Stress Testing the Respiratory System: Too Little or Too Much?
Stress Testing the Respiratory System: Too Little or Too Much?
复制标题
呼吸系统压力测试：太少还是太多？
DOI：
10.1164/rccm.201611-2283ed
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发表时间：
2017
影响因子：
24.7
通讯作者：
Parthasarathy,Sairam
中科院分区：
文献类型：
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作者：
Parthasarathy,Sairam
在美国，近27%的住院治疗需要留在重症监护室（ICU），占医院费用的48%（1）。大多数此类ICU使用者为呼吸系统疾病患者，需要机械通气支持的时间少于96小时（1）。因此，在美国需要机械通气的患者每年造成的负担相当于2005年估计的80万例住院治疗，花费近270亿美元（2）。自2005年以来，在需求的推动下，美国重症监护病床的数量增长了15%（3）。需要机械通气的患者对重症监护服务的需求迅速增长，住院时间更是雪上加霜。在接受机械通气的患者中，在ICU中花费的总时间的40-50%用于脱机过程，这反过来又受到重症监护医师能够预测患者在拔管后维持自主呼吸的能力的限制（4，5）。可在拔管前通过尝试模拟拔管后条件进行自主呼吸准备度评估，方法是提供低水平压力支持（5-7 cm H2O），呼气末正压（PEEP）为5 cm H2O，持续气道正压通气（CPAP）或T型管。以压力支持的形式提供低水平吸气辅助的基本原理是对抗气管内插管阻力和自插管首次插入以来可能累积的结石所施加的气道阻力，因此最好地模拟拔管后的生理状态（6）。然而，其他研究表明，拔管后气管内导管的阻力负荷被水肿的上气道所施加的阻力负荷所取代，并且在T形件试验后拔管前后的呼吸功是相同的（7）。此外，存在慢性阻塞性肺疾病（COPD）患者中由内源性PEEP呈现的“隐性”弹性负荷的问题，其被CPAP或PEEP的应用有效地“掩盖”。在COPD患者中，与T形管相比，CPAP设置为接近固有PEEP水平（z 8 cm H2O）可将膈肌的压力时间乘积（呼吸努力的测量）降低45%（8）。因此，CPAP有效地掩盖了COPD患者隐性内源性PEEP施加的弹性负荷。隐匿性是一个可以使用的术语，因为尽管有大量关于内源性PEEP和动态过度充气对COPD危重患者呼吸努力的影响的科学文献，但在研究环境之外对这些生理变量进行准确测量是不可行的（9，10）。在接受CPAP或压力支持联合PEEP时似乎呼吸正常的患者中，拔管后停用CPAP/PEEP可能导致弹性负荷显著增加，与危重患者的呼吸肌无力结合，可能导致快速呼吸功能受损，从而避免重新插管（5，11，12）。可以认为，上述逻辑引用了个体生理学研究，样本量非常小，受试者群体的变异性很高。正是由于这个原因，Sklar及其同事在本期杂志中通过严格的荟萃分析进行了汇总分析。1477-1485）是最受欢迎的（13）。Sklar及其同事回顾了4，138篇引文，产生了涉及239名患者的16项研究（13）。他们证明了在拔管后状态下，当使用压力支持和CPAP进行呼吸试验时，系统性低估了呼吸功。
Nearly 27% of hospitalizations in the United States require stays in the intensive care unit (ICU), and they account for 48% of hospital charges (1). A majority of such ICU use was by patients with respiratory conditions requiring mechanical ventilation support for less than 96 hours (1). As a consequence, the annual burden imposed by patients requiring mechanical ventilation in the United States amounts to a 2005 estimate of 800,000 hospitalizations that cost nearly $27 billion (2). Since 2005, the number of critical care beds in the United States, driven by demand, has grown by 15%(3). Such a burgeoning demand for critical care services in patients requiring mechanical ventilation is further compounded by their length of stay. In patients receiving mechanical ventilation, 40–50% of the total time spent in the ICU is devoted to the weaning process, which, in turn, is rate-limited by the intensivist being able to predict the patient’s ability to sustain spontaneous breathing postextubation (4, 5). Assessment of readiness to breathe spontaneously can be performed before extubation by trying to mimic the postextubation conditions by providing low levels of pressure support (5–7 cm H2O) with positive end-expiratory pressure (PEEP) of 5 cm H2O, continuous positive airway pressure (CPAP), or T-piece. The rationale for providing the low levels of inspiratory assistance in the form of pressure support is to counter the airway resistance imposed by resistance of the endotracheal tube and concretions that may have accumulated since the time the tube was first inserted, and therefore to best simulate the postextubation physiological state (6). However, others have shown that the resistive load of the endotracheal tube is supplanted by that imposed by edematous upper airways after extubation, and that the work of breathing is identical both before and after extubation after a T-piece trial (7). Moreover, there is the issue of “occult” elastic load presented by intrinsic PEEP in patients with chronic obstructive pulmonary disease (COPD) that is effectively “masked” by application of CPAP or PEEP. In patients with COPD, CPAP set close to the intrinsic PEEP levels (z8 cm H2O) reduces the pressure time product of the diaphragm (a measure of respiratory effort) by 45% when compared with T-piece (8). CPAP therefore effectively masks the elastic load imposed by occult intrinsic PEEP in patients with COPD. Occult is a term that could be used because, despite a significant body of scientific literature on the effect of intrinsic PEEP and dynamic hyperinflation on breathing effort in critically ill patients with COPD, it is infeasible to perform accurate measures of such physiological variables outside the research setting (9, 10). Potentially, in such patients who seemingly appear to breathe normally when receiving CPAP or pressure support combined with PEEP, withdrawal of CPAP/PEEP after extubation could lead to marked increase in elastic load, which, combined with respiratory muscle weakness in critically ill patients, could potentially lead to rapid respiratory compromise, warranting reintubation (5, 11, 12).It can be argued that the aforementioned logic cites individual physiological studies, with very small sample size and high variability in the subject population. It is for this reason that the pooled analysis by the rigorous metaanalysis in this issue of the Journal by Sklar and colleagues (pp. 1477–1485) is most welcome (13). Sklar and colleagues reviewed 4,138 citations to yield 16 studies involving 239 patients (13). They demonstrate a systematic underestimation of the work of breathing in the postextubation state when breathing trials are performed, using pressure support and CPAP …