Cereda Maurizio, Xin Yi, Kadlecek Stephen, Hamedani Hooman, Rajaei Jennia, Clapp Justin, Rizi Rahim R
Department of Anesthesiology and Critical Care, University of Pennsylvania, Philadelphia, PA, USA.
NMR Biomed. 2014 Dec;27(12):1468-78. doi: 10.1002/nbm.3136. Epub 2014 Jun 11.
Considerable uncertainty remains about the best ventilator strategies for the mitigation of atelectasis and associated airspace stretch in patients with acute respiratory distress syndrome (ARDS). In addition to several immediate physiological effects, atelectasis increases the risk of ventilator-associated lung injury, which has been shown to significantly worsen ARDS outcomes. A number of lung imaging techniques have made substantial headway in clarifying the mechanisms of atelectasis. This paper reviews the contributions of computed tomography, positron emission tomography, and conventional MRI to understanding this phenomenon. In doing so, it also reveals several important shortcomings inherent to each of these approaches. Once these shortcomings have been made apparent, we describe how hyperpolarized (HP) gas MRI--a technique that is uniquely able to assess responses to mechanical ventilation and lung injury in peripheral airspaces--is poised to fill several of these knowledge gaps. The HP-MRI-derived apparent diffusion coefficient (ADC) quantifies the restriction of (3) He diffusion by peripheral airspaces, thereby obtaining pulmonary structural information at an extremely small scale. Lastly, this paper reports the results of a series of experiments that measured ADC in mechanically ventilated rats in order to investigate (i) the effect of atelectasis on ventilated airspaces, (ii) the relationship between positive end-expiratory pressure (PEEP), hysteresis, and the dimensions of peripheral airspaces, and (iii) the ability of PEEP and surfactant to reduce airspace dimensions after lung injury. An increase in ADC was found to be a marker of atelectasis-induced overdistension. With recruitment, higher airway pressures were shown to reduce stretch rather than worsen it. Moving forward, HP MRI has significant potential to shed further light on the atelectatic processes that occur during mechanical ventilation.
对于急性呼吸窘迫综合征（ARDS）患者，采用何种最佳通气策略来减轻肺不张及相关肺泡扩张，仍存在相当大的不确定性。除了一些即时的生理效应外，肺不张还会增加呼吸机相关性肺损伤的风险，而这已被证明会显著恶化ARDS的预后。多种肺部成像技术在阐明肺不张的机制方面取得了重大进展。本文回顾了计算机断层扫描、正电子发射断层扫描和传统磁共振成像在理解这一现象方面所做的贡献。在此过程中，还揭示了这些方法各自存在的几个重要缺点。一旦这些缺点显现出来，我们将描述超极化（HP）气体磁共振成像——一种能够独特地评估外周肺泡对机械通气和肺损伤反应的技术——如何有望填补其中的一些知识空白。HP - MRI衍生的表观扩散系数（ADC）可量化外周肺泡对³He扩散的限制，从而在极小尺度上获取肺部结构信息。最后，本文报告了一系列实验的结果，这些实验测量了机械通气大鼠的ADC，以研究（i）肺不张对通气肺泡的影响，（ii）呼气末正压（PEEP）、滞后现象与外周肺泡尺寸之间的关系，以及（iii）PEEP和表面活性剂在肺损伤后减小肺泡尺寸的能力。发现ADC升高是肺不张引起过度扩张的一个标志。随着肺复张，较高的气道压力显示可减少而非加重肺泡扩张。展望未来，HP MRI有很大潜力进一步阐明机械通气过程中发生的肺不张过程。
