Comparison of Respiratory Impedance in patients with Chronic Obstructive Pulmonary Disease and Bronchial Asthma

<bold>Introduction:</bold> This study aimed to compare respiratory impedance between COPD and asthma patients, stratified by severity of airflow obstruction. <bold>Methods:</bold> We reviewed spirometry and oscillometry data of 297 COPD and 118 asthma patients with an FEV₁/FVC ratio <0.7. The severity of airflow obstruction was stratified as follows: FEV₁ % predicted ≥70% mild; <70% to ≥50% moderate; and <50% severe. The z-scores were used to express the severity of impedance abnormalities. <bold>Results:</bold> Asthma patients were younger (43 vs. 63 yr, p<0.01) and had more females (44.9% vs. 8.4%, p<0.01). The mean FEV₁% predicted in asthma was higher (52.5% vs. 42.8%; p<0.01). Mild (13.6% vs. 7.0%, p<0.01) and moderate (44.1% vs. 22.1%, p<0.01) airflow obstruction were more prevalent in asthma. Impedance of COPD and asthma patients with mild obstruction was statistically not different. The R5 (3.27±2.39 vs. 2.04±1.9, p<0.01) and R19 (2.99±1.97 vs. 1.78±1.9, p<0.01) were higher in asthma with moderate obstruction. Asthma patients with severe obstruction also had higher R5 (5.6±2.91 vs. 3.71±2.18, p<0.01) and R19 (3.52± 2.21 vs. 2.36±1.83, p<0.01). There was no significant difference in the z-scores of X5, R5-19, resonant frequency (Fres), reactance area (AX), and ΔX5 between COPD and asthma across all severities of airflow obstruction. <bold>Conclusion:</bold> The severity of impedance abnormalities increases with the severity of airflow obstruction. The severity of heterogeneity in ventilation (X5) and small airway dysfunction (R5-19) is not specific to the disease. Asthma patients are characterized by higher total (R5) and proximal (R19) airway resistance.