Fraction of oxygenated haemoglobin

This chapter is most relevant to Section F7(i) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "describe the carriage of oxygen in blood", or it could refer to Section F9(iii) which is "interpret normal and abnormal blood gases". There is probably some utility in knowing about this, as many ABG machines report these variables, and knowing what they mean must have some sort of professional benefit. One might have thought that there is probably no benefit to it in terms of passing exams, as this specific narrow topic is unlikely to ever become the subject of a CICM exam question, but the examiners sprung this on us in Question 17 from the first paper of 2021, where they complained that only few of the candidates described the inability fo discriminate functional from fractional saturation as a limitation of pulse oximetry

In the briefest summary,

The fraction of oxygenated haemoglobin is a parameter that is calculated from values directly measured by absorption spectrophotometry. It is expressed as FO₂Hb.

As you can see, oxygen saturation and the fraction of oxygenated haemoglobin are not interchangeable.

Observe this crude diagram, which illustrates the difference between the various measures of haemoglobin oxygen content:

Functional saturation vs fractional saturation

The term "functional saturation" refers to the saturation of the haemoglobin which is capable of transporting oxygen, and corresponds to the sO₂. The term "fractional saturation" refers to the fraction of total haemoglobin which happens to be oxygenated, and refers to the FO₂Hb.

In healthy individuals, the two values will be very similar, as these people typically have little dyshaemoglobin circulating around. However, the sO₂ will be consistently a little higher, because everybody has tiny (~1%) fractions of FMetHb and FCOHb.

This has implications for the estimation of the total oxygen-carrying capacity of the blood. In the presence of large concentrations of dyshaemoglobin, the sO₂ will remain normal, while the actual oxygen transport will be severely impaired. The FO₂Hb will accurately reflect this impairment.

What is the point of measuring this value?

Which is the more trustworthy of the two indices, FO₂Hb or sO₂?

The answer is that both are useful in different circumstance.

One must remember that the end goal of all these measurements is the determination of the oxygen content of the blood.

If one does not have the ability to measure abnormal haemoglobin species, one is served with the ctHb value. This value represents the total haemoglobin. The FO₂Hb can be used together with the total haemoglobin to calculate the total oxygen content; and one would not need to know the concentration of dyshaemoglobin species.

However, one would not be able to determine the total oxygen carrying capacity in this way.

For that, the sO₂ would be the more relevant measure. The sO₂ value excludes haemoglobins which have lack the capacity for oxygen transport (it uses oxyhaemoglobin and deoxyhaemoglobin only).