Carvalho Alysson R, Spieth Peter M, Pelosi Paolo, Beda Alessandro, Lopes Agnaldo J, Neykova Boriana, Heller Axel R, Koch Thea, Gama de Abreu Marcelo
Clinic of Anesthesiology and Intensive Care Medicine, University Clinic Carl Gustav Carus, Dresden, Germany.
Anesth Analg. 2009 Sep;109(3):856-65. doi: 10.1213/ane.0b013e3181aff245.
Spontaneous breathing (SB) activity may improve gas exchange during mechanical ventilation mainly by the recruitment of previously collapsed regions. Pressure support ventilation (PSV) and biphasic positive airway pressure (BIPAP) are frequently used modes of SB, but little is known about the mechanisms of improvement of lung function during these modes of assisted mechanical ventilation. We evaluated the mechanisms behind the improvement of gas exchange with PSV and BIPAP.
Five pigs (25-29.3 kg) were mechanically ventilated in supine position, and acute lung injury (ALI) was induced by surfactant depletion. After stabilization, BIPAP was initiated with lower continuous positive airway pressure equal to 5 cm H2O and the higher continuous positive airway pressure titrated to achieve a tidal volume between 6 and 8 mL/kg. The depth of anesthesia was reduced, and when SB represented > or = 20% of total minute ventilation, PSV and BIPAP + SB were each performed for 1 h (random sequence). Whole chest helical computed tomography was performed during end-expiratory pauses and functional variables were obtained. Pulmonary blood flow (PBF) was marked with IV administered fluorescent microspheres, and spatial cluster analysis was used to determine the effects of each ventilatory mode on the distribution of PBF.
ALI led to impairment of lung function and increase of poorly and nonaerated areas in dependent lung regions (P < 0.05). PSV and BIPAP + SB similarly improved oxygenation and reduced venous admixture compared with controlled mechanical ventilation (P < 0.05). Despite that, a significant increase of nonaerated areas in dependent regions with a concomitant decrease of normally aerated areas was observed during SB. In five of six lung clusters, redistribution of PBF from dependent to nondependent, better aerated lung regions were observed during PSV and BIPAP + SB.
In this model of ALI, the improvements of oxygenation and venous admixture obtained during assisted mechanical ventilation with PSV and BIPAP + SB were explained by the redistribution of PBF toward nondependent lung regions rather than recruitment of dependent zones.
自主呼吸（SB）活动主要通过重新扩张先前萎陷的区域来改善机械通气期间的气体交换。压力支持通气（PSV）和双相气道正压通气（BIPAP）是常用的自主呼吸模式，但对于这些辅助机械通气模式下肺功能改善的机制了解甚少。我们评估了PSV和BIPAP改善气体交换背后的机制。
五只猪（体重25 - 29.3千克）仰卧位进行机械通气，通过表面活性剂耗竭诱导急性肺损伤（ALI）。稳定后，开始使用BIPAP，较低的持续气道正压等于5厘米水柱，较高的持续气道正压进行滴定以达到6至8毫升/千克的潮气量。降低麻醉深度，当自主呼吸占总分钟通气量的≥20%时，分别进行PSV和BIPAP + SB各1小时（随机顺序）。在呼气末暂停期间进行全胸部螺旋计算机断层扫描并获取功能变量。通过静脉注射荧光微球标记肺血流量（PBF），并使用空间聚类分析来确定每种通气模式对PBF分布的影响。
ALI导致肺功能受损以及依赖肺区未通气和通气不良区域增加（P < 0.05）。与控制机械通气相比，PSV和BIPAP + SB同样改善了氧合并减少了静脉血掺杂（P < 0.05）。尽管如此，在自主呼吸期间观察到依赖区域未通气区域显著增加，同时正常通气区域减少。在六个肺簇中的五个中，在PSV和BIPAP + SB期间观察到PBF从依赖肺区重新分布到非依赖、通气更好的肺区。
在这个ALI模型中，PSV和BIPAP + SB辅助机械通气期间氧合和静脉血掺杂的改善是由于PBF向非依赖肺区的重新分布，而不是依赖区域的重新扩张。
