Respiratory failure

Definition

P_aO₂<8kPa. There are two types which are defined based on the P_aCO₂.

Type I

P_aCO₂ is normal or low. Basically, hypoxia without hypercapnia (low oxygen, not high carbon dioxide). It doesn't matter if the CO₂ is low or normal - if it's not high and the patient is hypoxic, it's type I respiratory failure.

Anything which causes a ventilation/perfusion (V/Q) mismatch can cause type I respiratory failure. V/Q mismatch is where the amount of air coming into the lungs (ventilation) is just enough to oxygenate the amount of blood coming into the lungs (perfusion).

Type II

P_aCO₂ > 6kPa i.e. it's high. In words, it's hypoxia with hypercapnia (low oxygen with high carbon dioxide). There are a bunch of theories on how this works. The theory of hypoxic drive (where oxygen becomes the primary stimulus for breathing) has become more controversial recently. So in terms of pathophysiology, it's a bit difficult to be sure what's right or not. If nothing else, remember the "hypoxia with hypercapnia (P_aCO₂ > 6kPa)" bit and nothing else.

Epidemiology

Since both of these are caused by common disease like asthma and COPD, they're dead common. Learn about them!