How is total lung capacity (TLC) evaluated?

Total Lung Capacity Evaluation Methods

Total lung capacity (TLC) is primarily evaluated using two main methods: body plethysmography and gas dilution techniques (helium dilution or nitrogen washout), with plethysmography being the gold standard as it measures all lung volumes including trapped gas. 1

Primary Measurement Techniques

Body Plethysmography

• Principle: Measures all gas in the thorax including trapped gas using Boyle's law
• Procedure:
  • Patient sits in an airtight chamber (body box)
  • Breathes through a mouthpiece with an occlusion valve
  • During airway occlusion at FRC, patient makes respiratory efforts
  • Changes in box pressure and mouth pressure are measured
  • Thoracic gas volume is calculated from these pressure changes
• Advantages:
  • Measures all lung gas including trapped gas
  • Most accurate in obstructive lung disease
  • Considered the gold standard for TLC measurement 1, 2

Gas Dilution Techniques

1. Helium Dilution

   • Principle: Closed-circuit system with known helium concentration
   • Procedure:
     • Patient breathes from a reservoir with known helium concentration
     • Helium equilibrates throughout the accessible lung volume
     • Final helium concentration is measured
     • FRC is calculated from the dilution of helium
     • TLC = FRC + Inspiratory Capacity
   • Limitation: Underestimates TLC in patients with air trapping 1, 2

2. Nitrogen Washout

   • Principle: Measures the volume of nitrogen washed out from the lungs
   • Procedure:
     • Patient breathes 100% oxygen
     • Nitrogen is washed out from the lungs
     • Total volume of nitrogen exhaled is measured
     • FRC is calculated, then TLC = FRC + Inspiratory Capacity
   • Limitation: Also underestimates TLC in obstructive disease 1

Differences Between Techniques

• Measurement Discrepancies:
  • In healthy adults, plethysmography and gas dilution yield similar results 1
  • In obstructive diseases (COPD), plethysmography can measure TLC up to 2L greater than gas dilution methods 2, 3
  • The difference increases with severity of gas trapping (higher RV%, FRC%, TLC%) 2, 3
  • In infants, plethysmography values are approximately 13% greater than nitrogen dilution values 1

Alternative Methods

Radiographic Methods

• Chest Radiography:
  • TLC can be estimated using PA and lateral chest X-rays
  • Uses Barnhard method (radiographic ellipses)
  • Automated computer-based methods show high correlation (r=0.98) with manual Barnhard method 4, 5
  • Less commonly used in clinical practice

High-Resolution Computed Tomography (HRCT)

• Can measure TLC with specialized software
• Shows strong correlation with plethysmography (r=0.92)
• Generally yields smaller values than plethysmography 3

Standardized Reporting

The American Thoracic Society recommends standardized reporting of lung volumes including:

• Measured values
• Predicted values
• Lower limit of normal (LLN)
• Z-scores
• Percent of predicted values 1

Clinical Applications

• Restrictive Disease: Reduced TLC defines chest restriction 6
• Obstructive Disease: Increased TLC, FRC, and RV are often seen 6
• Mixed Disease: Lung volume measurements are especially helpful when spirometry results are difficult to interpret 6

Common Pitfalls

• Method Selection: Using gas dilution in patients with obstructive disease may significantly underestimate TLC
• Reference Values: Different reference equations exist for different age groups and techniques
• Statistical Models: In adults, linear regression models are appropriate, while in children and infants, logarithmic transformations are required 1
• Technical Factors: Proper technique is essential; poor patient effort or cooperation can affect results

Body plethysmography should be the preferred method when evaluating patients with suspected air trapping or obstructive lung disease, while either technique may be appropriate for patients with suspected restrictive disease.