Law of Plethysmography
The law of plethysmography is fundamentally based on Boyle's Law, which states that pressure and volume are inversely related when temperature remains constant. 1, 2
Core Principle
The plethysmographic technique operates on the principle that when a patient breathes inside a sealed, rigid chamber, any compression or decompression of thoracic gas creates mirror-image pressure changes in the chamber itself. 1 This relationship allows for the measurement of:
- Functional residual capacity (FRC) - by relating changes in alveolar pressure to changes in plethysmographic volume during airway occlusion 1
- Airway resistance (Raw) - by relating changes in box pressure (reflecting alveolar pressure) to simultaneous airflow changes 1
Fundamental Relationships
For Lung Volume Measurement
During airway occlusion, the patient makes respiratory efforts against a closed shutter, compressing and rarefying thoracic gas. 1 The total volume of gas within the thorax (TOGV) is determined from the ratio of change in plethysmographic volume (ΔVpleth) to change in airway opening pressure (ΔPao) during these respiratory efforts. 1
The key assumption is that during periods of zero airflow, pressure at the airway opening reflects changes in alveolar pressure. 1
For Airway Resistance Measurement
When respired gas is maintained at BTPS (Body Temperature and Pressure, Saturated) conditions, changes in plethysmographic pressure during spontaneous breathing are inversely proportional to changes in alveolar pressure. 1 By relating these box pressure changes to simultaneous airflow changes at the airway opening, airway resistance can be calculated. 1
Critical Technical Requirements
The measurement relies on the fact that generation of airflow requires pressure generation, which means a mass of air is compressed or decompressed relative to its equilibrium volume—this difference is called "shift volume." 2 Since the body box is sealed with rigid walls, its free volume experiences the same mirror-image shift volume as the lung. 2
Essential Conditions
- The chamber must be completely sealed and rigid to ensure accurate pressure equilibration 1
- Gas must be maintained at BTPS conditions for resistance measurements using heated rebreathing bags 1
- Drift correction must be applied to account for thermal changes during measurements 1
Clinical Significance
The plethysmographic method measures true airway resistance (relating alveolar pressure to airflow), which differs from pulmonary resistance (lung tissue plus airway) or respiratory resistance (airway plus tissue plus chest wall). 1 This distinction is critical for accurate interpretation of obstructive airway disease. 2
The technique also provides specific airway resistance (sRaw), which represents the work performed by volume displacement to establish a given flow rate, offering complementary information to Raw measurements. 2