Respiratory Failure = Failure of the lungs to maintain their primary function - gas exchange.
Type 1 Respiratory Failure - Hypoxaemic
= Hypoxaemia with a normal or low PCO2
This type of respiratory failure is caused by functional failure of the lung itself.
CO2 is much more soluble and requires less functioning tissue to be exchanged efficiently, so compensatory ventilation can keep the CO2 normal / low.
E.g. Pneumonia, Pulmonary Oedema.
Type 2 Respiratory Failure - Hypercapnic (also known as ventilatory failure)
= Hypoxaemia with a high PCO2
Caused by mechanical disadvantage, like lung hyperinflation in COPD, CNS abnormalities or respiratory muscle dysfunction.
PCO2 is high because the alveoli are hypoventilated and both O2 and CO2 exchange is impaired.
Most commonly caused by COPD but also by e.g. chest wall deformaties, Guillian-Barre syndrome.