Mechanism of phase V in the single breath washout curve

The underlying mechanism of the occurrence of a downward phase V at the end of a bolus washout curve was investigated in three healthy subjects. Phase V appeared not to originate from apical zones with low concentration of tracer gas but from dependent lung zones, which were not pathological. Indeed: 1) a reversed, upward, terminal slope was obtained when the bolus had been directed to the lung base by imposing a transition of the body through 180 degrees between inspiration and expiration or by inhaling a bolus close to TLC; 2) phase V was also present on N2 washout curves and, thus, originated from zones with smaller RV/TLC than the apical ones. Phase V increased with flow rate.