Taylor laminar dispersion in human airways

Healthy subjects took breaths of approximately 200 ml or approximately 600 ml, from functional residual capacity, of a mixture of He, Ar, SF6 (approximately 7% each), 21% O2, balance N2. Without breath-holding they expired steadily to residual volume. Concentrations of He, Ar and SF6 were measured continuously near the lips and expressed in each case as a fraction of the inspired concentration. After the bigger breaths the order in phase 2 was SF6 greater than Ar greater than He and at the end of phase 3 He greater than SF6, in agreement with other workers. After small breaths, and especially with low inspiratory flows, the early part of phase 2 showed, in a highly significant number of experiments, the order He greater than Ar greater than SF6; in these cases phase 3 still showed He greater than SF6. These results are fully consistent with the occurrence, during inspiration, of Taylor laminar dispersion (TLD) in airways situated within about 200 ml from the lips. They show that the end of phase 3 cannot be used to infer the presence or absence of TLD, and that correction for the response of the mass spectrometer is of crucial importance in the interpretation of phase 2.