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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 2.92 (0.86)  versus  2.94 (0.89) L, p=0.75), 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.

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