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Ann Thorac Surg 2000;70:690-691
© 2000 The Society of Thoracic Surgeons
a 2nd Department of Surgery, Yamagata University School of Medicine, Yamagata, Japan
e-mail: takwatan{at}med.id.yamagata-u.ac.jp
To the Editor
We appreciate the keen attention paid by Miyamoto and Miyamoto to our recent article. We agree with the title and the essence of their letter in which the importance of gas management for deep hypothermic perfusion is emphasized and some criticism is revealed. Excessive use of microspheres may disturb the brain microcirculation. Therefore, we injected microspheres only two times during deep hypothermia and avoided normothermia as a baseline. A control group at normothermia might have been helpful, as recommended. The 60-minute data were obtained by injecting microspheres at 60 minutes of deep hypothermic perfusion as described in the Methods section. We used an arterial carbon dioxide tension that was a little higher than strict alpha-stat strategy throughout the protocol to avoid cerebral vasoconstriction. The pH measured was lower by 0.1 than that measured in vivo by nuclear magnetic resonance elsewhere [1]. Our method with vital stain requires several minutes for slicing the entire brain, which may contribute a decrease of about 0.1 to the pH. Therefore, the pH of 7.2 in our study, compatible with other measurements [1], may not suggest any metabolic derangement. Higami and coworkers [2] evaluated the safe limit of retrograde cerebral perfusion to be 70 minutes and that of antegrade cerebral perfusion, to be much longer.
Glucose utilization failure after rewarming [3] can be a result of low-flow perfusion during cooling and rewarming and luxury perfusion during deep hypothermia, as the perfusion flow rate is fixed over a wide temperature range. Hindman and associates [4] showed stable cerebral blood flow and oxygen consumption during 27°C perfusion using both the alpha-stat and pH-stat strategies, where the latter was not supported exclusively.
Nature may have coped with hypoxia by hypercarbic acidosis (pH-stat management). However, there is a long natural history of alpha-stat respiration as long as oxygen deprivation does not occur [5]. With reference to deep hypothermic cardiopulmonary bypass, the following hypothesis would apply: pH-stat strategy for core cooling and total circulatory arrest and alpha-stat strategy for continuous deep hypothermic perfusion.
As a goal of our study, we showed the effect of perfusion flow rate and pressure on cerebral blood flow and metabolism during 120 minutes of continuous perfusion. Because the Bohr effect cannot be ignored, we carefully equalized the cooling condition. As Miyamoto and Miyamoto stressed with possibly inadequate examples, the optimal strategy for core cooling has long been a target of investigation. Recent observations [6] may support the use of pH-stat management at least during cooling perfusion for total circulatory arrest. The brain may obtain additional storage of high-energy phosphates to deal with hypoxia when cooled by a higher-hematocrit perfusate and a higher cerebral blood flow with pH-stat management [1, 6]. The effect of a gaseous strategy should also be tested during continuous deep hypothermic perfusion with antegrade flow and with retrograde flow and during rewarming perfusion. Though widely used, deep hypothermic perfusion still raises many queries with clinical implications that need to be investigated.
References
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