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Ann Thorac Surg 1996;61:1043-1044
© 1996 The Society of Thoracic Surgeons
Department of Cardiovascular Surgery, Tenri Hospital, 200Mishima, Tenri, Nara 632 Japan
To the Editor:
In the August 1995 issue of The Annals, Boeckxstanens and Flameng [1] raise a valid question about retrograde cerebral perfusion (RCP), which does not perfuse the brain in an experiment using nonhuman primates. They described that their experimental setting was designed to be as close as possible to the clinical situation to obtain the answer. There was, however, a major modification of the method of RCP that differed from our clinical report. In their experiment, inferior caval blood was drained and the aorta was not opened during RCP. Consequently, the pressure of the inferior vena cava and right atrium must be lower than jugular vein pressure, nearly 0 mm Hg or less. On the other hand, the pressure in the brain system can be kept high, as Dr Bradley S. Allen pointed out in his discussion. This might promote massive shunt of the oxygenated blood from the internal jugular vein to the venous system of the body. Retrograde cerebral perfusion blood escaped to the area of lower pressure instead of the brain and the opened aorta. Therefore, Boeckxstanens and Flameng concluded that in nonhuman primates, RCP does not perfuse the brain because of venovenous shunting.
We regard their experimental model as not suitable for evaluation of brain perfusion during RCP. Actually, we clamp the inferior vena caval cannula during RCP as described in our reports [2–4]. The pressure of the inferior vena cava and the right atrium was gradually increasing and reached almost the level of jugular venous pressure during RCP in our clinical practice. Rich venovenous anastomoses at various levels are naturally present; however, shunt flow was minimized by this small pressure gradient between the superior and inferior venae cavae. The open aorta was the only space at the pressure of the atmosphere, ie, 0 mm Hg, and the rest of the body including the venous system was kept at 15 to 20 mm Hg of pressure during RCP. We conclude that retrograde flow to the brain will increase in this situation, even though large venovenous anastomoses are present. Safi [5] also pointed out this issue in an invited commentary on a report by Pagano and colleagues. The crucial point of RCP is that the whole venous system should be higher than the open aorta.
Another issue is competency of the internal jugular valve, which is the only venous valve between the right atrium and brain in humans. The competency and existence of the internal jugular vein valve is controversial. The flow rate of RCP seems to depend on the competence of the valve. However, rich collaterals promote shunting of flow to the brain in the same way as mentioned above. Our clinical results show that RCP has protective effects for the brain under deep hypothermic circulatory arrest [4]. We believe that RCP does perfuse the brain in humans, even if it is only a part of the RCP flow.
References
Department of Cardiac Surgery, Katholieke Universiteit Leuven, Herestraat 49, B-3000 Leuven, Belgium
To the Editor:
We appreciate the comments of Ueda and Miki on our experimental study of retrograde cerebral perfusion in the baboon. They might be quite correct by saying that our experimental model does not correspond to the clinical setting of RCP they use. If you close the inferior caval vein and perfuse via the superior caval vein, you are dealing with whole-body retrograde perfusion because of the large anastomoses between both caval vein systems. It was not the aim of our study to assess brain perfusion during whole-body retrograde perfusion but during isolated retrograde perfusion via the internal jugular vein. Our experimental setup is based on the protocol used by Usui and associates. In their first study investigating RCP in dogs [1], they mentioned that approximately 80% of RCP inflow returned via the inferior caval vein. In later studies they closed the inferior caval vein and reported a dramatic increase in the pressure in the lower part of the body and the portal system with creation of ascites [2]. In another study on dogs, Yoshima and colleagues [3] did not mention the outflow during RCP, but the drawing of the circuit suggests that they worked with an open caval vein.
From our pilot experiments we also learned that when RCP was instilled via the superior caval vein after the inferior caval vein was clamped, after a few minutes the blood reservoir of the cardiopulmonary bypass was empty, but the volume returned immediately after the inferior caval vein was opened.
Most clinical studies fail to mention whether the inferior caval vein is closed; only Pagano and associates [4] in their clinical report, mentioned clearly drainage via the inferior caval vein. Despite the remark made by Safi on the study by Pagano, a real consensus whether the inferior caval vein has to be opened or closed cannot be recognized when reviewing the literature on RCP.
It is not correct that our experiments are performed with a closed aorta; the aorta drained freely, and the amount of outflow was measured. This is clearly stated in our article. Obviously, with open inferior caval drainage the blood flow prefers to shunt away from the brain tissue to the inferior caval vein system, in spite of the pressure in the arterial system being zero.
We believe that Ueda and Miki's letter can help to unravel the confusion about the different techniques used for RCP. Perhaps the brain will be perfused retrogradely with the use of whole-body retrograde perfusion, but this still remains to be proved.
References
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