压力支持通气和双相气道正压通气通过肺血流重新分布从而提高氧合

背景机械通气时保留自主呼吸（spontaneousbreathing，SB）可改善气体交换，主要是因为这种通气方式可使萎陷的肺复张。在保留SB的机械通气中，常选择压力支持通气（pressuresupportventilation，PSV）和双相气道正压通气（biphasicpositiveairwaypressure，BIPAP）。但医师对这些辅助性机械通气方式改善肺功能的机制却不甚了解。我们评价了采用PSV和BIPAP改善氧合的机制。方法5头猪（25-29．3kg）于仰卧位行机械通气，并通过清除肺表面活性物质诱发其发生急性肺损伤（acutelung砸jury，Au）。状态稳定后，给予BIPAP通气，初期较低持续气道正压设置为5cmH2O，随后逐渐增加持续气道正压至维持潮气量在6-8ml／kg。减浅麻醉深度，当SB≥每分通气量的20％时，随机进行1小时的PSV或BIPAP＋SB通气。于呼气末行全胸螺旋式计算机体层扫描，并记录肺功能参数。静脉给予荧光微球体来记录肺血流（pulmonarybloodflow，PBF），并利用空间聚类分析来评估每种通气方式对PBF重新分布的影响。结果Au损伤肺功能并加大了下肺萎陷或不张组织的面积（P〈0．05）。和对照组比较，PSV和BIPAP＋SB的通气模式提高了氧合并减少了静脉血掺杂（P〈0．05）。尽管如此，我们观察到自主呼吸时下肺的不张肺组织或通气不良组织明显增加，而可正常通气的肺组织减少。观察到6例中有5例行PSV或sB＋BIPAP通气时PBF由下至上重新分布，肺通气也较好。结论在ALI模型中采用PSV或BIPAP＋SB行机械通气可改善氧合，减少静脉血掺杂的原因为PBF由下至上的重新分布，而不是因为下肺复张。

BACKGROUND： Spontaneous breathing （SB） activity may improve gas exchange during mechanical ventila- tion mainly by the recruitment of previously collapsed regions. Pressure support ventilation （PSV） and biphasic positive air- way 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. METHODS： Five pigs （25 -29. 3 kg） were mechanically ventilated in supine position, and a- cute lung injury （ALI） was induced by surfactant depletion. After stabilization, BIPAP was initiated with lower continuous positive airway pressure equal to 5 cmH20 and the higher continuous positive airway pressure titrated to achieve a tidal vol- ume between 6 and 8 ml/kg. The depth of anesthesia was reduced, and when SB represented ≥20% of total minute ventila- tion, 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 duster analysis was used to deterlrfine the effects of each ventilato- ry mode on the distribution of PBF, RESULTS： ALl led to impairment of lung function and increase of poorly and nonaerat- ed 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 are-as in dependent regions with a concomitant decrease of normally aerated areas was observed during SB. In five of six lung dusters, redistribution of PBF from dependent to nondependent, better aerated lung regions were observed during PSV and BIPAP ＋ SB. CONCLUSIONS： In this model of ALI, the improvements of oxygenation and venous admixture obtained dur- ing assisted mechanical ventilation with PSV and BIPAP ＋SB were explained by the redistribution of PBF toward nondepen- dent lung regions rather than recruitment of dependent zones.