46 - 50 Physiology SBAs for the Primary FRCA
Question 46
You have just anaesthetised a patient for incision and drainage of an abscess, and controversially decide to use a 50:50 mix of nitrous oxide and oxygen.
Your consultant mutters something about the second gas effect and 'what's wrong with Des' before wandering off to find some droperidol.
Upon permeating the alveolus, which of the following gases reaches equilibrium with the pulmonary capillary blood fastest?
- Nitrogen
- Carbon dioxide
- Carbon monoxide
- Oxygen
- Nitrous Oxide
Answer
- Nitrous Oxide
A gas will diffuse down a partial pressure gradient from the alveolus into the pulmonary capillary.
The rate at which it reaches equilibrium between the two is largely determined by what the gas does once it gets into the blood.
If it reacts with another molecule, or gets bound to something like haemoglobin, then the partial pressure gradient is maintained and equilibrium is not reached for much longer.
If it just sits there, however, and doesn't react, then equilibrium is reached much sooner.
- Nitrous oxide reaches equilibrium very rapidly because it diffuses quickly and doesn't react with anything in the blood
- Nitrogen doesn't diffuse all that quickly
- CO2 dissolves on the other side, maintaining a partial pressure gradient
- Carbon monoxide binds rapidly and avidly to haemoglobin, and can't diffuse fast enough to reach equilibrium (diffusion limited)
- Oxygen binds pretty quickly, but may reach equilibrium depending on blood flow (perfusion limited)