Effect of positive end-expiratory pressure ventidation on organ blood volumes

在实际应用中，呼气末正压通气（PEEP）开始时，常观察到心出气量有所下降。然而，其机制仍不清楚。有一种假说认为，正压可能使循环血液从中心区域（如心脏和肺部）转移到周围血管，因此正压可能减少静脉回流，这种假说基本上得到了一致的支持。我们打算在本研究中证实这一假设，并找到血液淤积发生最密集的外周区域。此外，我们试图观察在PEEP后池化将持续多长时间，并采取如何防止池化的措施。实验用氯胺酮- n22o - o_2麻醉，维库溴铵完全麻痹的小猎犬。拖mililiters的自体红细胞包装标签与^ < 99 > Tc (2 mCi)静脉注射和红细胞的密度是决定在大脑中,胸器官(主要是h ... eart和肺),肝脏,splen,后肢肌肉内的一股静脉与固定腔伽马射线探测器,分别,然后血液量计算计算放射性和血细胞比容血液的器官。随机负荷0、5、10、15、20 cmH_2O PEEP时，循环血容量和胸脏器血容量随PEEP变化呈负相关。另一方面，肝、脾和骨骼肌的血容量随PEEP的变化而增加。热稀释法测心输出量随PEEP变化而降低。在第二个实验系列中，用空气止血带压迫下腹部，每次PEEP随机升高腹内压至10和20 cmH_2O。通过这种方法，上述器官血容量的所有变化都减弱了，特别是循环血容量的减少和骨骼肌血容量的增加都消失了。然而，心输出量的减少仍然持续存在。在第三个系列中，观察了多巴胺（5杯，kg^<-1>, min^<-1>）或硝酸异山药（2杯，kg^<-1>, min^<-1>）在15 cmH_2O PEEP下，伴或不伴20 cmH_2O腹内压时对器官血容量和心输出量变化的影响。输注多巴胺或硝酸异山梨酯后，器官血容量均无明显变化。然而，心输出量的减少在多巴胺输注期间恢复，而硝酸异山梨酯输注期间没有恢复。根据以上结果，我们推测PEEP下心排血量的减少不是单一因素引起的，而是多种因素共同作用的结果。在未来，它似乎需要更多的实质性研究，特别是与内分泌方法。少

At initiation of positive end-expiratory pressure ventilation (PEEP), some decreases in cardiac out are usually observed in practice. However its mechanism remains still obscure. A hypothesis, that the PEEP may move the circulating blood from the central area, such as the heart and lung, to peripheral vessels and thus the PEEP may decrease the venous return, has been supported mostly consistently. We intended to confirm this hypothesis in this study and to find the peripheral area where the blood pooling would occur most intensively. Further we attempted to observe how long the pooling would be continued after the PEEP and to adopt how we could prevent the pooling.Experiments were carried out in beagles which were anesthetized with ketamine-N_2O-O_2 and fully paralyzed with vecuronium. Tow mililiters of autologous packed red cells which were labelled with ^<99m>Tc (2 mCi) were injected intravenously and density of the red cells was determined in the brain, thoracic organs (mainly the h … More eart and lungs), liver, splen, adductor muscles in the hind limb and one of the femoral vein with the fixed lumen by gamma ray detectors, respectively, and then blood volumes were calculated with the counted radioactivities and hematocrit value for the blood in the organ.While zero, 5,10,15, and 20 cmH_2O PEEP were loaded randomly, decreases in the circulating blood volume and thoracic organ blood volume were observed inversely with the changes in PEEP.On the hand, the blood volumes in the liver, spleen and skeletal muscle increased paralleling with the changes in PEEP.Cardiac output measured with thermodilution technique decreased paralleling with the changes in PEEP.In the second experimented series, the lower part of abdomen was compressed by an air tourniquet and the intraabdominal pressure was elevated to 10 and 20 cmH_2O randomly for each PEEP.By this procedure, all changes in the organ blood volume described above were attenuated and, particularly, both decrease in the circulating blood volume and increase in the skeletal muscle blood volume disappeared. However the decrease in cardiac output had still persisted.In the third series, effects of dopamine (5 mug, kg^<-1>, min^<-1>) or isosorbid dinitrate (2 mug, kg^<-1>, min^<-1>) on the changes in organ blood volume and cardiac output were observed under 15 cmH_2O PEEP with or without 20 cmH_2O intraabdominal pressure. None of definite change was noted in the organ blood volumes either with the infusion of dopamine or isosorbide dinitrate. However the decrease in cardiac output recovered during the infusion of dopamine but not isosorbide dinitrate.From the results obtained above, we presumed that the decrease in cardiac output under PEEP is caused by not sole factor but by complexes of many factors. In the future it seems to need more substantial study, particularly with endocrinological approch. Less