Exam 3 - EtCO2, compliance, Bohr equation Flashcards
How do lung volumes change from sitting to supine?
FRC decreases b/c ERV decreases
IRV increases proportionally keeping TLC the same
EtCO2 should match what?
Arterial blood gas CO2
Why does a normal capnograph have a increasing slope?
Because blood is always moving through the lungs
During an expiration the EtCO2 concentration increases because there is no fresh gas coming in, but CO2 still being unloaded
When would you expect there to be highest and lowest CO2 on a capnograph?
Highest: Very end of expiration, just before inhalation
Lowest: During inspiration, just before exhaling
How can we calculate PACO2 flucuation during a breath?
1st: Calculate the FACO2 @ FRC = PPCO2 ÷ Ptotal
2nd: FACO2 × FRC = Volume of CO2
3rd: Add VA to FRC
4th: New [CO2] = Vol CO2 ÷ (VA + FRC)
5th: pCO2 = new [CO2] × Ptotal
Fluctuates between ~36 and 40 during nromal inspiration and exhalation
Describe what would cause a EtCO2 wave like this?
Late stage emphysema
- The base of the lung has a higher pCO2 than the apex
- Small airways in the base of the lung collapse quickly
- This means most of the air during an exhalation is coming from the apex (which is lower in CO2)
- This causes the inverted slope seen in the capnograph
Why doesn’t the capnograph change right when inspiration happens?
There is a slight delay because the air has to travel through a long length of tubing
Hoe does alveolar deadspace affect the capnograph?
Alveolar DS contains no CO2 (no gas exhange) - therefore it dilutes out the normal alveolar air and decreases the capnograph reading
Whats a quick way to figure out how much alveolar dead space you have?
Look at the difference between the EtCO2 and arterial pCO2
If the EtCO2 is lower than the pCO2 you likely have alveolar deadspace
What does the Bohr equation solve for?
Determines how much alveolar dead space do we have
What is physiologic deadspace, what is their CO2 concentration?
Physiologic is anatomical and alveolar deadspace
They should have no CO2
What is the descriptive Bohr equation?
Volume of CO2 in the meixed expired air = Volume of CO2 from deadspace + Volve of CO2 from alveoli
Why do we remove the deadspace volume of CO2 from the Bohr equation?
- Because deadspace should contain no CO2 and therefore is not necessary to solve for
- All of the CO2 in mixed expired air should come from ventilated and perfused alveoli
What is the formula to solve for Alveolar volume?
VA = VT - VDS
What is the final Bohr equation that we can use?
How can you derive the variables?
- PaCO2 should be the same as EtCO2 or can be pulled from an ABG
- Mixed expired can be taken from the anesthesia machine, or derived like we learned before
Once you calculate an answer from the Bohr equation, how do you determine alveolar deadspace?
The equation gives you physiologic dead space
If you know anatomic you can subtract anatomic DS from your answer
What 2 things are hindering us from filling the lungs?
- Compliance of the lungs (ER)
- Compliance of the chest wall
Describe the tendency of lungs and chest wall to recoil?
What does this create?
- The lungs want to recoil and become smaller PER
- The chest wall wants to recoil outwards
- This creates our -5 mmHg PPl
What happens to the chest wall if ER of the lungs decreases?
- There is less opposition to chest wall expansion - increases chest wall expansion
- This makes PPl more positive
- This can cause barrel chest found in COPD
This image shows that in emphysema the lungs have a huge VC with a relatively low TP pressure. How is this possible?
- Transpulmonary pressure (PL) = Alveolar Pressure (PA) - Intrapleural Pressure (Ppl)
- In emphysema, due to increased lung compliance, a given volume of air does not generate as much recoil pressure.
- Normally, elastic recoil creates a more negative intrapleural pressure, but in emphysema, the weakened lung tissue generates less recoil force, reducing the difference between alveolar and pleural pressure.
- This results in a lower transpulmonary pressure at a given lung volume.
Calculate the compliance of the lungs (pulmonary compliance)?
Do we typically need to factor in chest wall compliance into pulmonary compliance?
Generally no, because in a healthy person breathing at FRC the chest wall doesnt cause an obstacle to inspiration
The lungs and chest wall are a system in ____ ?
What does mean for the relationship between total compliance and individual compliance?
- Series
- The total compliance must be lower than each individual part
- Compliance = 1 / Resistance
What is the compliance of the chest wall?
0.2 L / cmH2O
How can you calculate total compliance of the lung?
How does a pneumothorax or blood in the pleural space affect the lung?
The outward pull of the chest wall holds the lungs open
An obstruction in the plueral space effectively takes away the pull of chest wall, causing loss of the PIP and lung collapse
How much O2 is in the lungs in between breaths in a healthy adult, at rest, and upright?
What does this tell you about how much time we can go without O2?
- This is expected because inspired O2 is 21% and is diluted the further down the lungs we look
- We require 250 mL/min, so we have about 2 mins of O2 without breathing
This is O2 in lungs at the end of expiration
Why is preoxygenation so important for anesthesia?
- Because when paralyzed and lying supine, our FRC can be below RV (< 1.0 L).
- This greatly decreases how much O2 we have in the lungs
- Preoxygenating prevents profound hypoxia during intubation
