Longitudinal mixing in pulmonary airways: comparison of inspiration and expiration.

The increase in dispersion of an inert tracer bolus of helium (He) or sulfur hexafluoride (SF6) was used as a direct and noninvasive measure of longitudinal mixing in the conducting airways. Over a 0- to 260-ml range of bolus penetration, a change in the inspiratory flow from 0.35 to 1.2 l/s did not affect the dispersion of He and SF6, this implies that pure convective dispersion is the dominant inspiratory mixing process. The same change in expiratory flow caused a significant increase in SF6 dispersion only; this indicates that Taylor dispersion is important during expiration. In experiments performed at a fixed penetration of 160 ml and with SF6 only, the inspiratory and the expiratory flows were independently varied from 0.18 to 5.4 l/s. Though the bolus dispersion exhibited a mildly negative correlation with inspiratory flow, it was linearly correlated with expiratory flow. From the former result we conclude that although inspiratory mixing occurs primarily by convective dispersion, there is also a small degree of turbulent dispersion. The latter result confirms that expiratory mixing is primarily due to Taylor dispersion.