摘要
严重慢性阻塞性肺疾病(COPD)和急性呼吸衰竭(AVF)患者机械通气时,呼气末肺容积偏离呼吸系统弹性平衡容积是一个重要特征。动态恶性膨胀的存在意味着肺泡压力在呼气过程中保持正。在呼气结束时,这种正压被称为内在呼气末正压(PEEPi)。最近的研究表明,在有呼气流量限制的COPD患者中,在辅助机械通气时应用外置PEEP,或在自主呼吸患者中应用持续气道正压(continuous positive airway pressure, CPAP),可以平衡和降低PEEPi施加的吸气阈值负荷。而不会引起肺体积、肺泡和胸腔内压力的进一步增加,直到达到临界值PEEP (Pcrit)。超过这个临界值,就会出现进一步的恶性通货膨胀。PEEPi在AVF期间损害COPD患者心功能的特异性和特征性作用可以通过以下因素确定:1)肺过度膨胀导致右心室阻抗增加;2)由于吸气阈值负荷引起的胸内压力较大的负偏转,导致静脉回流至右心室增加,从而导致室间隔左移;3)回流至右心室的静脉进一步增加,最终因腹肌呼气补张引起腔静脉塌陷;4)急性呼吸衰竭引起的低氧和高碳酸血症,可进一步增加右心室阻抗和静脉回流到右心室。 All these phenomenon are directly correlated to the large negative intrathoracic pressure developed by the respiratory muscles to overcome the inspiratory threshold caused by intrinsic positive end-expiratory pressure (preload effect), and to the increase in lung volume (afterload effect). Application of positive end-expiratory pressure/continuous positive airway pressure in chronic obstructive pulmonary disease patients during acute ventilatory failure may, hence, unload the respiratory muscles as well as the heart.