Ventilation/Perfusion relationship Flashcards
What is ventilation?
The provision of gas to an area
What is perfusion?
The provision of blood to an area/ flow of blood to alveolar capillaries
What is the ‘ideal’ ratio of ventilation and perfusion?
1:1 ratio
What is the typical VQ ratio and why can we get away with it?
0.8:1 , but we can get away with it because we have functional reserves
Is there more gas or blood at the base of the heart?
More gas, so Q is higher
Is there more gas or blood at the apex of the heart?
More blood, so V is higher
What can disruption of the V/Q ratio lead to?
clinical hypoxia
What determines the partial pressure of oxygen?
the amount of oxygen entering the alveoli and the amount of oxygen being removed by capillary blood flow
What determines the partial pressure of carbon dioxide?
The amount of co2 that diffuses into the alveoli from the capillary blood and the amount of carbon dioxide being removed from the alveoli through alveolar ventilation
What is the effect of increasing/ decreasing ventilation on the partial pressure of oxygen?
As ventilation increases, more oxygen enters the alveolus therefore the ppO2 increases (and vice versa)
What is the effect of increasing/decreasing perfusion on the ppO2?
As perfusion increases, more oxygen is removed from the blood, therefore the ppO2 decreases and vice versa
What is the effect of increasing/ decreasing ventilation on PaCo2?
As ventilation increases, more Co2 is removed from the blood therefore ppCO2 decreases and vice versa
What is the effect of increasing/decreasing perfusion on PaCo2?
Increasing perfusion brings more Co2 carrying blood back for elimination therefore the PaCo2 increases and vice vers a
What is the consequence of the V/Q ratio increasing?
Overventilation or underperfusion
What is underperfusion?
Ventilating dead space
What is the consequence of V/Q decreasing?
Underventilation (shunting blood away) or over perfusion
What is V(d) dead space ventilation?
The volume of blood that is ventilated but not exchanged with pulmonary capillary blood
What occurs to the deoxygenated blood when VQ is too low?
deoxygenated blood flows to the left side of the heart and is then circulated meaning the patient will be hypoxemic,
What is true shunting?
Blood enters left circulation without an increase in oxygen concentration as the blood has not had a chance to gas exchange with functional alveoli
What is Hypoxic pulmonary vasoconstriction?
A physiological reflex when the pulmonary arteries constrict in the presence of hypoxia
Why do pulmonary arteries constrict during hypoxia?
So they can direct blood to the parts of the lung with more oxygen
Why do arteries in systemic circulation dilate during hypoxia?
So they can allow for increased blood flow
What is the end aim for HPV (hypoxic pulmonary vasoconstriction?)
Normalise the VQ ratio by redistributing blood from poorly to well ventilated areas
What causes the patient to be hypoxaemic?
Shunting, which occurs when the V/Q ratio is too low
deoxygenated blood is circulated around the body
What occurs to the alveoli when there is no ventilation at all (also known as the true shunt)
alveolar collapse
What are the two different types of shunting?
capillary and anatomic
