Pendelluft is a gas flow phenomenon occurring in parallel respiratory units. When the parallel respiratory units are forded by a periodic pressure, the flow in them becomes asynchronous. This phenomenon plays an important role in mixing and transport of air especially during high-frequency ventilation. Understanding of this phenomenon was based n a mechanical model proposed by Otis et al. (1956). In that model the respiratory units are modeled as electric circuits with lumped parameters. They found that unequal resistances and compliances in the parallel respiratory units could give rise to pendelluft. Ultman et al. (1988) and High, Ultman, and Karl (1991) observed that, for high frequency ventilation, the inertance should be included. They thus expanded the Otis R-C model to the R-I-C model of analogous electric circuits.