Department of Mechanical and Industrial Engineering, The University of Iowa, Iowa City, 52242, USA.
Ann Biomed Eng. 2010 Dec;38(12):3550-71. doi: 10.1007/s10439-010-0110-7. Epub 2010 Jul 8.
High-frequency oscillatory ventilation (HFOV) is considered an efficient and safe respiratory technique to ventilate neonates and patients with acute respiratory distress syndrome. HFOV has very different characteristics from normal breathing physiology, with a much smaller tidal volume and a higher breathing frequency. In this study, the high-frequency oscillatory flow is studied using a computational fluid dynamics analysis in three different geometrical models with increasing complexity: a straight tube, a single-bifurcation tube model, and a computed tomography (CT)-based human airway model of up to seven generations. We aim to understand the counter-flow phenomenon at flow reversal and its role in convective mixing in these models using sinusoidal waveforms of different frequencies and Reynolds (Re) numbers. Mixing is quantified by the stretch rate analysis. In the straight-tube model, coaxial counter flow with opposing fluid streams is formed around flow reversal, agreeing with an analytical Womersley solution. However, counter flow yields no net convective mixing at end cycle. In the single-bifurcation model, counter flow at high Re is intervened with secondary vortices in the parent (child) branch at end expiration (inspiration), resulting in an irreversible mixing process. For the CT-based airway model three cases are considered, consisting of the normal breathing case, the high-frequency-normal-Re (HFNR) case, and the HFOV case. The counter-flow structure is more evident in the HFNR case than the HFOV case. The instantaneous and time-averaged stretch rates at the end of two breathing cycles and in the vicinity of flow reversal are computed. It is found that counter flow contributes about 20% to mixing in HFOV.
高频振荡通气（HFOV）被认为是一种有效的、安全的呼吸技术，可以用于对新生儿和急性呼吸窘迫综合征患者进行通气。HFOV 的特点与正常呼吸生理有很大的不同，潮气量更小，呼吸频率更高。在这项研究中，我们使用计算流体动力学分析研究了三种不同几何模型中的高频振荡流，这些模型的复杂性逐渐增加：直管、单分叉管模型和基于 CT 的多达七个分支的人体气道模型。我们的目的是使用不同频率和雷诺数（Re）的正弦波来理解在流量反转时的逆流现象及其在这些模型中的对流混合作用。通过拉伸率分析来量化混合。在直管模型中，在流量反转周围形成了同轴逆流，与解析的沃默斯利解决方案一致。然而，在末端周期，逆流不会产生净对流混合。在单分叉模型中，在高 Re 时，逆流在分支（子分支）末端的呼气（吸气）时会被二次涡旋所干预，导致不可逆的混合过程。对于基于 CT 的气道模型，考虑了三种情况，分别是正常呼吸情况、高频正常 Re（HFNR）情况和 HFOV 情况。HFNR 情况下的逆流结构比 HFOV 情况下更明显。计算了两个呼吸周期末端和流量反转附近的瞬时和时均拉伸率。结果发现，逆流对 HFOV 中的混合贡献约为 20%。
