At a stretch, one might say that this chapter is relevant to some parts of Section F from the 2023 CICM Primary Syllabus, as it refers to something vaguely related to the respiratory system. Or it may correspond to the forgotten apocryphal codices of 2017, which expected the exam candidates to be able to "interpret normal and abnormal blood gases". One might be able to extend that old syllabus item to include the methods of measuring the blood gas variables, of which the oxygen saturation is an important representative. It has appeared at least once in the CICM First Part Exam, as Question 1 from the second paper of 2023, which asked the candidates to compare the pulse oximeter output with the "measured arterial value (SaO2)".
The principles underlying the use of absorption spectrophotometry to identify different haemoglobin species are discussed elsewhere. Suffice to say, it is possible to arrive at a measure of oxygen saturation, which the ABG machine expresses as sO2. Not SaO2, because that might be venous blood you are testing. In fact the whole nomenclature of saturation is carefully standardised.
This is a method of calculating the sO2 on the basis of spectrophotometric measurements.
In this definition, the oxygen saturation does not take into the equation all the demented cousin species of haemoglobin which cannot carry oxygen (eg, methaemoglobin and carboxyhaemoglobin). This is a sensible definition, as it assists one in calculating the total oxygen carrying capacity of the blood.
The influence of dyshaemoglobin species on the measurements of sO2 and FO2Hb (and how these values differ) is discussed in the chapter on FO2Hb (the fraction of oxygenated haemoglobin). In brief, it will suffice to say that oxygen saturation and the fraction of oxygenated haemoglobin are not interchangeable.
It is precisely because of its sophisticated approach to haemoglobin that the sO2 can get you into trouble. Hypothetically, there may exist a situation where all the haemoglobin is fully saturated (giving an sO2 of 100%) but only represents 10% of the total haemoglobin (because the rest has been disabled by carbon monoxide, to use a classical example). This means that the sO2 value can occasionally misrepresent the effectiveness of oxygen transport.
Furthermore, one needs to consider the possibility that the ABG machine is confabulating the sO2 value.
Perhaps confabulating is too strong a word. In the ABL800 Flex reference manual, sO2 is listed both among both the measured and derived parameters; "can also be calculated" says the company.
The sO2 value can be calculated from just the pO2 value and the pH. Certain ABG machines, it appears, are incapable of measuring all of the haemoglobin species, and are therefore incapable from generating a ceHb value. Thus, a lookup table is used: at this pH, with this pO2, what should the sO2 be?...
A number of equations are described and evaluated in the literature; only the Kelman, Sevinghaus and Siggaard-Andersen equations consistently generate numbers which agree with the measured values.
I will not complicate the issue by reproducing them here. Suffice to say the general aim of all of these is to generate a shape of the oxyhemoglobin dissociation curve, and fit as many parameters into it as possible in order to predict the direction of its shift.
In general, the measured sO2 and haemoglobin variables can be used to calculate the actual shape of the oxyhaemoglobin dissociation curve, which is a topic discussed elsewhere.