Flow Decay: A Novel Spirometric Index to Quantify Dynamic Airway Resistance

Area of Science:

• Pulmonary Medicine
• Respiratory Physiology
• Diagnostic Tools

Background:

• Obstructive lung diseases cause dynamic airway resistance, leading to a characteristic concavity in spirometry's mid-expiratory flow-volume loop.
• Current methods for quantifying this obstruction can be complex, necessitating simpler clinical tools.

Purpose of the Study:

• To develop and validate a straightforward model to measure exponential airflow decay during forced exhalation.
• To quantify dynamic airway obstruction and improve clinical detection of obstructive airway diseases.

Main Methods:

• Calculated flow decay as the slope of volume versus ln(1/flow) during mid-exhalation.
• Derived normal ranges in healthy volunteers and validated the upper limit of normal (mean + 2 × SD) in separate healthy and patient groups.

Main Results:

• The upper limit of normal for flow decay was established at 0.802 L⁻¹.
• The model demonstrated high sensitivity (94%) and specificity (92%) in distinguishing reversible airway obstruction and (92% sensitivity, 95% specificity) obstruction with hyperinflation and air trapping from healthy subjects.

Conclusions:

• Flow decay serves as a reproducible index for quantifying dynamic airway obstruction.
• This spirometry-derived measure effectively differentiates individuals with obstructive lung defects from healthy controls, supporting clinical application.