Controlled versus free breathing for multiple breath nitrogen washout in healthy adults

Multiple breath nitrogen washout (MBNW) quantifies ventilation heterogeneity. Two distinct protocols are currently used for MBNW testing: "controlled breathing", with targeted tidal volume (V _T) and respiratory rate (RR); and "free breathing", with no constraints on breathing pattern. Indices derived from the two protocols (functional residual capacity (FRC), lung clearance index (LCI), S _cond, S _acin) have not been directly compared in adults. We aimed to determine whether MBNW indices are comparable between protocols, to identify factors underlying any between-protocol differences and to determine the between-session variabilities of each protocol. We performed MBNW testing by both protocols in 27 healthy adult volunteers, applying the currently proposed correction for V _T to S _cond and S _acin derived from free breathing. To establish between-session variability, we repeated testing in 15 volunteers within 3 months. While FRC was comparable between controlled versus free breathing (3.17 (0.98) versus 3.18 (0.94) L, p=0.88), indices of ventilation heterogeneity derived from the two protocols were not, with poor correlation for S _cond (r=0.18, p=0.36) and significant bias for S _acin (0.057 (0.021) L^-1 versus 0.085 (0.038) L^-1, p=0.0004). Between-protocol differences in S _acin were related to differences in the breathing pattern, i.e. V _T (p=0.004) and RR (p=0.01), rather than FRC. FRC and LCI showed good between-session repeatability, but S _cond and S _acin from free breathing showed poor repeatability with wide limits of agreement. These findings have implications for the ongoing clinical implementation of MBNW, as they demonstrate that S _cond and S _acin from free breathing, despite V _T correction, are not equivalent to the controlled breathing protocol. The poor between-session repeatability of S _cond during free breathing may limit its clinical utility.