To the Editor:—
We have considerable interest in the study by Nielsen et al. 1Their article provides additional insights into the incredible physiologic tolerance to severe isovolemic anemia. We were particularly struck by the arterial blood gas data, which revealed an increase in arterial partial pressure of oxygen (PaO2) with hemodilution and an apparent strong inverse correlation between PaO2and hematocrit. Even the relatively small increase in PaO2in the control group was associated with a decrease in hematocrit from 33% to 28%. However, despite rather large increases in PaO2with hemodilution (mean increases of 50, 98, and 72 mmHg in the three groups), the authors suggest that the changes were not statistically significant. We realize that this was not a primary outcome measure of focus of the study, but an improvement in PaO2with hemodilution has been noted by us 2and other investigators. Perhaps a more robust test of significance involving this type of protocol would be repeated-measures analysis of variance between treatment (hemodilution) and control animals. The use of one-way analysis of variance to test for differences at individual points will result in considerable loss of power to detect differences between groups over time because of the between-subject variability at each time point.
An improvement in arterial partial pressure of carbon dioxide with anemia may occur because of changes in pulmonary blood flow distribution and improved ventilation–perfusion distribution. 2Given the general interest in this subject, we would welcome any further information the authors could provide.