Type 1 is hypoxaemia alone; type 2 adds hypercapnia
Mnemonic
Count the abnormalities:
- Type 1 has one problem: low oxygen with a normal or low carbon dioxide. A gas exchange failure
- Type 2 has two: low oxygen and high carbon dioxide, above 6 kPa. A ventilation failure
“Type 1 is the lung, type 2 is the pump.”
Type 1 comes from V/Q mismatch, shunt or diffusion failure: pneumonia, pulmonary oedema, embolism, fibrosis.
Type 2 comes from anywhere along the drive to bellows chain: opioids and sedation (drive), Guillain-Barre and myasthenia (nerve and junction), kyphoscoliosis and obesity (chest wall), and severe COPD or asthma (airways).
The bicarbonate dates it: a raised bicarbonate with a near normal pH means the hypercapnia is chronic and renally compensated; a normal bicarbonate with acidaemia means it is acute.
Expansion
- Type 1 - PaO2 below 8 kPa (60 mmHg) with a normal or low carbon dioxide. A failure of oxygenation, from V/Q mismatch or shunt. Causes: pneumonia, pulmonary oedema, pulmonary embolism, asthma, fibrosis, ARDS
- Type 2 - PaO2 below 8 kPa with carbon dioxide above 6.7 kPa (50 mmHg). A failure of ventilation. Causes: chronic obstructive pulmonary disease, severe asthma, opioid overdose, neuromuscular disease, chest wall deformity, obesity hypoventilation
The distinction is between the lung and the pump. In type 1 the lung cannot transfer oxygen; in type 2 the bellows cannot move enough gas.
Carbon dioxide is often low in early type 1 failure, because hypoxia drives hyperventilation. A rising carbon dioxide in a patient with severe asthma is therefore an ominous sign of exhaustion, not reassurance.
Treatment differs accordingly: oxygen and treatment of the lung pathology in type 1; ventilatory support, typically non-invasive ventilation, in type 2.