Chapter 25 – Ventilatory Failure




Abstract




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.





Chapter 25 Ventilatory Failure




What is meant by the term ‘respiratory failure’?


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:




  • Type 1 respiratory failure is defined as a PaO2 < 8.0 kPa with a normal or low PaCO2.



  • Type 2 respiratory failure is defined as a PaO2 < 8.0 kPa with a raised PaCO2 > 6.0 kPa.



What is the difference between oxygenation and ventilation?


The respiratory system can be considered as two parts: a gas-exchange system and a ‘bellows’.




  • The gas-exchange system is made up of:




    1. Alveolarcapillary units;



    2. Pulmonary circulation.

    The gas-exchange system is responsible for oxygenation; deficiency leads to hypoxaemia and type 1 respiratory failure.



  • The bellows system is made up of:




    1. Chest wall and pleura;



    2. Respiratory muscles;



    3. Airways;



    4. Nerves;



    5. Respiratory centre.

    The bellows system is responsible for ventilation: moving air from the atmosphere to the alveoli on inspiration and from the alveoli to the atmosphere on expiration.


Importantly, A facilitates the diffusion of CO2 from the pulmonary capillaries to the alveoli: A (and not E) 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. E and A both increase.



  • Owing to the inverse relationship between PaCO2 and A, PaCO2 decreases.


In health, this system is very sensitive: PaCO2 is kept within tight limits. If PaCO2 rises above 6 kPa, A must be inadequate and one of the components of ventilation must be malfunctioning:




  • Failure of the respiratory centre to respond appropriately. This may be due to:




    1. Respiratory centre depression by opioids or general anaesthesia;



    2. Reflex desensitisation of the respiratory centre to high PaCO2 in order to prevent respiratory muscle fatigue.




  • A problem with chest wall movement. This could be:




    1. Mechanical; for example, flail chest;



    2. Neuropathic; for example, Guillain–Barré syndrome;



    3. 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.

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Sep 27, 2020 | Posted by in ANESTHESIA | Comments Off on Chapter 25 – Ventilatory Failure
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