Reference value for expiratory time constant calculated from the maximal expiratory flow-volume curve

Area of Science:

• Pulmonary Physiology
• Respiratory Mechanics
• Diagnostic Spirometry

Background:

• The expiratory time constant (RC_EXP) assesses respiratory mechanics in ventilated patients.
• Its application and reference values in spontaneously breathing individuals using maximal expiratory flow-volume (MEFV) curves are less understood.
• Investigating RC_EXP in relation to standard spirometry parameters is crucial for broader clinical application.

Purpose of the Study:

• To establish a reference value for RC_EXP derived from MEFV curves in a general patient population.
• To explore the correlation between RC_EXP and key spirometric indices of airflow limitation.
• To assess the feasibility of using RC_EXP for detecting airway obstruction in spontaneously breathing subjects.

Main Methods:

• Retrospective analysis of 777 spirometry records from patients aged 15+ undergoing preoperative pulmonary function testing.
• Calculation of RC_EXP from the MEFV curve's descending limb using MEF₅₀ and MEF₂₅.
• Definition of airway obstruction based on established Japanese spirometric reference values for FEV₁/FVC and FEV₁.

Main Results:

• A cut-off value for RC_EXP of 0.601 seconds was identified, with high diagnostic accuracy (AUC=0.934).
• RC_EXP demonstrated strong association with the FEV₁/FVC ratio.
• Moderate correlations were observed between RC_EXP and MMF/MEF₅₀, with weaker associations for MEF₂₅ and MEF₅₀/MEF₂₅.

Conclusions:

• An RC_EXP exceeding approximately 0.6 seconds may indicate the presence of airway obstruction.
• The study confirms the feasibility of applying the RC_EXP concept to spontaneously breathing individuals using a simplified calculation.
• This method offers a potential complementary tool for assessing respiratory mechanics and airflow limitation.