lecture 9 - gas exchange Flashcards
What are the 3 causes of a gas exchange problem?
Impaired diffusion, shunt, ventilation-perfusion (V/Q) mismatch
What is a gas exchange problem?
Where there is insufficient loading of O2 into arterial blood in the lungs
What are the 4 causes of impaired diffusion in the lungs?
- Thickened blood-air barrier
- Decreased surface area (e.g. emphysema)
- Exercise causing increased blood flow to lungs
- Breathing low PIO2
Why can exercise impair diffusion in the lungs?
Increased blood flow/transit through the lungs due to increased HR leaves less time for diffusion
Why is CO2 diffusion rarely impaired compared to O2 diffusion?
CO2 diffusion in many times faster and easier than O2 diffusion
What is shunt, in terms of gas exchange problems?
Venous blood that re-enters the arterial system before being oxygenated in the lungs, and therefore dilutes the oxygenated arterial blood leaving the lungs
What is the effect of shunt on the A-a gradient?
Increased by shunt
Why does pneumonia cause shunt?
Alveoli are filled with exudate and so are not ventilated. However, they are still perfused with blood, but no gas exchange occurs and the under-perfused blood re-enters the arterial circulation
What is V/Q mismatch?
When ventilation and perfusion are mismatched, resulting in reduced gas exchange
What 2 things can cause non-uniform alveolar ventilation, and therefore a V/Q mismatch?
- Uneven resistance to airflow (e.g. collapse in part of the lung)
- Uneven compliance due to fibrosis, etc.
What can cause non-uniform distribution of alveoli perfusion, and therefore a V/Q mismatch?
Blocked blood flow to a certain region of the lung - e.g. by embolisation or thrombosis, tumor, hypotensions, etc.
What are some of the causes of hypoventilation?
- Increased work of breathing (increased compliance/airways resistance)
- chest wall/muscle damage
- respiratory depressant drugs
- sleeping
What is always increased by hypoventilation?
PaCO2 (arterial pressure of CO2)
What scenario would allow PaO2 to remain normal despite hypoventilation?
Increased the PIO2 - allowing less air to be breathed but with more O2 so gas transfer is maintained. Can be done by putting a patient on oxygen
What are the 2 types of respiratory failure?
Type 1, Type 2
What is type 1 respiratory failure?
Failure due to gas exchange problems, resulting in low PaO2, but usually normal PaCO2
What is Type II respiratory failure?
Ventilatory failure resulting in low PaO2 as well as notably high PaCO2 due to failure to remove CO2 from the blood
