- This is an idealised value calculated from the measured PaO2 at a standard set of conditions (pH 7.40, pCO2 40mmHg, and in the absence of dyshaemoglobins), represented as p50(st).
- By "normalising" the potentially very abnormal environment of the critically ill patient's bloodstream, the p50(st) value eliminates the effect of everything except 2,3-DPG.
- In the adult, the normal p50(st) should be 24-28mmHg.
Unlike the "real" p50 which at least makes some attempt to adjust the shape of the oxygen-hemoglobin dissociation curve according to measured variables, the p50(st) makes no such attempt.
Observe the equation for p50:
Now, the equation for p50(st) would look exactly the same, with the exception of the calculated shift.
As has been previously discussed, this is a sum of all the influences on the x-axis position of the curve, all the "Bohr's effects" (pH and pCO2) as well as the concentration of "normal" dyshaemoglobins and temperature.
If all of these were null, the only remaining measured variables required would be 2,3-DPG and weird rate dyshaemoglobins (like sulfhaemoglobin or nitrosylhaemoglobin, for example).
Therefore, the real value of p50(st) lies in the comparison between itself and p50.
Observe, the oxygen-hemoglobin dissociation curve with a hypothetical left shift.
Thus, the difference between the two values reflects the influence of Bohr's effect and temperature on the shape of the dissociation curve. This is actually a valuable piece of information, as these variables are amenable to some sort of crude adjustment. In contrast, the concentration of 2,3 -DPG is not something you can immediately address.