Distal Airway Function Assessed by Oscillometry at Varying Respiratory Rate: Comparison with Dynamic Compliance

Abstract

Distal airways disease causes significant morbidity yet remains insufficiently identified. We hypothesize that: () when spirometry is normal impulse oscillometry may provide information about mechanical properties of the distal airways in a manner comparable to dynamic compliance and () variation of breathing frequency will influence oscillometric measurements similar to effects of breathing frequency on dynamic compliance. Fifty-three symptomatic subjects with normal large airway function (spirometry) were studied; distal airway dysfunction was identified by presence of frequency dependence of compliance (FDC). Oscillometric parameters evaluated were resistance at 20 Hz (R₂₀) and 5–20 Hz (R_{5 − 20}), reactance at 5Hz (X₅), and reactance area (AX). R₂₀ correlated with airway resistance by esophageal manometry (r = 0.74, p< 0.001); X₅ correlated with dynamic compliance at a respiratory rate of 60bpm (r = 0.61, p< 0.001); R_5-20 and AX correlated with FDC (r = 0.48, p< 0.001; r = 0.53, p< 0.01). IOS indices were further evaluated at increased respiratory rate (RR40). Oscillometric parameters changed minimally at RR40 in normal subjects ΔR₂₀ = 0.20 = 0.08 cmH2O/L/s, ΔR_5-20 = 0.10 ± 0.03 cmH₂O/L/s, Δ AX = 0.33 ± 0.19 cmH₂O/L). However, in symptomatic subjects, while R₂₀ increased minimallyat RR40 (Δ R₂₀ = 0.37 ± 0.10 cmH₂O/L/s), R_{5 − 20} and AX increased markedly (Δ R_{5 − 20} = 0.54 ± 0.06 cmH₂O/L/s, Δ AX = 4.28 ± 0.67 cmH₂O/L) and reversed post bronchodilator. IOS evaluates physiology of the distal airways in a manner comparable to dynamic compliance. Elevated respiratory rate influences oscillometric parameters and must be considered when interpreting oscillometric data. IOS provides a non-invasive tool for assessment of distal airway function when spirometry is normal, which can be applied to various clinical settings including early diagnosis of COPD (GOLD stage 0), asthma in clinical remission and occupational/ environmental irritant exposure.

Keywords:

• Airway Resistance
• Chronic Obstructive Lung Disease
• Distal Airways
• Lung Compliance
• Oscillometry
• Respiratory Mechanics