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Ann Thorac Surg 2000;69:974-975
© 2000 The Society of Thoracic Surgeons
a Department of Cardiac Surgery, University of Milan, Milan, Italy
To the Editor
We thank Drs Miyamoto and Miyamoto for their comments on our paper entitled "Cardiopulmonary bypass and oxygen consumption: oxygen delivery and hemodynamics."
In their letter, they claim that the reduction in oxygen extraction rates during routine hypothermic cardiopulmonary bypass (CPB) with alpha-stat acid base management has to be ascribed to increased oxygen affinity for hemoglobin, resulting both from hypothermia and to the alkalotic environment of the alpha-stat acid base management. They also suggest, that the best method to improve oxygen transport to tissues in such conditions could be the use of a more acidotic protocol of acid base management (pH-stat), to facilitate the release of oxygen from hemoglobin.
However, previous studies document no significant differences in oxygen consumption during moderately hypothermic perfusion with alpha-stat or pH-stat protocols [1, 2]. The behavior of the oxyhemoglobin dissociation curve did not change, and the values of P50 were very similar in the case of alpha-stat or pH-stat protocol use, because in these conditions the effect of hypothermia on the oxyhemoglobin dissociation is predominant over the Bohr effect [1]. In addition, a relative impairment in oxygen extraction, with respect to animal models, had previously been documented in anesthesized humans at normothermia [3], as well in acutely and chronically ill patients [4]. In these cases it cannot be ascribed to hypothermia nor to acid base management. Furthermore, another case of hypothermic (20°C) perfusion in humans, with the use of the alpha-stat method, documented oxygen extraction rates significantly higher, up to 55%, and in this case there was no impairment in oxygen extraction [5].
For the aforementioned reasons, we do not believe that the reduction in tissue capabilities of oxygen extraction, as documented in our study, should be ascribed to the pH management protocol, and the causes still need to be defined. However, the evidence of an impaired oxygen extraction that occurs during moderately hypothermic routine CPB should stimulate towards CPB protocols with lower vascular resistances and higher flow rates, so that the impairment of the tissues capability in oxygen extraction can, at least partially, be overcome.
In addition, we can not share the statement that the use of alpha-stat acid-base management protocol results in alkalosis. When hypothermia ensues, alpha-stat allows arterial blood pH to rise as expected for a fluid (ie, the pH of water at 37°C is 6.8, whereas at 20°C it is 7.4) or for blood in a closed system [6]. Alpha-stat also allows the maintenance of the constancy of the fraction of dissociated imidazole groups versus total imidazole groups of histidine, which is the main protein buffer intra and extracellularly. So, with alpha-stat regulation, arterial blood pH remains close to the value that represents the biologic neutrality of blood at normothermia.
Finally, we perfectly agree with Drs Miyamoto and Miyamoto on the need for additional studies to better define the pros and cons of the different protocols for acid-base management during routine hypothermic CPB.
References
-stat versus pH-stat strategy on oxyhemoglobin dissociation and whole-body oxygen consumption during hypothermic cardiopulmonary bypass. Anesth Analg 1992;74:32-37.Related Article
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