Oscillometry and pulmonary magnetic resonance imaging in asthma and COPD

Developed over six decades ago, pulmonary oscillometry has re-emerged as a noninvasive and effort-independent method for evaluating respiratory-system impedance in patients with obstructive lung disease. Here, we evaluated the relationships between hyperpolarized ³ He ventilation-defect-percent (VDP) and respiratory-system resistance, reactance and reactance area (A_X ) measurements in 175 participants including 42 never-smokers without respiratory disease, 56 ex-smokers with chronic-obstructive-pulmonary-disease (COPD), 28 ex-smokers without COPD and 49 asthmatic never-smokers. COPD participants were dichotomized based on x-ray computed-tomography (CT) evidence of emphysema (relative-area CT-density-histogram ≤ 950HU (RA₉₅₀ ) ≥ 6.8%). In asthma and COPD subgroups, MRI VDP was significantly related to the frequency-dependence of resistance (R_5-19 ; asthma: ρ = 0.48, P = 0.0005; COPD: ρ = 0.45, P = 0.0004), reactance at 5 Hz (X₅ : asthma, ρ = -0.41, P = 0.004; COPD: ρ = -0.38, P = 0.004) and A_X (asthma: ρ = 0.47, P = 0.0007; COPD: ρ = 0.43, P = 0.0009). MRI VDP was also significantly related to R_5-19 in COPD participants without emphysema (ρ = 0.54, P = 0.008), and to X₅ in COPD participants with emphysema (ρ = -0.36, P = 0.04). A_X was weakly related to VDP in asthma (ρ = 0.47, P = 0.0007) and COPD participants with (ρ = 0.39, P = 0.02) and without (ρ = 0.43, P = 0.04) emphysema. A_X is sensitive to obstruction but not specific to the type of obstruction, whereas the different relationships for MRI VDP with R_5-19 and X₅ may reflect the different airway and parenchymal disease-specific biomechanical abnormalities that lead to ventilation defects.