Diagnosing ventilation inhomogeneity after COVID-19 by multiple-breath nitrogen washout test

Abstract

The aim was to evaluate the ventilation inhomogeneity (VIH) by the multiple-breath nitrogen washout test (MBNW) after COVID-19 and to identify the relationship of the lung clearance index (LCI) with other functional parameters of the respiratory system. Methods. The cross-sectional study included 35 patients (97% men); the median age was 44 years. Spirometry, body plethysmography, diffusing capacity of the lung for carbon monoxide (DL_co), MBNW test, and impulse oscillometry were performed. Dyspnea was evaluated by mMRC scale. MBNW test was performed using the Easy-one Pro, MBW Module (ndd Medizintechnik AG, Switzerland). Results. The patients were divided into 2 groups. Group 1 included 21 (60%) patients who were not diagnosed with VIH. Group 2 included 14 (40%) patients with VIH. The median testing period was 72 (47 - 109) days from the onset of COVID-19. The median of the maximum volume of lung damage determined by high-resolution computed tomography (CT_max) was 50% in the acute period of the disease and 12% during the study. The medians of all analyzed parameters remained normal in the study cohort as well as in groups 1 and 2, except the resonance frequency (f_res) in group 2. Statistically significant differences were found between groups 1 and 2 in the absolute frequency dependence of resistance (R5 - R20), reactance area (A_X), f_res. Significant differences were also found in pathological changes of vital capacity, forced expiratory volume in the first second (FEV₁), (R5 - R20). The abnormalities were more common in group 2. A significant correlation was shown between LCI with the ratio of residual lung volume to total lung capacity, (R5 - R20), AX, fres, relative frequency dependence of resistance, CTmax, FEV1 and trasfer-factor (DL_co). Conclusion. Seventy-two days after the onset of CoVID-19, the ventilation inhomogeneity was detected in 40% of the patients, decreased DL_co - in 23%, airway obstruction - in 11.4%, and restrictive ventilatory defect - in 8.6%. Correlations were found between LCI and DL_co, spirometry parameters, body plethysmography, impulse oscillometry, and CT_max.