Numerous past paper SAQs have presented the candidate with an ABG featuring an oxygenation or ventilation problem:
As one can plainly see, these seem to take a fairly standard form. The candidate calculates the A-a gradient, or the A-eT CO2 gradient, or discovers a shift of the oxyhaemoglobin dissociation curve. Chapters which might be relevant to this exist locally:
However, these are much too verbose for casual revision. This chapter is a summary of the necessary material for the time-poor exam candidate.
Thus, one can say that PAO2 = (FiO2) × (713) - (PaCO2 × 1.25).
Unfortunately, this plays a major role in the SAQs, and probably needs to be committed to memory. The only use for this clinically is in excluding the possibility of a hypoventilation-associated hypoxia (i.e. where the alveolar concentration of CO2 is so high that it abuses Dalton's Law to crowd oxygen out of the alveolar gas mixture). Another, more esoteric application is reminded by Question 6.1 from the second paper of 2010, where by calculating the alveolar gas mixture we arrive at the conclusion that the atmospheric gas must contain an unusually low concentration of oxygen, and that therefore the patient is either close to the summit of Mount Everest or is sitting in some sort of sadistic vacuum chamber.
In the adult, the normal p50 should be 24-28mmHg.