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We continue our discussion, this week we will focus on the mechanisms of gas exchange.
1.How is gas exchanged in conventional ventilation, and why is this mechanism not important in HFV?
常规机械通气的气体交换机制是什么，为什么此种机制在高频通气中不重要
2.What are some of the theories to explain how does gas get exchanged during High Frequency Ventilation ?
解释高频通气的气体交换原理的理论有哪些？

参考答案
常规机械通气的气体交换机制是什么，为什么此种机制在高频通气中不重要
在正常呼吸和常规机械通气方式中，气体交换以总体流量对流（convective bulk flow）的方式进行。吸入的混合气体被运送到肺泡，肺泡以分子扩散方式进行氧气和二氧化碳的交换，由于肺和胸壁的弹性回缩，自主呼吸时，呼气可能是主动的或被动的。进行气体交换时潮气量必须大于气道的死腔气体量。分钟通气量(MV)等于呼吸频率和潮气量的乘积(VT)，即MV= f * (VT)。在高频通气中，潮气量很小，此类型气体的总体流量在气体交换过程中不起主要的作用，然而正是这些小容积气体通过一些机制使大气道周围的肺泡能够正常通气，进行气体交换。
解释高频通气的气体交换原理的理论有哪些？
高频通气气体交换的确切机制理论上依据肺通气机制，有不同的解释，仍处于探索阶段。肺泡充气和排空的速率依据于持续时间（TC），肺泡顺应性(C )和肺泡阻力（R）
TC (seconds)= R (cm H2O/l/second) * C (l/cm H2O)
肺泡间持续时间不同，所以吸气时气体从一肺泡流向另一肺泡，气体将从持续时间长的肺泡流向持续时间短的肺泡，在吸气末，此过程以相反的顺序进行。这种钟摆式效应已经在实验中得到证明，此机制在高频震荡通气中尤为重要。非对称性流速图可能对气体交换有影响，早期实验表明，吸气相与呼气相相比，气体通过笔直管道时，流速图的偏移更大些。这意味着一均一气流将在吸气相形成一抛物线式流速图，这样气道中心的气流将以脉冲式传向气道深处，而气道边缘的气体运行距离相对较短。在呼气相，气流方向突然反转，反向流速图是平坦的，气道边缘和中心气流一起均一流出。最终效应是气道中心气流被运送到了气道的更远端。Chang (1)证实此效应在气道的分叉处是最重要的。伴随着浓度梯度导致的气体辐射状分布，轴向抛物线式流速图引起的气体在气道中的分布是此效应的拓展，也就是说，脉冲式气流形成了一长的矛状抛物线形气柱，气柱中心气体浓度很高，向前移动的气体在吸气末停止，此时气道中气体迅速通过分子扩散与气道外进行气体交换，增加了气体交换速率，此泰勒型扩散是高频通气被认为是通过扩散分布进行气体交换的理论基础。另外一些原因可能是由于心脏的跳动，使肺处于自然的震荡状态，此种心源性影响只有在心脏运动产生肺部气流才有意义，此气流最易在肺周边部产生。总而言之，由于氧气的浓度梯度导致的分子扩散和二氧化碳横穿肺毛细血管膜是高频通气的气体交换的最终阶段是不容置疑的。
How is gas exchanged in conventional ventilation, and why is this mechanism not important in HFV?
In both normal breathing and conventional ventilation, gas exchange occurs through convective bulk flow. The inhaled gas mixture is delivered to the alveoli, where oxygen and carbon dioxide are exchanged by molecular diffusion. Expiration may be active during spontaneous breaths, or passive, due to recoil of the lung and chest wall. The tidal volume of inspired (or delivered) gas must be larger than the dead space of the airways for exchange to occur. The minute ventilation (MV) is equal to the breath frequency  times this tidal volume (VT), or MV= f * (VT). In high frequency ventilation, very small tidal volumes are employed, so this type of bulk flow doesn't play a major role in gas exchange. It is true, however, that these small tidal volumes may be large enough to affect the ventilation of alveoli close to the large airways, and thereby contribute in some small way to gas exchange (1).
What are some of the theories to explain how does gas get exchanged during High Frequency Ventilation ?
The exact mechanism of gas exchange in HFV remains a matter of theory and modeling based on lung mechanics. The rate at which an alveolus fills and empties depends on its time constant TC, itself a function of compliance (C ) and resistance R.
TC (seconds)= R (cm H2O/l/second) * C (l/cm H2O)
These time constants vary between alveoli, so gas will flow from one to another. At end expiration, gas should flow from slow alveoli to fast ones, and the opposite will occur at the end of inspiration. This Penduluft effect has been demonstrated experimentally (2), and is particularly important at higher oscillatory frequencies.
Gas exchange may be effected through asymmetrical velocity profiles (3). Early experiments showed that gas flowing down a straight tube will show more skewing in the velocity profile in inspiration than in expiration. This means that a uniform bolus of gas will develop a parabolic velocity profile in inspiration, so that the center of the gas pulse will travel further down the tube than the gas at the edges. When flow is abruptly reversed in expiration, the reverse velocity profile is flat, so that both the center and the edges of the pulse move backwards uniformly . The net effect is that material in the center of the tube will be transported further down the tube than material at the edges, and this effect will be enhanced with each breath, leading to transport of gas down the tube. Chang (1) showed that this effect is most pronounced at bifurcations of airways.
An extended aspect of this effect is that the dispersion of gas in a tube results from both its axial parabolic velocity profile, followed by radial diffusion along the concentration gradient. In other words, the pulse of gas forms a long parabolic spike, with a high central concentration. As the forward movement of gas ceases at the end of the inspiratory pulse, molecular diffusion will occur radially from the center of the tube outwards, resulting in increased gas mixing. This Taylor Dispersion (4) is why HFV has been considered to work via gas exchange by augmented diffusion.
Some contribution may be due to the motion of the beating heart, which subjects the lung to a natural oscillation. The impact of this Cardiogenic Mixing is probably only significant when the motion of the heart produces some pulmonary airflow, likely at the periphery of the lung. Finally, there is no reason to doubt that Molecular Diffusion due to the concentration gradients of oxygen and carbon dioxide across the alveolar capillary membrane is the final stage of gas exchange in HFV.
References:
1. Chang H. Mechanisms of gas transport during ventilation by high frequency oscillation. J Appl Physiol 1984; 56:553-563.
2.Lehr J, Butler J, Westerman P, et al. Photographic measurements of pleural surface motion during lung oscillation. J Appl Physiol 1985;57: 623-633.
3.Haselton FR, Scherer PW. Bronchial bifurcations and respiratory mass transport. Science 1980; 208:69-71.
4.Taylor G. The dispersion of matter in turbulent flow through a pipe. Proc R Soc Lond 1954;223:446-468.