Assessment of oxygenation on the basis of arterial blood gases

Numerous past paper SAQs have presented the candidate with an ABG featuring an oxygenation or ventilation problem:

• Question 13.4 from the first  paper of 2014 (increased dead space in PE)
• Question 30.3 from the first  paper of 2014 (a left-shifted p50)
• Question 3.1 from the second  paper of 2012 (hypoventilation with a normal A-a gradient)
• Question 6.1 from the second paper of 2010 (a gas taken at high altitude)
• Question 6.2 from the second paper of 2010 (a left-shifted p50)
• Question 6.3 from the second paper of 2010 (another pulmonary embolism)
• Question 6 from the second paper of 2001 (a right-shifted p50)

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:

• The alveolar gas equation
• The oxyhaemoglobin dissociation curve
• Factors which influence the affinity of haemoglobin for oxygen
• The p50 value of a blood gas sample
• Oxygen tension - based indices of oxygenation
• The shunt equation and content - based indices of oxygenation

However, these are much too verbose for casual revision. This chapter is a summary of the necessary material for the time-poor exam candidate.

The A-a gradient

The alveolar gas equation:

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.

The p50 value and the oxygen-haemoglobin dissociation curve

In the adult, the normal p50 should be 24-28mmHg.

• Causes of a right shift in the oxygen-hemoglobin dissociation curve
  • Acidosis
  • Increased PaCO₂ (the Bohr Effect)
  • Increased temperature
  • Increased 2,3-DPG (eg. in pregnancy)
  • Sulfhaemoglobin
• Causes of a left shift in the oxygen-hemoglobin dissociation curve
  • Alkalosis
  • decreased PaCO₂
  • Decreased temperature
  • Decreased 2,3-DPG (eg. in stored blood)
  • Carboxyhaemoglobin
  • Methaemoglobin