Relationships between respiratory impedance and prognosis in patients with end-stage pulmonary lymphangioleiomyomatosis: A concept based on the Helmholtz resonator

Background: Patients with lymphangioleiomyomatosis (LAM) are evaluated with multi-directional assessments in the same manner as those with COPD, but the pulmonary physiological features in patients with end-stage pulmonary LAM are complicated because the severe condition of patients caused by disease progression prohibits extensive testing. To complement this, we applied a forced oscillation technique (FOT) for the assessment of patients with LAM, and evaluated the relationship between respiratory impedance and disease severity. Methods: 9 subjects with LAM were studied cross-sectionally. Lung function tests and respiratory impedance were measured. A concept based on the Helmholts Resonator was used to interpret those respiratory mechanics. A simple 3-element series (RIC) model was used to estimate airway resistance (Raw), airway inertance (Iaw), respiratory capacitance (Crs) and resonant frequency (fres). For the estimation of disease severity, a multifunctional index (BODE index) was used. Results: The respiratory impedance spectra were fitted to the RIC model, and each value of goodness of fit from the 9 subjects was over 0.9. Raw, Iaw and fres correlated with%FEV1, RV/TLC,%DLCO/VA & the BODE index. Crs correlated with RV/TLC. Conclusions: The RIC model corresponding with the Helmholtz Resonator provided a reasonable physiological interpretation in which the mechanical parameters reflected airway obstruction and a decrease in capacitive behavior. Thus, non-invasive FOT are useful and represent a potential monitoring tool to evaluate disease severity and prognosis in a cohort of patients with LAM.