Mean distribution time (MDT). MDT is the mean time during which consecutive fractions of inspired tidal volume remain in the respiratory zone of the lung (that is, the time available for distribution and mixing by diffusion of inspired gas with resident alveolar gas. The graph shows flow (black) and volume change (blue) of a breath against time. Until airway dead space (VDaw) has been inhaled (shaded area), no fresh gas arrives to alveoli, and this volume does not contribute to MDT. The following fractions of inhaled volume, N° 1 to N° n, (vertically striped area) have different distribution times in alveoli. For N° 1 distribution time is marked DT1 and for N° n DTn. MDT is the volume-weighted mean of DT1 to DTn.