Lecture 24 - Diffusion Flashcards
What is the limitation of NO2 in the alveolus? Why?
Perfusion limited because there is no diffusion limitation and it equilibrates VERY fast
In order to obtain more NO2 in blood, you would need to perfuse the alveolus more
Is NO2 normally found in blood?
NOPE (just used as an example here)
What is the limitation of CO in the alveolus? Why?
Diffusion limited because it has a very high affinity to Hb (240x more than O2) and the only obstacle that is limiting more CO from entering into the bloodstream is therefore the properties of the membrane through which it is diffusing
CO will never equilibrate: it never exerts any back-pressure on more CO gas that is entering the blood
What is the limitation of O2 in the alveolus? Why? Which portion of the curve does each represent?
Both perfusion and diffusion limited:
- In the first 0.25 seconds it takes for O2 to equilibrate, O2 is diffusion limited (slope)
- After 0.25 seconds, O2 is perfusion limited (plateau)
Curve representing the diffusion or perfusion limitations of gases? How will the O2 curve be affected if the alveolar membrane is diseased? How will this affect the limitations of O2 in the alveolus?
- X-axis: time in capillary
- Y-axis: partial pressure
Slope would be flatter because it will take more time for O2 to pass the membrane and equilibrate => more diffusion limited
3 factors that can challenge the diffusion of O2 into the pulmonary capillaries?
- Exercise
- Alveolar hypoxia
- Thickening of the blood-gas barrier
Describe how exercise can challenge the diffusion of O2 into the pulmonary capillaries.
Exercise => CO increases => RBC only spends 0.25 seconds in the pulmonary capillaries => if someone has a diffusion abnormality that causes O2 to equilibrate in more than 0.25 seconds, exercise will be difficult for them and they will become short of breath
Under what circumstances could alveolar hypoxia occur?
High altitude
How is diffusion of a certain gas measured?
Transfer factor = DL = A x D / T = V°gas / (P1 - P2)
Transfer factor of CO?
DL = V°CO / P1 (since P2 is negligible) = V°CO / PACO
How to measure diffusion capability of the lungs?
- Have a person inhale a small amount of CO, hold their breathe for 10 seconds, and then exhale (CO is used to measure diffusion capacity because it is a gas that is purely diffusion limited)
- Whatever amount of CO that went in but did not come out must have diffused into the blood stream
- Measure volume of CO that was exhaled
- Calculate the minute diffusion of CO, V°CO by multiplying measured volume by 6
- Measure PaCO
- Calculate the transfer factor of CO
What are the 5 physiological causes of hypoxemia?
- Low inspired PiO2
- Hypoventilation
- Diffusion limitations
- Shunt
- V/Q imbalance
If the diffusion capability experiment was conducted on someone whose lungs are bleeding, aka there are RBCs in the alveoli, what would happen?
Less CO would be exhaled because the Hb molecules in the alveoli would take up some of it on top of the CO diffusing to pulmonary capillaries => diffusion capability is increased
If the diffusion capability experiment was conducted on someone with CO in their blood, what would happen? What patients would have CO in their blood?
Less CO would diffuse to blood => diffusion capability is decreased
SMOKERS!
What does hypoxemia mean?
Low PaO2
Other name for V/Q imbalance?
Ventilation-perfusion mismatch
How can you test if a patient with lung disease will need extra O2 when they fly on a plane?
Put a mask on them with only 15% O2 and see how they react
How can hypoxemia due to low PiO2 be treated?
Give patient 100% O2
What does hypoventilation mean?
Low ALVEOLAR minute ventilation, NOT total minute ventilation
Hallmark of hypoventilation?
High PaCO2
How can hypoxemia due to hypoventilation be treated?
Give patient 100% O2
Hallmark of hypoxemia due to diffusion limitations?
PcO2 < PAO2
How can hypoxemia due to diffusion limitations be treated?
Give patient 100% O2
2 examples of diffusion abnormalities?
- Thickened alveolar membrane
2. Decreased alveolar surface area
What 2 diseases have thickened alveolar membranes?
- Pulmonary fibrosis
2. Sarcoidosis
What is hypoxemia due to shunt?
Venous blood bypasses reoxygenation and enters into arterial systemic circulation without coming in contact with a functioning alveolar-capillary membrane: perfusion, but no ventilation
What are the 2 normal shunts of the body?
- Bronchial circulation
2. Thebesian circulation
Describe the shunt of the bronchial circulation.
Bronchial arteries carry oxygenated blood to cells of bronchi & trachea => blood becomes deoxygenated => some deoxygenated blood returns via pulmonary veins to the LA => deoxygenated blood mixes with oxygenated blood & pumped to body
Describe the shunt of the Thebesian circulation.
Coronary arteries branch off of aorta => deliver oxygenated blood to heart => blood becomes deoxygenated => some deoxygenated blood is dumped into LA and LV via Thebesian veins => deoxygenated blood mixes with oxygenated blood & pumped to body
Why is PaO2 = 95 mmHg?
Because of normal circulatory shunts
What are 5 possible abnormal shunts? Describe each.
ANATOMIC SHUNTS: Right to left flow in heart:
1. Ventricular Septal Defect (VSD): a hole in the LV will cause the RV to get very strong. Over time, the RV will become a higher pressure system and push deoxygenated blood into LV
- Atrial-Septal Defect (ASD): hole in atrial septum with RA pressure greater than LA pressure => deoxygenated blood in RA will pass into LA, then LV, and out to the entire body
- Arterial-Venous Malformation (AVM): congenital abnormality where an artery goes right into a vein without a capillary in between
- Patent Ductus Arteriosus (PDA): ductus arteriosus remains open, flow of deoxygenated blood from pulmonary trunk to aorta into systemic circulation
PHYSIOLOGIC SHUNTS:
4 Anatomical shunts
+
- Blood is coming in contact with alveolus but no gas exchange (e.g.: mucus blocking a bronchus)
How can hypoxemia due to a shunt be treated?
NOT by giving the patient 100% O2 as blood will still escape being re-oxygentated
Would still help, but can’t fix the shunt issue like this
V/Q ratio in shunt?
= 0
What kind of patients have alveolar-venous malformations?
Patients with liver disease will have some in the lungs
What is the A-a gradient? What is it normally?
PAO2-PaO2 due to shunts
Usually between 0 and 10 mmHg WHEN BREATHING ROOM AIR in a healthy person
What is hypoxemia due to V/Q imbalance?
The rate of ventilation does not match the rate of perfusion: V/Q < 1
Will there ever be a diseased stated where TOTAL V/Q > 1? Why?
NOPE, because perfusion will always move to another area of the lung to perfuse
V/Q ratio of dead space?
= infinity
No perfusion, normal ventilation
PAO2 and PACO2 as V/Q decreases?
PAO2 decreases and equilibrates to 40 mmHg
PACO2 increases and equilibrates to 45 mmHg
PAO2 and PACO2 as V/Q approaches infinity?
PAO2 increases and equilibrates to 150 mmHg
PACO2 decreases and equilibrates to 0 mmHg
What experiment can be done to visualize where most ventilation goes when a person inhales? Result? Why?
We have our patient inhale a given amount of nuclear-tagged gas, and then have a radiation counter behind them that will count where these particles go in the lung
Upon doing this experiment, we will get a distribution that shows that most ventilation occurs at the bottom of the lung
The reason for this is due to the fact that the alveoli at the top of the lung are stretched wide open because the weight of the lungs hang down and therefore pulls them open so since they are already full of air they do not have a lot of the air flow during respiration
Describe the effect of gravity on pulmonary blood flow.
Lungs are 25 cm long and blood flow enters them at about the middle of their height, so with gravity:
- Blood flow is the greatest at the bottom of the lung due to gravity
- Blood flow at the top of the lung is lowest due to collapse of vessels
In graphs showing the perfusion and ventilation from top of lung to bottom of lung, which slope is steepest?
Perfusion slope
What does the fact that most ventilation occurs at the bottom of the lung mean a for regional pressure gradient?
One exists inside the lung with intrapleural pressure at the top of the lungs being more negative than at the bottom
If both ventilation and perfusion are plotted on one graph with x-axis as bottom to top of lung, at what point would both curves intersect?
At V/Q = 1 around the 3rd rib
V/Q ratio at top of lung? What to note?
> 1
BUT summation of V/Qs in all of the regions of the lung will be 1 in a healthy individual
V/Q ratio at bottom of lung? What to note?
< 1
BUT summation of V/Qs in all of the regions of the lung will be 1 in a healthy individual
What are West Zones of the lung?
They divide the lung in 3 regions based on difference in PA, Pa, and Pv:
- Zone 1 = apex: ventilation is better than perfusion so PA > Pa > Pv
- Zone 2 = middle: Pa > PA > Pv
- Zone 3 = bottom: perfusion is better than ventilation so Pa > Pv > PA
In which West Zone of the lung can you insert a Swan-Ganz catheter? Why? What happens if you place it in the other 2 zones?
Zone 3 only because only zone where venous pressure is greater than the alveolar and the balloon will stay open and keep capillary occluded
- Zone 1: only measuring airway pressure because capillaries are collapsed
- Zone 2: balloon will not stay open and you will only measure airway pressure because PA>Pv
A-a gradient for low FiO2 state?
Normal
A-a gradient for hypoventilation state?
Normal
A-a gradient for diffusion limitation state?
Abnormal: elevated
How can hypoxemia due to V-Q mismatch be treated?
100% O2 => ALL PAO2 will rise, including in the alveoli that have ventilation issues
A-a gradient for V-Q mismatch state?
Abnormal: elevated
A-a gradient for shunt state?
Abnormal: elevated
If the diffusion capability experiment was conducted on someone with anemia what would happen?
Less CO would diffuse to blood because back pressure would build up in the blood due to low Hb => diffusion capability is decreased
Out of the 5 physiological causes of hypoxemia, which ones will be corrected to over 500-600 mmHg by increasing FiO2?
All, except for shunt
