Question 18

College answer

This question was poorly answered, with many candidates not answering the questions asked. For example, in part a), many candidates listed the clinical effects of hypothermia rather than its effects on the ABG analysis. Knowledge of sodium measurements and hyponatraemia were either at a superficial level, incorrect, or omitted.

Discussion

a) 

i)

 Problems with the interpretation of an ABG from a severely hypothermic patient:

• The lower the temperature, the higher the gas solubility.
• The higher the solubility, the lower the partial pressure.
• Thus:
  • PaO₂ drops by 5mmHg for every degree below 37°C.
  • PaCO₂ drops by 2mmHg for every degree below 37°C.
• pH increases by 0.015 for every degree below 37°C because temperature affects the ionisation constant of water
• Additionally, one needs to decide whether one uses the alpha-stat or the pH-stat models of ABG interpretation, i.e:
  • Alpha-stat: you warm or correct all samples to 37°C 
  • pH-stat: you correct all samples to the patient's actual body temperature
  • Of these, alpha-stat is preferred by cardiac anaesthetists for mild hypothermia, and pH-stat for deep hypothermic circulatory arrest so that the extra CO2 helps cerebral blood flow (according to Baraka, 2004)

ii)

Problems with the interpretation of an ABG which suffered a 45 minute delay in processing:

• Escape of cellular contents:
  • Potassium increases
• Compartment shift:
  • Sodium decreases
• Metabolism by live cells:
  • pH decreases
  • HCO₃^- decreases
  • SBE decreases
  • Lactate increases
  • Glucose decreases
• Sample errors:
  • Clots which have formed in the sample may completely occlude the sampling channel of the ABG machine, or cause a failure to aspirate the sample
• Meaninglessness
  • A lot can happen in 45 minutes; one might be able to legitimately claim that the sample is now worthless because the events have moved on and the patient has changed.

b)

i)

Causes of pseudohypernatremia:

• Decreased total blood protein, eg. hypoalbuminaemia in critical illness
• Extremely low serum lipids, eg. following plasma exchange
• Pseudonormonatremia would be these same things observed in a patient with a "true" hyponatremia, where the sodium is falsely elevated and therefore appears normal

ii)

Causes of factitious hyponatremia ("pseudohyponatremia" is also an accepted term)

• Abnormally elevated lipids, eg. familial hypercholesterolaemia
• Abnormally elevated protein, eg. paraproteinaemia of multiple myeloma, or high dose IV immunoglobulin
• Other large molecules (eg. accidental heparin) in the blood sample