Respiratory failure occurs when the respiratory system fails in one or both of its main functions; namely, the oxygenation of blood and the elimination of CO2. Respiratory failure is categorised as ‘type 1’ or ‘type 2’ on the basis of blood gas analysis.
Respiratory failure occurs when the respiratory system fails in one or both of its main functions; namely, the oxygenation of blood and the elimination of CO2. Respiratory failure is categorised as ‘type 1’ or ‘type 2’ on the basis of blood gas analysis:
The respiratory system can be considered as two parts: a gas-exchange system and a ‘bellows’.
The gas-exchange system is made up of:
– Alveolar–capillary units;
– Pulmonary circulation.
The bellows system is made up of:
– Chest wall and pleura;
– Respiratory muscles;
– Respiratory centre.
Importantly, V̇A facilitates the diffusion of CO2 from the pulmonary capillaries to the alveoli: V̇A (and not V̇E) is inversely proportional to PaCO2 (see Chapter 11). Failure of alveolar ventilation leads to increased PaCO2; that is, type 2 respiratory failure.
Which pathophysiological processes cause type 2 respiratory failure?
Normally, ventilation is controlled by a negative-feedback mechanism:
A rise in PaCO2 stimulates the respiratory centre in the medulla oblongata via the peripheral and central chemoreceptors (see Chapter 22).
The respiratory centre sends excitatory impulses to the respiratory muscles to increase the rate and depth of inspiration. V̇E and V̇A both increase.
Owing to the inverse relationship between PaCO2 and V̇A, PaCO2 decreases.
In health, this system is very sensitive: PaCO2 is kept within tight limits. If PaCO2 rises above 6 kPa, V̇A must be inadequate and one of the components of ventilation must be malfunctioning:
– Respiratory centre depression by opioids or general anaesthesia;
– Reflex desensitisation of the respiratory centre to high PaCO2 in order to prevent respiratory muscle fatigue.
– Mechanical; for example, flail chest;
– Neuropathic; for example, Guillain–Barré syndrome;
– Muscular; for example, myopathies.
Respiratory muscle fatigue. Fatigue occurs when the respiratory muscles cannot synthesise sufficient ATP to meet the demands of muscle contraction despite an intact respiratory drive and chest wall.