这句与全段的关系

It has long been known that, in contrast to the situation in intensive care, PEEP often does little to improve the arterial Po2 during anaesthesia.There are two reasons why PEEP is not associated with improved oxygenation. First, the decrease in cardiac output associated with PEEP reduces the oxygen saturation of the blood traversing the remaining shunt, and so reduces arterial Po2. Second, in the supine position PEEP increases ventilation in dependent areas, but causes similar changes in perfusion such that regional V˙/Q˙ ratios are virtually unchanged (Fig. 21.20). The essential difference from the patient undergoing critical care is probably the lack of protection of intrathoracic blood vessels from raised airway pressure that is afforded by stiff lungs in most patients requiring critical care.

呼气末正压的作用

众所周知，相对于重症监护，PEEP常对改善麻醉期间的动脉PO2几无作用。PEEP与改善氧合无关有两个原因。首先，与PEEP相关的心输出量减少就降低了流经剩余分流区域的血氧饱和度，因此降低了动脉PO2。其次，仰卧位时PEEP增加重力依赖区的通气，但也引起类似的重力依赖区的灌注增加，使局部性V/Q比值几乎不变（图21.20）。麻醉患者与重症监护患者的本质差异可能是，大多数需要重症监护的患者中，肺顺应性的下降需提高气道压，这可能会损害胸内血管。

这句与全段的关系？怎么感觉与全段的意思相反