Description

Respiratory failure occurs when the respiratory system fails in oxygenation and/or carbon dioxide (CO₂) elimination. The two types are:

• Hypoxaemic (type I)
• Hypercapnic (type II)

Hypoxaemic: PaO₂ is less than 60 mm/Hg (8 kPa), with a normal or low PaCO₂, caused by ventilation-perfusion mismatch, with either/both:

• Under-ventilated alveoli (pulmonary oedema, pneumonia, or acute asthma).
• Venous blood bypasses ventilated alveoli (right to left cardiac shunts).

Hyperventilation increases CO₂ removal, but does not increase oxygenation, because the blood leaving unaffected alveoli is almost fully saturated.

Hypercapnic: PaCO₂ is more than 50 mm/Hg (6.5 kPa), which suggests inadequate alveolar ventilation. Any ventilation-perfusion mismatch will affect PaO₂, so arterial blood may be low, or normal.