Lecture 22 - Ventilation and Gas Exchange Flashcards
Functions of conducting zone?
- Warms and humidifies the air as it flows through
- Secretes mucus to trap particles in the inspired air
- Expectorates phlegm with cilia
Describe how spirometry works.
Water filled bucket with an inverted bucket placed inside
Water filled bucket connected to an air tube through which the patient breathes
Inverted bucket connected to pulley system connected to a pen on paper recording changes in rise/fall
How many lung volumes are there? Describe each. WHICH ONES ARE DIRECTLY MEASURED via spirometry?
- ***Tidal volume (TV) = amount of air breathed out/in under normal conditions
- ***Inspiratory reserve volume (IRV) = max amount inspired above the tidal volume
- ***Expiratory reserve volume (ERV) = max amount expired below the tidal volume
- Residual volume (RV) = remaining amount of air left after max expiration
How many lung capacities are there? Describe each. WHICH ONES ARE DIRECTLY MEASURED via spirometry?
- Total lung capacity (TLC) = sum of all lung volumes = TV + IRV + ERV + RV
- ***Vital capacity (VC) = largest breath you can take = TV + IRV + ERV
- ***Inspiratory capacity (IC) = maximal inspiratory volume = IRV + TV
- Functional residual capacity (FRC) = amount of air left after residual expiration = ERV + RV
During spirometry, does the bucket move up or down during expiration?
Up
During spirometry, does the bucket move up or down during inspiration?
Down
During spirometry, does the pen move up or down during inspiration?
Up
During spirometry, does the pen move up or down during expiration?
Down
What is a lung capacity?
Summation of 2 or more lung volumes
How is RV measured?
- Via helium dilution
- Via nitrogen washout
- Via body plethysmography
Does the residual volume ever leave the chest?
NOPE
What is special about helium gas? What does this mean?
It is inert = it cannot pass alveolar capillary membranes
Describe the helium dilution technique to calculate RV.
- Add a certain quantity of helium gas in the upside down bucket (C1 x V1)
- Let the patient breath until the helium equilibrates throughout the lungs, the tube, and the bucket (aka at FRC)
- Record C2 in the bucket
- Calculate VT = V1 + V2 = C1 x V1 / C2
- VT - V1 = V2 = FRC
- FRC - ERV = RV
Describe the nitrogen washout technique to calculate RV.
- Patient starts at FRC
- Measure [N2] expired
- Use 100% O2 gas to wash all of the N2 out into a collecting bag
- Measure volume and N2 concentration in bag once it has reached a steady state and no more N2 is exiting the lungs = quantity of N2 which was in the FRC
- FRC x 0.8 = VB x FN2 (in bag) OR FRC = VB x FN2 / 0.8
- FRC - ERV = RV
What is FN2 at FRC?
0.8
What is the caveat of both GAS techniques to indirectly measure RV? Does this apply to body plethysmography?
Both techniques assume that gases reach all sections of the lungs evenly, so if there are pathologies causing some alveoli to be occluded or stop functioning (asthma, emphysema, COPD), the helium will not reach all of them OR all of the N2 will not be expelled, and therefore the RV will be underestimated
NOPE, it does not because body plethysmography measures all of the gases in the lungs whether it is communicating or not
What is Boyle’s Law?
P x V will stay constant at the same temperature
Describe the body plethysmography technique to calculate RV.
- Place patient in a box with a pressure monitor and a tube they can breath through
- As patient inspires, the chest expands and the pressure within the box increases and the volume will decrease
- Boyle’s Law:
- P1 x V1 = P2 x (V1 - ΔV)
- P3 x V2 = P4 x (V2 + ΔV)
ΔV = volume change caused by expanded chest P3/P4 = pressure at patient's mouth V2 = FRC = volume inside the chest
In which case is the body plethysmography technique not valid?
If a child has an ear tube
What is minute ventilation?
Volume of gas moved in and out of the body during regular breathing = VE = TV x respiratory rate = dead space minute ventilation + alveolar minute ventilation
Normal respiratory rate?
15 breaths/min
Normal minute ventilation?
500 mL x 15 breaths/min = 7,500 mL/min
What is the tidal volume equal to? Describe the proportions.
VT = VD (40%) + VA (60%) = 150 mL + 350 mL = 500 mL
VD = Dead space volume VA = Alveolar volume
What is the ventilation to perfusion ratio?
V/Q ~ 1
What is the alveolar ventilation?
350 mL x 15 breaths/min = 5,250 mL/min
PvO2 in pulmonary artery?
40 mmHg
PvCO2 in pulmonary artery?
45 mmHg
PaO2 in pulmonary vein?
100 mmHg
PaCO2 in pulmonary vein?
40 mmHg
PAO2?
100 mmHg
PACO2?
40 mmHg
How to measure the volume of the anatomical dead space aka the conducting zone? Describe this technique.
Fowler’s method:
- Ask patient to inhale 100% O2 in one maximal inspiration
- Measure amount of O2 exhaled: first portion will include 100% O2
- As more air travels up from the respiratory zone we will also detect CO2/N2
- As soon as CO2/N2 starts to appear, we can deduce it is the end of the anatomical dead space => on a graph use the midpoint of the rise to measure the anatomical dead space volume because at the dead space/respiratory zone limit some CO2 will diffuse into the dead space and mix with the pure O2
What is the physiological dead space?
Regions of alveoli that are not capable of exchanging gas + anatomical dead space = volume of gas that does not eliminate CO2
How to measure the volume of the physiological dead space? What to note?
The Bohr method: you can measure TV, exhaled volume of CO2 (=VT x FeCO2), and alveolar CO2 (aka arterial CO2) to calculate alveolar volume (VA) and then VD = VT - VA
VT x PeCO2 = VA x PACO2 VT x PeCO2 = VA x PaCO2 VA = VT – VD VT x PeCO2 = (VT-VD) x PACO2 Bohr equation: VD/VT= PACO2- PeCO2/PACO2
Note: since physiological dead space includes alveoli that are not capable of exchanging gas, the CO2 that we expire only comes from alveolar gas space
What is an example of a case in which an alveolus would be incapable of exchanging gas?
Pulmonary embolism
What is the minute ventilation of CO2 equal to in a steady state?
oVeCO2 = oVA x PaCO2 / 0.8
How to relate alveolar minute ventilation to VD and VT?
◦VA = VA x RR = (VT - VD) x RR = ◦Ve x (1 - VD/VT)
1 - VD/VT = % of TV that is alveolar
What does VD/VT represent?
% of tidal volume that is dead space
What can be said of the CO2 production during stead state?
It is constant
When is minute ventilation Ve x PaCO2 constant? What does this mean?
When CO2 production and VD/VT are both constant:
Ve x PaCO2 = VeCO2 x 0.862 / (1 - VD/VT)
Means that if you double the minute ventilation BY ADJUSTING RESPIRATORY RATE you are halving the PaCO2, therefore PACO2
If the PaCO2 increases but the minute ventilation stayed constant, what happened?
One of 2 things:
- Either the CO2 production rose
- Or the volume of physiological dead space increased
What is total ventilation equal to?
= VT x RR
Are physiologic and anatomical dead space usually the same value?
Yes, but physiologic can be increased in many lung diseases
Normal anatomical dead space?
150 mL
