Some criteria for laminar conditions during HFV

Assessment of the type of flow regime-laminar, transitional or turbulent-present in the central airways during high-frequency ventilation (HFV) can assist in identification of the predominant gas transport mechanisms for a particular species and set of HFV conditions. In this study, we use published empirical relationships, developed to identify the initial change from oscillating laminar flow to oscillating turbulent flow in tubes, to derive the limiting relationships between a dimensionless stroke volume and the Womersley number for maintenance of laminar conditions. When used with morphometric lung models for man and dog, these limiting relationships predicted maximum stroke volumes at experimental frequencies that either exceeded or agreed within 20% with the stroke volumes reported for steady-state ventilation of humans and dogs using HFV. Based on these limiting relationships, stroke volume-frequency combinations reported to demarcate the decline of PaO2 and alveolar ventilation were associated with nonlaminar conditions. It is expected that this approach may be useful in selecting the stroke volume-frequency pairs for HFV when a specific type of flow regime is desired as well as for analysis of HFV data.