Dear editor, We read with great interest of the report by Coppola and colleagues [1] about the presence and possible factors of dynamic hyperinflation and intrinsic positive endexpiratory pressure (PEEP) in severe acute respiratory distress syndrome (ARDS) patients. They suggested that in sedated, paralyzed ARDS patients without a known obstructive disease, the amount of intrinsic PEEP during lung-protective ventilation is negligible and does not influence respiratory mechanical properties. However, some details about who, when, and how for monitoring and managing dynamic hyperinflation and intrinsic PEEP in ARDS patients is still needed to be defined. First, who needs more attention for monitoring dynamic hyperinflation and intrinsic PEEP with ARDS? During lung-protective ventilation in ARDS, when a respiratory rate (RR) up to a maximal of 35 breaths/min was needed to provide a minute ventilation that minimized hypercapnia and respiratory acidosis, the shortening of the expiratory time consequent to the higher RR may generate substantial intrinsic PEEP [2, 3]. In the study by Coppola et al. [1], average RR was only 16 breaths/min. Therefore, severe ARDS patients with a high RR requirement for hypercapnia and respiratory acidosis are at high risk of dynamic hyperinflation and intrinsic PEEP, which needs more investigation. The second question is when should we focus on dynamic hyperinflation and intrinsic PEEP with ARDS? In the article reported by Coppola et al. [1], only paralyzed patients in the very early stage of ARDS were investigated. However, in deeply sedated ARDS patients without paralysis, the respiratory entrainment with reverse triggering may cause breath stacking in deeply sedated non-paralyzed ARDS patient, which led to volumes and pressures that were incompatible with lung-protective ventilation, and may lead to intrinsic PEEP and dynamic hyperinflation [4]. Finally, how to recognize and calculate intrinsic PEEP in patients with ARDS, especially in patients without neuromuscular blocking agents. In Coppola’s report [1], intrinsic PEEP was defined as the total PEEP minus the external PEEP, and intrinsic PEEP decreased when external PEEP was raised. Expiratory flow limitation and airway closure may be two factors mainly responsible for the development of intrinsic PEEP in ARDS patients, and the response to a raising external PEEP might be due to airway closure and flow limitation at low PEEP and an airway opening pressure at high PEEP. Thus, reliability of the calculation for intrinsic PEEP in Coppola’s study may not be valid under condition of flow limitation, which can occur in patients with ARDS. A measurement of intrinsic PEEP at zero PEEP should be more accurate. Furthermore, for patients with spontaneous breath, an increasing number of reports indicate that measurement of intrinsic PEEP can be obtained both with advanced monitoring systems (esophageal and gastric manometry, diaphragm electromyography, electrical impedance tomography) and, with some limitations, with simple airways occlusion maneuvers in patients with spontaneous breath [5]. Therefore, details about who, when, and how to investigate intrinsic PEEP and dynamic hyperinflation in ARDS patients are still needed to be evaluated. Further researches are needed for early recognition and better measurement of intrinsic PEEP and dynamic hyperinflation in this population.

中文翻译：

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ARDS 患者的动态恶性通货膨胀和内在 PEEP：谁、何时以及如何需要更多关注？

尊敬的编辑： 我们饶有兴趣地阅读了 Coppola 及其同事的报告 [1]，该报告涉及严重急性呼吸窘迫综合征 (ARDS) 患者动态过度充气和内在呼气末正压 (PEEP) 的存在及其可能因素。他们认为，在没有已知阻塞性疾病的镇静、瘫痪 ARDS 患者中，肺保护性通气期间的内在 PEEP 量可以忽略不计，并且不会影响呼吸机械特性。然而，仍然需要确定有关谁、何时以及如何监测和管理 ARDS 患者动态恶性通货膨胀和内在 PEEP 的一些细节。首先，谁需要更多关注监测 ARDS 的动态恶性通货膨胀和内在 PEEP？在 ARDS 的肺保护性通气期间，当呼吸频率 (RR) 需要达到最大 35 次呼吸/分钟才能提供每分钟通气量以最大限度地减少高碳酸血症和呼吸性酸中毒时，较高 RR 导致的呼气时间缩短可能会导致呼气时间缩短。产生大量的内在 PEEP [2, 3]。在科波拉等人的研究中。[1]，平均RR仅为16次/分钟。因此，对高碳酸血症和呼吸性酸中毒有较高 RR 要求的严重 ARDS 患者存在动态过度充气和内源性 PEEP 的高风险，需要更多研究。第二个问题是我们什么时候应该关注 ARDS 的动态恶性通货膨胀和内在 PEEP？在科波拉等人报道的文章中。[1]，仅对ARDS早期的瘫痪患者进行了调查。然而，在深度镇静且无瘫痪的 ARDS 患者中，反向触发的呼吸夹带可能会导致深度镇静的非瘫痪 ARDS 患者出现呼吸堆积，从而导致与肺保护性通气不相容的容量和压力，并可能导致内源性 PEEP和动态恶性通货膨胀[4]。最后，如何识别和计算ARDS患者的内在PEEP，特别是没有神经肌肉阻滞剂的患者。在Coppola的报告中[1]，内在PEEP被定义为总PEEP减去外部PEEP，当外部PEEP升高时，内在PEEP降低。呼气流量限制和气道关闭可能是 ARDS 患者发生内源性 PEEP 的两个主要因素，而对外部 PEEP 升高的反应可能是由于低 PEEP 时的气道关闭和流量限制以及高 PEEP 时的气道开放压力。因此，Coppola 研究中内在 PEEP 计算的可靠性在流量受限的情况下可能无效，这种情况可能发生在 ARDS 患者中。零 PEEP 时的内在 PEEP 测量应该更准确。此外，对于自主呼吸的患者，越来越多的报告表明，可以通过先进的监测系统（食管和胃测压、隔膜肌电图、电阻抗断层扫描），并且在有一定局限性的情况下，对自主呼吸的患者进行简单的气道阻塞操作[5]。因此，仍然需要评估有关谁、何时以及如何研究 ARDS 患者的内在 PEEP 和动态过度充气的详细信息。需要进一步的研究来早期识别和更好地测量该人群的内在 PEEP 和动态恶性通货膨胀。