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

点击翻译按钮获取中文摘要

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