- 期刊
The Myth of Positive End-expiratory Pressure Setting Using the Low Inflection Point in the Pressure-Volume Curve
以壓力－容積曲線之下轉折點設定呼氣末正壓之迷思
摘要
急性呼吸窘迫症候群(acute respiratory distress syndrome，ARDS)的病人常常需要呼吸器治療以維持生命，然而呼吸器設定失當將加重肺部發炎，稱為呼吸器導致肺傷害(ventilator-induced lung injury，VILI)，目前公認的VILI發生機制包括：高期氣容積(tidal volume，V(下標 T))導致肺泡過度膨脹(overdistension)與肺泡反覆開閉(cyclic alveolar closing and reopening)。呼氣末正壓(positive end-expiratory pressure，PEEP)對ARDS的治療十分重要，藉由塌陷肺泡的動員(alveolar recruitment)可以改善氣體交換功能，避免肺泡反覆開閉，進而預防VILI的發生，但是過高的PEEP將損及血行動力與造成肺泡過度膨脹，反而促進VILI產生。多年來對於如何設定理想的(optimal)PEEP仍多所爭議，臨床上習慣以動脈血氣(arterial oxygenation)來調整PEEP值，但此法完全不考慮PEEP所造成的肺部機械特性(lung mechanics)改變，忽略了VILI的發生機制。若考慮PEEP對肺部機械特性的影響，傳統觀念建議將PEEP設定在壓力－容積曲線(PV curve)的下轉折點(lower inflection point，LIP)之上，認為如此肺泡將可完全動員，使肺部擴張性(compliance)達到最佳程度。然而，新近許多研究證實：LIP以上的區段仍存在有塌陷肺泡，肺泡動員在高壓區仍持續進行，LIP與肺泡的臨界開啟壓力(threshold opening pressure，TOP)的關係較密切，與預防肺泡塌陷的臨界關閉壓力(threshold closing pressure，TCP)關係不大，故以吸氣段的LIP來設定PEEP似乎沒有生理根據。本文主要探討以LIP設定PEEP的適用性及其限制，並介紹PV curve的新觀念，強調肺泡動員對PV curve之重大影響，除了LIP之外，希望能找到其他運用PV curve的方法，使ARDS病人之PEEP設定能更符合呼吸生理需求。
並列摘要
Acute respiratory distress syndrome (ARDS) is characterized by severe hypoxemia and impaired pulmonary mechanics. Positive end-expiratory pressure (PEEP) is very important in the treatment of patients with ARDS because PEEP can improve the gas exchange function and prevent cyclic alveolar collapse and reopening, thus preventing ventilator-induced lung injury (VILI). However, to date there is no consensus on setting the optimal PEEP. Most clinicians adjust PEEP according to arterial oxygenation without considering the pulmonary structural changes that may exaggerate lung injury due to alveolar over-distension by the high airway pressure. The pressure-volume (PV) curve, which can be performed at bedside, is used generally to assess pulmonary mechanics. Many investigators suggest setting PEEP above the lower inflection point (LIP) on the inflation limb of the PV curve to avoid end-expiratory alveolar collapse. This review analyzes the feasibility and limitations of this method and introduces new insights into PV curves.