Respiratory Failure

Respiratory Failure

Respiratory failure can be divided into two categories:

• Type 1: hypoxaemia (<8.0 kPa PaO₂) with normocapnia (PaCO₂ <6.0 kPa)
• Type 2: hypoxaemia (PaO₂ <8.0 kPa) with hypercapnia (PaCO₂ >6.0 kPa)

Let’s briefly think about the respiratory system. Two things need to happen for it to function correctly:

1. The pump: air needs to be ‘pumped’ into (and out of) the alveoli. This involves the airways, the chest wall, and the central nervous system.
2. Gas exchange: gases need to be able to transfer from the alveoli to the blood, and vice versa. This involves the lung tissue and the surrounding capillaries.

Type 1 Respiratory Failure

Type 1 respiratory failure is caused by a gas exchange issue affecting oxygen transfer to the blood. The most common of these is ventilation/perfusion (V/Q) mismatch, where parts of the lung receive insufficient oxygen or insufficient blood flow.

Some examples of this include pulmonary oedema reducing gas transfer in the affected alveoli, and pulmonary embolism reducing blood flow to an area of lung tissue. The result is a low PaO₂.

PaCO₂ remains unaffected (or in some cases is reduced) because the 'pump' is still working effectively and can compensate for the areas that have reduced gas transfer. CO₂ dissolves more readily across the alveolar membrane and is easier to get rid of.

Type 2 Respiratory Failure

Type 2 respiratory failure is due to a problem with the ‘pump’, resulting in alveolar hypoventilation (insufficient air getting into and out of the alveoli) throughout the lungs.

In this case it doesn’t really matter how efficient gas-exchange is – enough new oxygen is not being provided to the alveoli and enough carbon dioxide is not being removed.

The outcome is a low PaO₂ and high PaCO₂. Some examples of this include COPD, chest wall deformities, and central nervous system depressants such as opiates.