Abstract
Roles of lung volumes in asthma remain controversial. We aimed to evaluate the efficacy of lung volumes in differentiating asthma severity levels. Consecutive outpatients with chronic persistent asthma were enrolled, and body plethysmography (BP) and helium dilution (HD) were performed simultaneously to extract RV%pred, TLC%pred, and RV/TLC. Significant negative correlations were found between FEV1%pred and RV%pred (r = −0.557, P < 0.001), TLC%pred (r = −0.387, P < 0.001), and RV/TLC (r = −0.485, P < 0.001) measured by BP, as well as difference in volumes between these two techniques (ΔRV%pred, ΔTLC%pred and ΔRV/TLC). In mild and moderate asthma, AUC of RV%pred detected by BP and ΔTLC%pred was 0.723 (95%CI 0.571–0.874, P = 0.005) and 0.739 (95%CI 0.607–0.872, P = 0.002) with sensitivity and specificity being 79.41% and 88.24%, and 65.22% and 56.52% at cut-off of 145.40% and 14.23%, respectively. In moderate and severe asthma, AUC of RV%pred detected by BP and ΔTLC%pred was 0.782 (95%CI 0.671–0.893, P < 0.001) and 0.788 (95%CI 0.681–0.894, P < 0.002) with sensitivity and specificity being 77.78% and 97.22%, and 73.53% and 52.94% at cut-off of 179.85% and 20.22%, respectively. In conclusion, lung volumes are reliable complement of FEV1 in identifying asthma severity levels.
Highlights
RVpleth%pred, predicted percentage of residual volume measured by body plethysmography; RV/TLChe, ratio of residual volume to total lung capacity measured by helium dilution method; RV/TLCpleth, ratio of residual volume to total lung capacity measured by body plethysmography; TLChe%pred, predicted percentage of total lung capacity measured by helium dilution method; TLCpleth%pred, predicted percentage of total lung capacity measured by body plethysmography; ΔRV%pred, predicted percentage of difference of residual volume between body plethysmography and helium dilution method; ΔRV/TLC, difference of ration of residual volume to total lung capacity between body plethysmography and helium dilution method; ΔTLC%pred, predicted percentage of difference of total lung capacity between body plethysmography and helium dilution method
We found that lung volumes including RV%pred, TLC%pred and RV to TLC (RV/TLC) measured by BP were significantly higher than those measured by Helium dilution (HD), and that the volumes measured by BP as well as the difference in volume between these techniques were positively correlated with increasing severity of asthma but negatively correlated with FEV1%pred
BP measures thoracic gas volumes (TGV) on the basis of Boyle’s Law, which states that, under isothermal conditions, the product of gas volume and pressure is constant at any given moment, and results in an equation of TGV =−(ΔV/ΔP) ×PA2 ×(PA1/PB), of which ΔVis the change in volume of the thorax before and after compression or rarefraction of the gas in thorax, ΔP is the change in the alveolar pressure measured at the airway opening under conditions of no flow during the panting maneuver, PA1 and PA2 are the alveolar pressure before and after compression or rarefraction of the gas in thorax under the assumption that pressure measured at the airway opening is representative of alveolar pressure, and PB indicates the barometric pressure[17]
Summary
RVpleth%pred, predicted percentage of residual volume measured by body plethysmography; RV/TLChe, ratio of residual volume to total lung capacity measured by helium dilution method; RV/TLCpleth, ratio of residual volume to total lung capacity measured by body plethysmography; TLChe%pred, predicted percentage of total lung capacity measured by helium dilution method; TLCpleth%pred, predicted percentage of total lung capacity measured by body plethysmography; ΔRV%pred, predicted percentage of difference of residual volume between body plethysmography and helium dilution method; ΔRV/TLC, difference of ration of residual volume to total lung capacity between body plethysmography and helium dilution method; ΔTLC%pred, predicted percentage of difference of total lung capacity between body plethysmography and helium dilution method. In a small study by Woolcock and colleagues, functional residual capacity (FRC) and TLC were found to be significantly higher by plethysmography than those by dilution method, and the differences between these methods were the greatest when the FEV1 was lowest, and these differences decreased during clinical recovery[16]. We hypothesized that the difference in volume between these two methods may provide additional clinical value in identifying individuals with differing asthma severity. To test this hypothesis, we conducted a prospective correlation and diagnosis analysis in an attempt to further assess the correlation between lung volumes and FEV1, and the values of individual lung volumes as well as the corresponding differences between the two methods in distinguishing asthma severity
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