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Department of Cardiac Anesthesia CHU Charleroi Charleroi, Belgium
Department of Intensive Care CUB Erasme Brussels, Belgium
In the article by Schou et al. (1) regarding the effects of acute normovolemic hemodilution on the tolerance of anesthetized pigs to uncompensated blood loss, in their discussion, they state, "we could not confirm the findings of Van der Linden and coworkers that hemodilution increases the tolerance to hemorrhage." We cannot accept the interpretation the authors have placed on our data. Indeed, contrary to their interpretation, we specifically stated in our article (2) "the present data do not imply that the reduced hemoglobin is beneficial at the onset of acute hemorrhage." In our study (2), we tested the hypothesis that hemodilution is associated with an increase in tissue oxygen extraction capabilities, and for this purpose, we used a model of progressive hemorrhage, which has also been used by other groups to evaluate the effects of different conditions on tissue oxygen extraction capabilities in anesthetized animals (3,4). It was not the purpose of our study to assess the tolerance of hemodiluted animals to hemorrhage, as in the study by Schou et al. (1). We believe that it is, in fact, a logical assumption that hemodilution will decrease the tolerance to blood loss as the oxygen reserves decrease, and that animals will not tolerate acute blood loss under conditions of extreme hemodilution.
Furthermore, at least some of the animals studied by Schou et al. (1) had already developed tissue dysoxia before the hemorrhagic sequence was started, because of the high degree of hemodilution. This is illustrated by the significant increase in serum lactate reported by the authors after the hemodilution procedure and before hemorrhage. In the same animal species, the pig, Trouwborst et al. (5) and Räsänen (6) showed that the critical level of hemodilution below which tissue oxygen delivery did not match oxygen demand, with resultant tissue hypoxia, was reached at a hemoglobin level of 4.0 g/dL, with an oxygen extraction ratio of 56%–57%. Before the hemorrhagic sequence, animals in the study by Schou et al. (1) had a hemoglobin level of 3.3 ± 0.3 g/dL and an oxygen extraction ratio of 65% ± 6%. In this situation, the ability of tissue to extract more oxygen to compensate for a further decrease in oxygen delivery due to the decreased blood volume could certainly be expected to be very limited.
References
Department of Anesthesia and Intensive Care University Hospital S 221 85 Lund, Sweden
We thank Van der Linden and Vincent for their letter concerning our article (1) and note that, if we read the letter right, we are in factual agreement, namely that hemodilution will probably decrease the tolerance to any degree of hypovolemia. We regret if they believe that we have not correctly related the findings in their article (2) and therefore cite the concluding sentence of the summary of that article in toto: "In the conditions of our study, moderate hemodilution was associated with an improvement of the O2 extraction capabilities of the body, probably related to the reduction in blood viscosity."
References
P, et al. Influence of hematocrit on tissue O2 extraction capabilities during acute hemorrhage. Am J Physiol 1993;264:H1942–7.
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