Respiratory Failure and VQ matching: Part II Flashcards
Ventilation-Perfusion VQ matching
what is the most common cause of hypoxia in respiratory diseases and respiratory failure
Ventilation (V) and Perfusion (Q) mismatching of alveolar capillaries
ventilation-perfusion VQ matching
what 2 things does correlating alveolar ventilation and perfusion do?
think gases
it maximises gas exchange and efficiency.
ideally should be 1:1
ventilation-perfusion VQ matching
how do you calculation VQ ratio
Ventilation (V) divided by Perfusion (Q)
- VQ ratio = V/Q
VQ mismatching
what does VQ mismatching increase
think space
increases alveolar dead space (space not used for gas exchange)
VQ mismatching
what are 3 causes of VQ mismatching
Think:
- oxygen
- circulation
- dysfunctions
- lack of inspiring oxygen
- lack of circulation/blood flow (shunts)
- respiratory dysfunctions
VQ mismatching
what is the consequence of VQ mismatching regarding oxygen saturation and what does this lead to?
think:
- alveoli
- systemic circulation
- there will be a much lower oxygen saturation in blood leaving the alveoli
- will lead to hypoxaemia when they mix with the systemic circulation
how come Type 1 respiratory failure always occur before type 2 respiratory failure and explain why?
this is because you get hypoxia before hypercapnia, due to the limited solubility of Oxygen
what are 6 diseases/disorders that commonly cause V/Q mismatches?
Think:
- C
- a
- e
- p
- p
- p
- Covid
- acute asthma
- exacerbation COPD
- pneumonia
- pulmonary oedema
- pulmonary embolism
Regulation of V/Q
what is the relationship between resistance and radius (Poiseullle’s Law)
resistance to flow is inversely proportional to the radius^4
regulation of V/Q
what do medium-sized bronchioles provide most resistance for and what do arterioles provide the most resistance for
Medium-sized Bronchioles:
- most resistance to airflow
Arterioles:
- most resistance to perfusion
regulation of V/Q
what are the 2 equations of Poiseulle’s law
resistance = 8nl / πr^4
Where:
- n = viscocity
- l = length of tube
- r = radius
OR
Q = ΔP/R
Where:
- Q = flow
- ΔP = change in pressure
- R = resistance
VQ matching - continuous local changes
what do the bronchioles do in response to raised PaCO2 (hypercapnia) and why
- bronchioles dilate in response to increased PaCO2
- does this to improve airflow so CO2 has more space to be eliminated
VQ matching - continuous local changes
what do pulmonary arterioles do in response to low PaO2 (hypoxia) and why
- pulmonary arterioles constrict in response to hypoxia
- does this to reduce flow and redirect blood to better perfused areas
state what would happen in these vessels if O2 decreases or increases and describe in one sentence the relationship between the 2 vessels:
- Pulmonary arterioles
- systemic arterioles
Oxygen decrease:
- pulmonary arterioles: vasoconstrict
- systemic arterioles: vasodilate
this is vice versa for an oxygen increase
Ventialtion and perfusion
what causes ventilation and perfusion to not be uniform throughout the lungs and what is also NOT uniform as a result
- Gravity
- as a result, VQ (V/Q ratio) is also not uniform
perfusion but not ventilation and ventilation but no perfusion
what is perfusion but no ventilation called and what is the V/Q ratio for this
- called a “shunt”
- V/Q = 0 as “V” has a value of 0
perfusion but not ventilation and ventilation but no perfusion
what is ventilation but no perfusion called and what is the V/Q ratio for this
- called “dead space”
- V/Q is infinity as “Q” is 0, and anything divided by 0 is infinity
Gravity and compliance
why does pulmonary ventilation increase from the apex of the lung compared to the base
because of gravity and compliance
Gravity and compliance
outline 2 reasons as to why lots of gas exchange occur at the base of the lungs
think:
- intrapleural pressure
- alveoli and compliance
- intrapleural pressure is less negative towards the base of lungs
- Alveoli at the base are highly compliant so can accommodate more easily
where is intrapleural pressure more negative in the lungs?
intrapleural pressure is more negative at the apex.
pulmonary blood flow
describe and explain the different pressures in Zone I of the lungs (the apex), state what this leads to
think:
- alveolar pressure
- blood vessels
Alveolar pressure is greater than both local pulmonary arterial and venous pressures in zone 1.
This is because the blood vessels are compressed by the high alveolar pressure and there is only intermittent flow if Pa increases during the breathing cycle
- this leads to little blood flow at the lung apex
Pressure hierachy: P.alv > P.a > P.v
outline a size hierachy of partial pressures between the alveoli (Palv/PA) , arteries (Pa) and veins (Pv) in zone I of the lungs (the apex)
Pressure hierachy: P.alv > P.a > P.v
pulmonary blood flow
what can the normal pressure gradient form arteries to veins be disrupted by in Zone II of the lungs?
can be disrupted by intermittnet high alveolar pressures
outline a size hierachy of partial pressures between the alveoli (Palv/PA) , arteries (Pa) and veins (Pv) in zone II of the lungs (the midsection)
Pressure hierachy: Pa > Palv > Pv
what does flow depend on in zone II (the midsection) of the lungs and what is it independent of?
dependent on:
- the difference between pulmonary arterial pressure and alveolar pressure
independent of:
- venous pressure
pulmonary blood flow
what is the pressure gradient like between arteries and veins in zone III (the base) of the lungs?
- normal pressure gradient from arteries to veins
how is perfusion ensured in zone III (the base) of the lungs?
pulmonary artery pressure is greater than venous pressure and alveolar pressure ensuring perfusion
outline a size hierachy of partial pressures between the alveoli (Palv/PA) , arteries (Pa) and veins (Pv) in zone III of the lungs (the midsection)
Pressure hierachy: Pa > Pv > Palv
what happens to ventilation and perfusion between the base and the apex when a patient is in the supine (lying down) position
it equals out, so the base perfusion is no longer much greater than the apex
What receptors are involved in vasoconstriction and vasodilation in the arterioles?
a1 and a2 receptors
