Dynamic compliance in flow-controlled ventilation,Intensive Care Medicine Experimental

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Dynamic compliance in flow-controlled ventilation
Intensive Care Medicine Experimental Pub Date : 2021-05-31 , DOI: 10.1186/s40635-021-00392-w
Dietmar Enk , Julia Abram , Patrick Spraider , Tom Barnes

The recent paper by Wittenstein et al. “Comparative effects of flow- vs. volume-controlled one-lung ventilation on gas exchange and respiratory system mechanics in pigs” [1] contains some intriguing observations we would like to discuss.

In flow-controlled ventilation (FCV), the gas flow is constant during both inspiration and expiration [2, 3]. This significantly differs from volume-controlled ventilation (VCV) where inspiratory flow is constant, but exhalation is passively driven by lung–chest elasticity resulting in a decelerating flow profile. Furthermore, in contrast to VCV (and any other ventilation mode) in FCV gas is always moving either into or out of the lungs without any pause phases in an accurately controlled way. This causes a continuous pressure drop across the airway resistance. Consequently, during inspiration tracheal pressure must be higher than alveolar pressure, whereas during expiration the opposite pertains. The alveolar driving pressure (ΔP) in FCV is therefore lower than the measured tracheal ΔP.

Compliance (which necessarily means dynamic compliance considering the nature of FCV) calculated from tracheal ΔP will lead to an underestimation of actual (alveolar) lung compliance because tracheal ΔP is higher than the alveolar pressure swing. Figure 1 shows how it is possible to estimate the difference between tracheal ΔP and aggregate alveolar pressure swing using the measured airway resistance reported by Wittenstein et al. [1] for FCV. The calculated difference between the two pressures (partially) accounts for the difference of the compliance in both groups of the Wittenstein study. Using alveolar rather than tracheal ΔP for the calculation of the dynamic compliance therefore results in similar compliance in both groups.

Gas exchange in mechanical ventilation is strongly related to dynamic compliance. If a larger volume of respiratory gas is shifted at the same compliance pressure (i.e., the part of the total pressure distending the alveolar periphery), alveolar gas exchange/turnover improves. Assuming the ventilated compartments of the lungs are perfused, so carbon dioxide is delivered, any higher alveolar gas exchange/turnover should result in increased carbon dioxide elimination. Wittenstein et al. compared FCV with VCV during one-lung ventilation (OLV) in hypovolemic and normovolemic pigs. They reported significantly better carbon dioxide elimination with FCV in normovolaemia, but substantially lower compliance—this seems counterintuitive.

Further, they reported an airway resistance of approx. 8 cmH₂O*s/L with FCV but approx. 34 cmH₂O*s/L with VCV—more than four times larger. Even taking into account the difference in flows (14 L/min in FCV vs. 24 L/min in VCV) and the use of a double-lumen tube with a small inner diameter as a prerequisite to perform OLV, this vast difference is surprising. In a similar double-lumen tube our own measurements show a pressure drop of 2.5 cmH₂O at a flow of 24 L/min across the bronchial lumen (= resistance of 6.25 cmH₂O*s/L). Obviously, this cannot explain the large difference in airway resistance between VCV and FCV.

To compare the dynamic compliance in FCV (which is calculated by the ventilator based on bronchial pressure measurements) with dynamic compliance in VCV, Wittenstein et al. had to convert the airway pressure data measured proximally of the bronchial lumen of the double-lumen tube into bronchial values using measurements of proximal airway pressure, flow, and tube Rohrer resistance. Systematic error in any of these measurements (e.g., differing flow conditions between the Rohrer resistance determination and the experiment) could cause differences between measured and actual driving pressure which may have significant effect on measured compliance, resistance, and calculated mechanical power (e.g., with a tidal volume of 220 mL at 11 mL/cmH₂O compliance, only 2 cmH₂O difference translates to 10% change in measured compliance). If the resistive pressure (i.e., the part of the total pressure needed to overcome resistance) is falsely high in VCV, compliance pressure amplitude will be underestimated and thus dynamic compliance (or aggregate alveolar dynamic compliance as provided for FCV in this letter) overestimated. Furthermore, the energy applied to and stored in the elastic lung tissue (i.e., elastic mechanical power) in VCV will be underestimated.

Because of the constant flow used, FCV allows accurate measurement of relative pressure swings and delivered volumes. It offers a more individualized approach to ventilation, allowing compliance-guided setting of both positive end-expiratory pressure (PEEP) and peak pressure. In our experiments, it improved lung aeration homogeneity and gas exchange efficiency without detectable regional overinflation [4]. In contrast to the fixed FCV ventilator settings used in the study of Wittenstein et al., this approach may be also applicable in OLV, possibly further reducing respiratory rate and dead space ventilation while ventilating at substantially lower levels of mechanical power and dissipated energy compared to conventional ventilation modes.

Not applicable.

1.
Wittenstein J, Scharffenberg M, Ran X et al (2020) Comparative effects of flow- vs. volume-controlled one-lung ventilation on gas exchange and respiratory system mechanics in pigs. Intensive Care Med Exp 8(Suppl1):24
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2.
Barnes T, van Asseldonk T, Enk D (2018) Minimisation of dissipated energy in the airways during mechanical ventilation by using constant inspiratory and expiratory flows—flow-controlled ventilation (FCV). Med Hypotheses 121:167–176
Article Google Scholar
3.
Barnes T, Enk D (2019) Ventilation for low dissipated energy achieved using flow control during both inspiration and expiration. Trends Anaesth Crit Care 24(2):5–12
Article Google Scholar
4.
Spraider P, Martini J, Abram J et al (2020) Individualized flow-controlled ventilation compared to best clinical practice pressure-controlled ventilation: a prospective randomized porcine study. Crit Care 24(1):662
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Affiliations

Faculty of Medicine, University of Münster, Münster, Germany
Dietmar Enk
Department of Anaesthesiology and Intensive Care Medicine, Medical University of Innsbruck, Innsbruck, Austria
Julia Abram & Patrick Spraider
University of Greenwich, London, UK
Tom Barnes

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Dietmar EnkView author publications
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Julia AbramView author publications
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Patrick SpraiderView author publications
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Tom BarnesView author publications
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Contributions

DE, JA, PS, TB: writing—original draft preparation; DE, JA, PS, TB: writing—review and editing. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Julia Abram.

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Competing interests

D. Enk: inventor of EVA and FCV technology (Ventrain, Tritube, Evone), royalties for EVA and FCV technology (Ventrain, Tritube, Evone), patent applications on minimizing dissipated energy and on calculating and displaying dissipated energy, (paid) consultant to Ventinova Medical. T. Barnes: patent application on calculating and displaying dissipated energy, (paid) consultant to Ventinova Medical.

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Cite this article

Enk, D., Abram, J., Spraider, P. et al. Dynamic compliance in flow-controlled ventilation. ICMx 9, 26 (2021). https://doi.org/10.1186/s40635-021-00392-w

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Received: 12 March 2021
Accepted: 28 April 2021
Published: 31 May 2021
DOI: https://doi.org/10.1186/s40635-021-00392-w

中文翻译：

流量控制通气的动态顺应性

Wittenstein 等人最近的论文。“流量控制与容量控制单肺通气对猪的气体交换和呼吸系统力学的比较影响”[1] 包含一些我们想讨论的有趣观察结果。

在流控制通气（FCV），气体流量是恒定期间既吸气和呼气[2，3]。这与容积控制通气 (VCV) 显着不同，其中吸气流量是恒定的，但呼气是由肺胸弹性被动驱动的，导致流量曲线减速。此外，与 VCV（和任何其他通气模式）相比，FCV 中的气体总是以精确控制的方式进入或离开肺部，没有任何暂停阶段。这会导致气道阻力的持续压降。因此，吸气时气管压力必须高于肺泡压力，而呼气时则相反。肺泡驱动压（ΔP) 因此在 FCV 中低于测量的气管 Δ P。

从气管 Δ P计算的顺应性（考虑到 FCV 的性质，这必然意味着动态顺应性）将导致对实际（肺泡）肺顺应性的低估，因为气管 Δ P高于肺泡压力波动。图 1 显示了如何使用 Wittenstein 等人报告的测量的气道阻力来估计气管ΔP和总肺泡压力波动之间的差异。[1] 用于燃料电池汽车。两个压力之间的计算差异（部分）解释了 Wittenstein 研究的两组的顺应性差异。使用肺泡而不是气管 Δ P计算动态顺应性，因此导致两组的顺应性相似。

机械通气中的气体交换与动态顺应性密切相关。如果在相同的顺应性压力下（即，使肺泡周边扩张的总压力的一部分）转移了更大体积的呼吸气体，则肺泡气体交换/周转会得到改善。假设肺的通风隔室被灌注，因此输送二氧化碳，任何更高的肺泡气体交换/周转率都应导致二氧化碳消除增加。维滕斯坦等人。在低血容量和血容量正常的猪的单肺通气 (OLV) 期间比较了 FCV 和 VCV。他们报告说，在血容量正常的情况下，FCV 的二氧化碳消除效果明显更好，但依从性大大降低——这似乎违反直觉。

此外，他们报告的气道阻力约为。8 cmH ₂ O*s/L 使用 FCV 但大约 34 cmH ₂ O*s/L 与 VCV — 大四倍以上。即使考虑到流量的差异（FCV 中的 14 L/min 与 VCV 中的 24 L/min）以及使用具有小内径的双腔管作为执行 OLV 的先决条件，这种巨大的差异也令人惊讶. 在类似的双腔管中，我们自己的测量结果显示，以 24 L/min 的流量穿过支气管腔时，压降为 2.5 cmH ₂ O（= 阻力为 6.25 cmH ₂ O*s/L）。显然，这不能解释 VCV 和 FCV 之间气道阻力的巨大差异。

为了比较 FCV 的动态顺应性（由呼吸机根据支气管压力测量值计算）与 VCV 的动态顺应性，Wittenstein 等人。必须使用近端气道压力、流量和管 Rohrer 阻力的测量值将双腔管的支气管腔近端测量的气道压力数据转换为支气管值。任何这些测量中的系统误差（例如，Rohrer 阻力测定和实验之间的不同流动条件）可能导致测量的驱动压力和实际驱动压力之间存在差异，这可能对测量的顺应性、阻力和计算的机械功率（例如，与潮气量为 220 mL，符合11 mL/cmH ₂ O，仅 2 cmH ₂O 差异转化为测量的顺应性变化 10%）。如果阻力压力（即克服阻力所需的总压力的一部分）在 VCV 中错误地高，则顺应性压力幅度将被低估，因此动态顺应性（或在这封信中为 FCV 提供的总肺泡动态顺应性）被高估。此外，在 VCV 中施加到弹性肺组织并储存在弹性肺组织中的能量（即弹性机械功率）将被低估。

由于使用恒定流量，FCV 允许准确测量相对压力波动和输送量。它提供了更个性化的方法来通风，允许合规性引导设置两个正面呼气末正压（PEEP）和峰值压力。在我们的实验中，它提高了肺通气的均匀性和气体交换效率，而不会检测到区域过度充气 [4]。与 Wittenstein 等人的研究中使用的固定 FCV 呼吸机设置相比，这种方法也可能适用于 OLV，可能会进一步降低呼吸频率和死腔通气，同时以显着较低水平的机械功率和耗散能量进行通气。常规通风模式。

不适用。

1.
Wittenstein J、Scharffenberg M、Ran X 等 (2020) 流量控制与容量控制的单肺通气对猪的气体交换和呼吸系统力学的比较影响。重症监护医学实验 8(Suppl1):24
文章谷歌学术
2.
Barnes T、van Asseldonk T、Enk D (2018) 通过使用恒定的吸气和呼气流量——流量控制通气 (FCV)，最大限度地减少机械通气期间气道中的耗散能量。医学假设121：167-176
文章谷歌学术
3.
Barnes T, Enk D (2019) 在吸气和呼气期间使用流量控制实现低耗散能量通气。趋势麻醉暴击护理 24(2):5–12
文章谷歌学术
4.
Spraider P、Martini J、Abram J 等人 (2020) 个性化流量控制通气与最佳临床实践压力控制通气的比较：一项前瞻性随机猪研究。暴击护理 24(1):662
文章谷歌学术

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隶属关系

德国明斯特大学医学院
迪特玛·恩克
奥地利因斯布鲁克医科大学麻醉学和重症监护医学系
朱莉娅·艾布拉姆和帕特里克·斯普拉德
英国伦敦格林威治大学
汤姆·巴恩斯

作者

Dietmar Enk查看作者出版物
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Julia Abram查看作者出版物
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Patrick Spraider查看作者出版物
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Tom Barnes查看作者出版物
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贡献

DE、JA、PS、TB：写作——原稿准备；DE、JA、PS、TB：写作——审查和编辑。所有作者阅读并认可的终稿。

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利益争夺

D. Enk：EVA 和 FCV 技术（Ventrain、Tritube、Evone）的发明者，EVA 和 FCV 技术（Ventrain、Tritube、Evone）的专利使用费，关于最小化耗散能量和计算和显示耗散能量的专利申请，（付费）顾问到 Ventinova Medical。T. Barnes：关于计算和显示耗散能量的专利申请，Ventinova Medical 的（付费）顾问。

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