studies show that <60% of patients with a restrictive pattern on spirometry have true restriction confirmed by lung volume measurements 6

2. Missing underlying obstructive component

   • The elevated RV/TLC ratio strongly suggests an obstructive component despite the normal FEV1/FVC
   • Research shows that reversible restrictive patterns often represent variants of obstructive lung disease with early airway closure 3

3. Overlooking neuromuscular causes

   • Complex restrictive patterns are commonly associated with neuromuscular weakness
   • Failure to assess respiratory muscle strength can miss treatable conditions 1, 2

This PFT pattern requires careful clinical correlation and potentially additional testing to determine the underlying cause, as the physiologic mechanism appears to involve reduced lung volumes with concurrent air trapping despite normal large airway function.