Assessing static lung hyperinflation by whole-body plethysmography, helium dilution, and impulse oscillometry system (IOS) in patients with COPD

Abstract

Purpose: Lung hyperinflation is a feature of chronic obstructive pulmonary disease (COPD) and can determine pivotal consequence on symptoms, exercise tolerance and quality of life. Despite the relevance of assessing lung hyperinflation, there is still no single consensus as to what volume should be taken into account. We investigate which spirometric measurement is more reliable in assessing static lung hyperinflation and which is more related with impulse oscillometry system (IOS) measurements in COPD. Patients and Methods: Fifty-five COPD patients were enrolled. TLC, RV and RV:TLC ratio were obtained both with helium and plethysmography techniques. IOS measurements (X5, Fres and R5-R20) were performed. Pearson and Spearman correlation determined the relationships between the functional parameters that evaluate static hyperinflation (RV: TLC, TLC, RV) and IOS measurements. Results: As expected, we reported a statistically significant difference between these two techniques in terms of mean percentage values of TLC (7.57 ± 3.26 L; p= 0.02) and RV (15.24 ± 7.51 L; p=0.04), while RV:TLC measured with the two methods was similar (5.21 ± 4.69%; p=0.27). The correlation analysis showed that IOS parameters, such as difference in resistance between 5 Hz and 20 Hz (R(5–20)) and resonant frequency (Fres), were positively correlated with RV:TLC ratio, while reactance at 5 Hz (X(5)) was negatively correlated with it. In particular, we pointed out a weak correlation between RV:TLC (%) (Pleth) and R(5–20) (r=0.3, p=0.04), Fres (r=0.3; p=0.03), while X5 had a mild correlation with RV:TLC (%) (r= −0.5;p<0.0001). Moreover, we noticed a strong relationship between RV:TLC (%)(He) and X5 (r=−0.7; p=0.0001) and a mild correlation between RV:TLC (%) (He) and Fres (r=0.4; p=0.003). Between R5-R20 and RV:TLC, there was a weak correlation (r=0.3; p=0.001). No correlation between TLC, RV (L,%) (both helium and Pleth derived) and IOS parameters (R (5–20), X5, Fres) was found. Conclusion: RV:TLC can represent the most reliable parameter in the assessment of hyperinflation, considering the absence of significant difference in its measurement between the two techniques. IOS provides supplementary information in the assessment of static hyperinflation.