What is the interpretation of a pulmonary function test (PFT) pattern 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?

Interpretation of Pulmonary Function Test Pattern

The PFT pattern demonstrates a restrictive ventilatory defect with air trapping, most consistent with a complex restrictive pattern that may be due to neuromuscular weakness, obesity, or early small airway closure. 1, 2

Analysis of Key PFT Parameters

Lung Volumes

• TLC: 84% of predicted - mildly reduced
• RV: 137% of predicted - significantly elevated
• RV/TLC: 170% of predicted - markedly elevated
• FVC: 63% of predicted - moderately reduced
• VC IN: 66% of predicted - moderately reduced

Flow Parameters

• FEV1: 71% of predicted - mildly reduced
• FEV1/FVC ratio: 112% - elevated (supranormal)
• PEF: 120% of predicted - elevated
• MEF values (75,50,25): all elevated (138-186% of predicted)

Physiologic Interpretation

This pattern shows several key abnormalities:

1. Restrictive Component:

   • Reduced TLC (84% of predicted)
   • Reduced FVC (63% of predicted)
   • Reduced VC IN (66% of predicted)

2. Air Trapping:

   • Elevated RV (137% of predicted)
   • Markedly elevated RV/TLC ratio (170% of predicted)

3. Preserved or Enhanced Flows:

   • Elevated FEV1/FVC ratio (112%)
   • Normal to increased flow rates (PEF, MEF values)
   • Normal DLCO

Differential Diagnosis

This pattern represents what has been termed a "complex restrictive" pattern 3, characterized by:

• Disproportionate reduction in FVC compared to TLC
• Air trapping (elevated RV and RV/TLC)
• Normal to high flow rates

Potential Mechanisms

1. Neuromuscular Weakness:

   • Weakness of respiratory muscles can cause reduced TLC and FVC
   • Inability to fully exhale leads to air trapping and elevated RV
   • The European Respiratory Society guidelines note this pattern in neuromuscular disorders 2

2. Obesity or Chest Wall Restriction:

   • Can cause restrictive pattern with reduced lung volumes
   • May be associated with small airway closure and air trapping 2, 3

3. Early Small Airway Disease:

   • Early closure of small airways during exhalation
   • Results in air trapping despite normal flow rates in larger airways
   • The high FEV1/FVC ratio and normal DLCO argue against significant obstructive disease 4

4. Upper Airway Dysfunction:

   • The elevated PEF and preserved flow rates could suggest variable extrathoracic obstruction
   • This can present with a restrictive pattern on spirometry 5

Clinical Implications

The normal DLCO and normal inspiratory HRCT are important findings that help exclude:

• Significant interstitial lung disease
• Emphysema
• Pulmonary vascular disease

The combination of reduced TLC, elevated RV/TLC ratio, and preserved flow rates is not typical of classic obstructive or restrictive diseases, suggesting a more complex pathophysiology.

Recommended Further Evaluation

1. Complete clinical assessment:

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

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
   • Consider bronchodilator reversibility testing 6

Pitfalls to Avoid

1. Misdiagnosing as pure restriction: The elevated RV and RV/TLC ratio indicate air trapping, which is not typical of pure restrictive disease 1, 2

2. Overlooking neuromuscular weakness: This pattern can be seen in early neuromuscular disease before obvious clinical symptoms 2, 3

3. Assuming normal parenchyma based only on inspiratory HRCT: Air trapping may only be visible on expiratory imaging 2

4. Relying solely on spirometry: As shown in this case, comprehensive lung volumes are essential for proper interpretation of complex patterns 7