Exam 3 - Blood Gas Cont/VQ ratios Flashcards
How much CO2 is delivered to the lungs per minute?
Venous CO2 content = 52.5 mL CO2 / dL
Arterial CO2 content = 48 mL CO2 / dL
52.5 - 48 = 4.5 mL CO2 / dL / min
How much O2 is picked up in the lungs per minute?
PaO2 = 20 mL / dL
PvO2 = 15 ml / dL
20-15= 5 mL O2/dL/min
What is the Haldane effect?
Describes how deoxygenated blood has more room to carry CO2
OR
OxyHb saturation levels can change Hb carrying capacity for CO2
How would you compare the acidity between oxyHb and deoxyHb?
DeoxyHb buffers (accepts) protons better and is therefore a weaker acid than oxyHb
Draw/describe the pathway of tissue gas transport from the erythrocyte.
What helps with O2 uloading from the RBC?
H+ being formed in the RBC from CO2 absorption helps knock O2 off Hb, and the more acidic environment causes O2 to unload to the tissues
Draw/describe pulmonary gas transport onto an erythrocyte?
Essentially the reverse of what happens in the tissues
Note the HCO3-/Cl- exchanger is in reverse here from the periphery
How does the unloading of CO2 and H+ in the lungs improve O2 binding?
The loss of H+ creates a less acidic environment on the RBC and increases the affinity of the RBC for oxygen
Normally, how long does it take for gas exchange to occur under normal conditions in the lungs?
0.25 seconds
What would increase the time needed for gas exchange?
Heavy exertion: decreases the pO2 of pulmonary. artery blood and requires more time to load O2 d/t the increased difference in PA and Pcap
Normally, how long does blood stay in the alveolus?
0.75 s
What happens to the time blood spends in the alveolus under increased CO?
How can this be a problem?
Time in the alveolus will be decreased, which for a normal person is okay since diffusion happens so rapidly
But in somone who has fluid in the lungs or other problem, they will not have enough time for complete gas exchange = diffusion problem
Describe the N2O line below?
N2O is less soluble in blood than O2 and equilbrates to PA rapidly.
This is why N2O has a quick onset and offset
How is carbon monoxide used in PFTs?
- CO has similar diffusion charectarestics to O2
- CO is used in low amounts to determine if a patient has a diffusion problem
- Slow diffusion = lung problem
What would gas exchange look like if diffusion capacity was only 25% of normal?
What about 12.5% normal?
- At 25% of normal, complete gas exchange is still possibly because there is a buffer in time that blood is still in the alveoi where gas exchange can occur.
- At 12.5% of normal, there is not enough time for gas exchange leading to diffusion abnormalities.
- Decrease in diffusion capacities is usually caused by fluid in the lungs (gases less soluble in water)
What is perfusion-limited gas?
Gas that equilibrates to the alveolar gas concentrations
Gas absorption is dependent on how much blood is moving through the lungs (more deoxyHb = more O2 can be loaded on)
N2O, O2
What is diffusion-limited gas?
Gas that does not equilibrate to alveolar gas concentrations (CO)
What is Fick’s Law of gas diffusion?
Describe the components?
T = thickness; greater thickness decreases diffusion
A: surface area; increased SA (more recruitment) increases diffusion
D: diffusivity
P: partial pressure difference of the gas across the barrier
What is diffusivity?
Diffusion of a gas based on MW and solubility
Describe the diffusivity of CO2 and O2?
- CO2 is 24x more soluble than O2
- CO2 weighs more than O2
- √MW O2/√MW CO2 = 0.85 = relative rate of diffusion, but have inverse relationship = 1/0.85 = 1.17
- This means O2 is more diffusable based on weight
- But CO2 being 24x more soluble = 0.85 x 24 = 20x more diffusable than O2
What are these equations telling us?
Based on MW O2 has faster diffusion
But when solubility taken into account CO2 is 20x more soluble
What is a normal V/Q?
4,200 mL air (alveolar ventilation) / 5,000 mL blood = 0.85
What would be the V/Q for unventilated alveoli?
What are gas concentrations?
What is this called?
- Alveolar gas concentrations should mimic pulmonary capillary concentrations (pCO2: 45mmHg, pO2: 40mmHg)
- Ventilation is decreased so the ratio is smaller (limit is 0)
- Shunt
What would be the V/Q for unperfused alveoli?
What would the gas concentrations be?
What is this called?
- Alveolar gas would mimic inspired gas (pO2: 150 mmHg, pCO2: 0 mmHg)
- Decreased perfusion increases the V/Q ratio (limit is ∞)
- Alveolar deadspace
Describe V/Q in the bottom and top of the lung?
The base has more flow than ventilation (under ventilated, ↓V/Q)
The top has more ventilation than flow (over ventilated, ↑V/Q)
What would you expect pO2 and pCO2 in the base and apex of the lung?
The base has a lower V/Q ratio = ↑ pCO2, ↓pO2
Apex has a higher V/Q ratio = ↓pCO2, ↑pO2
What are some concerns with V/Q and anesthesia?
- The creation of alveolar deadspace from PPV
- Creates increased R in alveolar capillaries
What is the pattern of V/Q and the effect of age?
V/Q matching gets worse with age
What happens instantly after intubation if we have no PEEP?
There is an instant V/Q mismatch and atelectasis
Describe LaPlace’s Law?
- Two connected spheres with differying radius; the smaller will have a higher pressure than the larger sphere (assuming not 100% full)
- This causes air to move to the larger sphere where there is a lower pressure
- Predicts that open alveoli recieve air before unfilled alveoli
Would be bad for pnemothorax
Why doesn’t LaPlace’s Law hold up in the alveoli?
Surfactant!
As alveoli get more full the surfactant is thinned and concentration is reduced
In unfilled alveoli the surfactant concentration is increased (makes it easier to fill less filled alveoli)
What happens if we don’t have surfactant?
Creates uneven ventilation
Becomes very hard to open up unused portions of the lung
What is mixed expired air?
A collection of anatomical deadspace and alveolar air (all expired gases)
What is the expected PEO2?
120 mmHg
In between dead space O2 (150 mmHg) and alveolar O2 100 mmHg
Closer to 100 because there is more volume in alveolar air than anatomical deadspace
What is the expected PECO2?
27 mmHg
In between alveolar air (40 mmHg) and anatomical dead space (0 mmHg)
Closer to alveolar because there is more volume than anatomical dead space
What happens to PECO2 as anatomical dead space increases?
PECO2 decreaes (less gas exchange, more dilution)
How can you calculate the pressure of CO2 in mixed expired air?
