Session 6: Ventilation-Perfusion Relationships in the Lung Flashcards
Define pulmonary ventilation.
Movement of air in and out of the lungs. Total amount.
Define pulmonary gas exchange.
Diffusion of O2 from lungs into blood and CO2 from blood into lungs.
Define pulmonary perfusion.
Flow of blood through the pulmonary capillaries surrounding the alveoli. Typically expressed as Q ml/min
Define alveolar ventilation.
The amount of air reaching alveoli in a given amount of time as a function of the tidal volume and the amount of air in the anatomical dead space as well as the respiratory rate.
Someone breathing 14 times a minute with a tidal volume of 500 ml and anatomical dead space of 150 ml.
What will the alveolar ventilation be?
14 x (500-150) = 4900 ml air/minute.
This is typically expressed as V
What would the normal V/Q ratio be in a healthy adult?
Between 0.8 to 1.2. With an average of 1
What does a mismatch of V/Q ratio result in?
The most common cause of hypoxaemia.
What happens when PaO2 is low?
Hypoxic vasoconstriction occur of the pulmonary arterioles to divert blood to better ventilated alveoli.
What happens when PaCO2 is low?
Bronchoconstriction occurs to divert air to better perfused lung.
What happens to PaCO2 and PaO2 when there is inadequate ventilation vs perfusion.
V/Q<1
PaCO2 rises and PaO2 falls.
Give examples of conditions where V/Q<1
Asthma COPD early stages Pneumonia RDS in newborn Pulmonary oedema
Why is the PO2 lower in than simply the average of the two partial pressures of alveoli?
Because partial pressure of O2 in the arteries is determined by the diffused O2 and not the O2 bound to haemoglobin. By looking at the oxygen-haemoglobin dissociation curve you will find that the total oxygen content will be the average of the two alveoli but the partial pressure will be significantly lower.
This is because the haemoglobin will take up oxygen until fully saturated. When fully saturated that is when partial pressure will start to increase significantly more.
Explain why hyperventilation does not correct the hypoxia in a mismatch V/Q.
Because increasing the partial pressure in the lungs will only result in haemoglobin taking up more oxygen and not contribute significantly to an increase in partial pressure of oxygen in the arteries.
The PO2 is very important once the blood arrives to tissues as a higher PO2 leads to haemoglobin being more likely to give up their oxygen to the tissue.
So even if there is a fairly high oxygen concentration the PO2 (with a significantly lower free O2 conc.) will determine whether oxygen can be given up to tissues.
Give an example of a V/Q mismatch where the perfusion is the problem.
Pulmonary embolism
Explain why a pulmonary embolism in the left upper lobar branch of the pulmonary artery can lead to mismatch V/Q in the right lung and lower lobar lung.
Perfusion is increased in the right lung and L/LL (the functioning parts) to match the V/Q ratio to remain to 1.
If hyperventilation cannot sufficiently increase alveolar ventilation to match the increased perfusion the V/Q ratio will be <1 and result in hypoxia.
