Matching lung ventilation and perfusion Flashcards
What is ventilation and how is it measured?
- Movement of gases in and out of the lungs
- Ventilation of the lungs occurs through airways
- Measured as change in volume per unit time
What is perfusion, where does it occur and how is it measured?
- Blood flow through any organ e.g. lungs
- Perfusion occurs through blood vessels
- Measured as flow of blood/unit time e.g. 650 mL/min
What are the regional differences in lung ventilation?
- Apical alveoli are about 4 times larger than basal alveoli in upright posture
- This is due to the effect of the gravity
- Basal alveoli can expand more than apical alveoli
- Basal regions of lungs have better ventilation than apical regions
What is the functional supply for the pulmonary circulation?
Main blood supply to the lungs- RV to pulmonary arteries to lungs to pulmonary veins to body organs to RV
What is structural supply of pulmonary circulation?
Nutrition to the lung tissues. Also called Bronchial circulation Bronchial artery from thoracic aorta Bronchial vein to superior vena cava Only a small amount of blood.
What is the equation for CO?
CO= SV x HR
What is SV?
SV is amount of blood that comes out every ventricle
Which ventricle is thicker and why?
Left ventricle 3 times thicker as pushing against more force- aorta has muscle and elastic tissue. P higher in LV and aorta.
What structures does oxygen travel through to become oxygenated?
Pulmonary artery carry deoxygenated blood from right ventricle; split into right and left branches to supply respective side lung.
Each branch then successively split into smaller branches closely following airway divisions. They are called extra-alveolar vessels while running through lung parenchyma.
What happens to the CO2 exchange when alveoli are damaged?
Can carry out CO2 exchange because it is easier as there are fewer barriers. Since alveoli are very badly damaged, they can’t always carry out oxygen exchange.
Where does blood go when it passes through capillaries?
Beyond terminal bronchioles, blood vessels form capillary beds (starting with smaller arterioles) where gas exchange starts. Blood flowing through rich network of capillaries forms a ‘sheet’ of blood with huge surface area. Very efficient system of gas exchange.
Oxygenated blood flows in to pulmonary venules which unite to form four large veins emptying in to left atrium.
What is emphysema?
– Widespread destruction and dilatation of distal airway (chronic inflammation)
– Regional destruction of vascular beds
– Poor gas exchange and hypoxia
– Chronic inflammation destroys alveoli
Which blood vessel carries deoxygenated blood from heart to lungs?
Pulmonary artery
Where in the pulmonary circulation does gas exchange start?
Small arterioles
Which factor is predominantly responsible for highly efficient gas exchange in the lungs?
Large surface area of exposure between blood and alveolar gases
Regional differences in lung perfusion- where is hydrostatic pressure the highest- top or bottom?
Max hydrostatic pressure at bottom
Hydrostatic pressure definiton
Force exerted by weight of a fluid (blood/ water) due to gravity
What is pulmonary circulation?
A column of blood exerting different hydrostatic pressure in different areas of lungs in relation to the position of the right ventricle – as that is where the blood is coming out of
Higher hydrostatic at the bottom of the lung compared to the top of the lung.
P above ventricle will be less, lower than the ventricle will be more.
What are the extra alveolar vessels?
– Running through lung parenchyma
– Their diameter is affected by the lung volume (via pull of the lung parenchyma)
– When breathing they get bigger and are pulled
Do the alveolar vessels get squashed?
– Diameter dependent
– They get smaller as they squashed
If P upstream is smaller then P out but is larger than P downstream will blood flow through?
No as the alveolar P is greater than the upstream pressure so the capillary will collapse.
If P upstream is larger then P out and is larger than P downstream will blood flow through?
Yes as the upstream P is greater than the alveolar P so capillary will stay open.
What does P upstream, out and downstream relate to?
P us is the pressure in the arterials.
P out is the pressure in the alveoli.
P ds is P in the venous.
What happens to the movement of blood when a patient bleeds?
If patient bleeds they lose blood and drop BP- P out will be more than P us- drop BP upstream P will drop below alveolar P- that zone that was zone of recruitment will turn into a zone where air is still there but no blood passes through.
What happens in zone 1 of the lungs?
Zone 1: Alveolar dead space
• Good ventilation but no perfusion
• No gas exchange
• Poor blood flow (lung apex 15 cm above right ventricle)
• Apices of the lungs, very small area in healthy people
• P alveolar >P arterial > P venous ( P out > P us > P ds)
• Blood flow determined by difference in Palveolar & Parterial
What happens in zone 2 of the lungs?
Zone 2: Recruitment zone- more and more blood is going through it as more capillaries
• Lower down the lung than zone 1
• Higher Parterial due to higher hydrostatic pressure
• Recruitment of more alveolar unit especially in systole
• Parterial > Palveolar > Pvenous ( P us > P out > P ds)
• Blood flow determined by difference in Parterial & Palveolar
What happens in zone 3 of the lungs?
Zone 3: Distension zone
• Lung bases
• Hydrostatic forces raise P arterial and P venous above P alveolar
• P arterial > P venous > P alveolar (P us > P ds> P out )
• Continuous blood flow
• Blood flow determined by difference in Parterial & Pvenous (because they are larger P- biggest difference= largest flow)
What happens when a haemorrhage occurs?
Drop in Parterial- loss of blood BP drops- area of lung that was participating in gas exchange, no participating in gas exchange anymore- patient gets hypoxic-more alveoli in recruitment zone as in arterial P higher than alveolus P- dynamic will change- recruitment zone will change into dead space
What happens during positive pressure ventilation?
Rise in P alveolar- when patient is asleep and mechanically ventilating- patient not able to make negative pressure- so air is forced through lungs through positive pressure- pressure in alveolus will be higher than normal. Some of the alveolus which were participating in gas exchange will not be because of pressure dynamic exchange.
What is Ventilation(V)- perfusion(Q) mismatch?
- The ventilation-perfusion (V/Q) ratio is commonly used to describe this mismatch
- If the amount of ventilation (VA) to an area of lung is equal to the amount of perfusion (Q) the ratio of VA/Q would be 1 (ideal situation)
- VA/Q ratio varies from 3.3 at the apex of the lungs to 0.6 in the base
- Normal average V/Q ratio is 0.8
What is anatomic dead space?
Conducting airways; no gas exchange
What is alveolar dead space?
Unperfused or poorly perfused alveoli
What is physiological dead space?
Part of each breath that does not take part in gas exchange
What is the equation for physiological dead space?
Physiological dead space = Anatomic dead space + Alveolar dead space
What is a pulmonary embolism?
– Medical emergency, potentially fatal
– Blood clot from deep veins of leg/pelvis or fat emboli
– Large/intermediate pulmonary arteries
– Severe hypoxia
– Lack of blood supply to that part of lung
– Enlarged alveolar dead space
– Areas where they is no blood but air is still there
What is a shunt? Give examples
– Deoxygenated blood reaching left side of the heart either bypassing lungs or failing to get oxygenated in the lungs
– Examples
• bronchial veins
• pneumothorax
• Age related changes: areas low V/Q ratio in the lungs
– Minimal effect of oxygen supplementation on a very large shunt
– Blood going to R to L side of heart picking up no oxygen
What is a pneumothorax?
A collapsed lung. Occurs when air leaks into the space between your lung and chest wall. This air pushes on the outside of your lung and makes it collapse. Lung collapses- no air in lung but same amount of blood is passing through
What is treatment for a pneumothorax?
- immediate needle decompression
* Chest drain connected to an underwater sealed system
What is the V/Q for dead space?
Dead space has good ventilation, no perfusion so V/Q ratio is infinite.
What is the V/Q for a shunt?
No ventilation, good perfusion. V/Q is 0.
What is alveolar dead space?
No perfusion, good ventilation.
What is the value of V/Q ratio in shunt?
Ventilation/perfusion ratio (V/Q)= 0- no ventilation
Which abnormality of oxygenation are you most likely to see in pneumonia?
Shunt- air being replaced but blood is still flowing through
