Ventilation-perfusion Relationships Flashcards
What is the hypothetical ideal lung situation ?
No difference between partial pressure of alveolar oxygen and arterial oxygen
The diffusion across the membrane proceeds to equilibrium in all lung units and all blood passing through is exposed to alveolar air
NEVER EXISTS: but difference is extreme,ly small
What is ventilation ?
Movement of air from the atmosphere through ventilatory system
What is hypoxaemia?
Any situation where partial pressure of oxygen in arterial blood is abnormally low
What are 4 basic causes of hypoxaemia ?
Hypoventilation
Diffusion limitation
Shunt
Ventilation-perfusion inequality
What 2 factors balance the partial pressure of oxygen in the alveoli ?
Rate of removal of oxygen by blood- determined by metabolic demands
Rate of replenishment of oxygen by alveolar ventilation.
What happens if the rate of replenishment at the alveoli falls but the rate of oxygen removal remains the same…
Partial pressure of oxygen in both arterial and alveoli falls while the carbon dioxide increases
What is hypoventilation ?
Situation where ventilation falls below demands of internal respiration
What are some causes of hypoventilation ?
Drugs which depress centra respiratory drive - opiates and barbiturates
Damage to the chest wall
Paralysis of respiratory muscles
When can the diffusion difference at the lungs get larger ?
During exercise due to reduced time and if the blood gas barrier thickens
What is shunted blood ?
Is blood that enters the arterial system without going through ventilated areas of lung
How do shunts occur and what do they cause ?
They reduce the amount of oxygen in the systemic arteries compared to pulmonary capillaries
Pulmonary arteriovenous fistula- some patients have abnormal vascular connections between pulmonary veins and pulmonary arteries
Cardiac atrial septal defect- causes arterial blood and venous blood to mix
How do we know that pulmonary vascular resistance is much lower ?
MAP = CO * TPR
The CO is the same and the MAP is much lower s therefore the resistance must be lower- a tenth of the systemic resistance
If the pulmonary arterial pressure rises it causes the pulmonary vascular resistance to decrease. Why ?
Recruitment: as pressure rises previously closed capillaries open and start conducting blood
Distension: pressure rises causes capillary lumen radius to increase which causes a decrease in resistance
How does blood flow exist in an upright lung ?
It decreases almost linearly from the base to the apex
Due to hydrostatic pressure difference within blood vessels - more recruitment and distension at the base
What are the 3 main forces that affect blood flow through the pulmonary capillary bed ?
Hydrostatic pressure
Arteriovenous pressure difference
Alveolar pressure
What is zone1 of the 3 flow zones in an upright lung ?
At the top of the lung
Capillaries are squashed flat so there is no flow
Alveolar pressure > arterial pressure > venous pressure
What is zone 2 of the 3 flow zones in an upright lung ?
Flow is determined by the difference between the arterial and alveolar pressures
Arterial pressure > alveolar pressure > venous pressure
What is zone 3 of the 3 flow zones ?
Flow is determined by the arteriovenous pressure difference
Arterial pressure > venous pressure > alveolar pressure
Why is ventilation greater at the base of the lungs compared to the apex ?
Due to the weight of the lungs and effects of gravity
- intrapleural pressure at the base is less negative so alveoli at base are less expanded - in inspiration there is a greater volume change in the alveoli at the base - this is partly due to reduced surfactant lining the alveoli
What is the ventilation-perfusion ratio ?
Ratio between alveolar ventilation and pulmonary blood flow
What is the ventilation perfusion ratio at the base ?
The pulmonary blood flow > alveolar ventilation so ratio is < 1
As you move up the lungs what happens to the ventilation- perfusion ratio ?
Increases towards 1 - > 1 towards top
Because as you move up the lungs the pulmonary blood flow and alveolar ventilation decrease but blood flow decreases faster
What happens if ventilation and blood flow are mismatched ?
Impairment of both oxygen and carbon dioxide transfer
What happens to the ventilation- perfusion ratio if ventilation decreases ?
Ratio decreases causing partial pressure of oxygen to decrease and partial pressure of carbon dioxide to increase
What happens to ventilation-perfusion ratio if perfusion decreases ?
Increases ratio causing an increase in partial pressure of oxygen and a decrease in partial pressure of carbon dioxide BUT it doesn’t lead to a huge increase in oxygen transport because hb is almost fully saturated
What happens when there is a low alveolar partial pressure of oxygen ?
Pulmonary arterioles constrict
Redirect blood to better ventilated areas
What happens in areas where perfusion is low but there is high alveolar partial pressure of oxygen ?
Pulmonary arterioles dilate
Increased pulmonary blood flow
Increased oxygen uptake
What happens in alveolar area withs high partial pressures of carbon dioxide ?
Bronchodilation - increase elimination of carbon dioxide
What happens in alveolar areas with low partial pressure of carbon dioxide ?
Bronchoconstriction - redirects blood to better perfused areas
What happens in high altitude pulmonary oedema ?
Vasoconstriction of arterioles is uneven
Produces an increase in pulmonary MAP with significant increases in pressure in capillaries downstream of non constricted arterioles
Initiates an inflammatory response which increase pulmonary capillary permeability making situation worse
What are the treatments of altitude sickness ?
To descend
Supplemental oxygen
Hyperbaric bag
Vasodilator drug. nifedipine- blocks calciu channels in vascular smooth muscle to reduce extent of pulmonary hypoxia vasoconstrictor response
