The buffer values and the Bohr effect of human fetal and adult whole blood in vitro in an acid range.

Abstract

Extract: The buffering properties and the Bohr effect were studied in vitro on 10 human fetal and 9 adult whole blood samples. At the beginning of the experiments, the fetal and adult blood samples were similarly acidotic (BE = - 9 mEq/liter).In order to obtain the respiratory and the metabolic buffer values, the blood was titrated either by varying the Pco2 at constant buffer base or by adding known amounts of base at constant Pco2. The mean fetal and adult in vitro apparent respiratory buffer values (β′) were 28.30 mEq HCO3-/liter·pH unit ± 2.23 sem and 23.66 mEq HCO3-/liter · pH unit ± 0.80 sem, respectively. These values were not statistically different. The in vitro apparent metabolic buffer value (λ) rose significantly when Pco2 increased. At Pco2 of 30, 40, and 60 mm Hg, the mean values were, respectively, for fetal blood: 55 ± 1.75 sem, 61.25 ± 1.41 sem, and 71.25 ± 3.21 sem mEq H+/liter·pH unit; for adult blood: 61.94 ± 2.76 sem, 65.83 ± 2.47 sem, and 71.94 ± 2.97 sem mEq H+/liter·pH unit. Only at a Pco2 of 30 mm Hg was the difference between adult and fetal blood statistically significant. In order to determine the magnitude of the Bohr effect, the iso-Pco2 pH change ($DTpHox) associated with the oxygenation of a desaturated blood sample was measured and the O2 contents were determined by Natelson's micromanometric method. Thus, by the following equation: $DTH+ = λ·$DTpH, the ratio - $DTH+/$DTO2 was calculated. The mean values obtained for Pco2 of 30, 40, and 60 mm Hg were, respectively, 0.330 ± 0.027 sem, 0.365 ± 0.028 sem, and 0.401 ± 0.032 sem mEq H+/mM O2 for fetal blood; and 0.391 ± 0.029 sem, 0.414 ± 0.029 sem and 0.445 ± 0.032 sem mEq H+/mM O2 for adult blood. No significant differences were found at each Pco2 between the adult and the fetal blood.Speculation: These results show that the in vitro buffering properties and the Bohr effect were similar in the adult and the fetal whole blood. The observed difference between the adult and fetal blood for the metabolic buffer value at Pco2 of 30 mm Hg could be explained by differences in hemoglobin (Hb) concentration or different carbamino reaction with fetal hemoglobin (HbF). However, the heterogeneity and the complexity of whole blood hinder simple interpretation. The practical implication of this study is that the use of the Siggaard-Andersen's nomogram for fetal blood is justified, at least in vitro.