Respiratory failure

The respiratory system has two jobs, and it is worth describing them separately because disease separates them. One is delivering oxygen to the body. The other is removing the carbon dioxide the body produces. That exchange of gases between the environment and the body is what makes aerobic metabolism possible, the kind that uses oxygen to obtain energy. When the system cannot carry out one of the two jobs, or neither of them, there is respiratory failure.

That is where the two types come from. In type 1 respiratory failure, what fails is oxygen delivery, and the result is called hypoxemia, meaning too little oxygen in the blood. In type 2, what fails is carbon dioxide removal, and the result is called hypercapnia, meaning carbon dioxide that has built up. One can fail without the other failing: they are like the two sets of pipes in a house, the one that brings clean water in and the one that takes dirty water out, and one can break while the other keeps working.

There is a second way of classifying it, which does not compete with the first but goes alongside it: by time. It can be acute, if it sets in suddenly; chronic, if it has been there for a long time; or acute on chronic, when someone already living with the chronic form has a sudden worsening.