Hemoglobin solution: Is a normal [Hb] or P50 more important?

Although stroma-free hemoglobin solution (SFH) supports normal levels of oxygen consumption in baboons exchange transfused to zero hematocrit, a significant decrease occurs in the mixed venous oxygen tension ($\text{P}\text{v}\text{O}{}_2$). The hemoglobin concentration (6–8 g%) and P₅₀ (12–17 Torr) are both low in the current SFH preparations, and it is likely that they contribute to the low $\text{P}\text{v}\text{O}{}_2$ values. A multiple linear regression analysis was used to quantitate the contribution of simultaneous changes in [Hb] and P₅₀ to the observed change in $\text{P}\text{v}\text{O}{}_2$ during total exchange transfusion with SFH. During the study the average $\text{P}\text{v}\text{O}{}_2$ decreased from 51.4 to 22.4 Torr; the [Hb] fell from 11.6 to 5.1 g%; and the P₅₀ decreased from 31 to 19.5 Torr. The resulting equation was $\text{P}\text{v}\text{O}{}_2\text{= 4.0[}\text{Hb}\text{] + 0.33 P}{}_{50}\text{- 4.2}$. The equation calculates that the 6.5 g% decrease in [Hb] was responsible for a fall in $\text{P}\text{v}\text{O}{}_2$ of 26 Torr, while the fall in P₅₀ of 11.5 Torr decreased the $\text{P}\text{v}\text{O}{}_2$ only 3.8 Torr. The data suggest that an increased [Hb] should be more effective in raising $\text{P}\text{v}\text{O}{}_2$ than a higher P₅₀. Since a low $\text{P}\text{v}\text{O}{}_2$ is an indication of a reduced oxygen reserve, future efforts should attempt to achieve a hemoglobin solution with a normal [Hb].