Lecture 22 - Ventilation and Gas Exchange

Question 1

Functions of conducting zone?

Answer 1

1. Warms and humidifies the air as it flows through
2. Secretes mucus to trap particles in the inspired air
3. Expectorates phlegm with cilia

Question 2

Describe how spirometry works.

Answer 2

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

Question 3

How many lung volumes are there? Describe each. WHICH ONES ARE DIRECTLY MEASURED via spirometry?

Answer 3

1. ***Tidal volume (TV) = amount of air breathed out/in under normal conditions
2. ***Inspiratory reserve volume (IRV) = max amount inspired above the tidal volume
3. ***Expiratory reserve volume (ERV) = max amount expired below the tidal volume
4. Residual volume (RV) = remaining amount of air left after max expiration

Question 4

How many lung capacities are there? Describe each. WHICH ONES ARE DIRECTLY MEASURED via spirometry?

Answer 4

1. Total lung capacity (TLC) = sum of all lung volumes = TV + IRV + ERV + RV
2. ***Vital capacity (VC) = largest breath you can take = TV + IRV + ERV
3. ***Inspiratory capacity (IC) = maximal inspiratory volume = IRV + TV
4. Functional residual capacity (FRC) = amount of air left after residual expiration = ERV + RV

Question 5

During spirometry, does the bucket move up or down during expiration?

Answer 5

Up

Question 6

During spirometry, does the bucket move up or down during inspiration?

Answer 6

Down

Question 7

During spirometry, does the pen move up or down during inspiration?

Answer 7

Up

Question 8

During spirometry, does the pen move up or down during expiration?

Answer 8

Down

Question 9

What is a lung capacity?

Answer 9

Summation of 2 or more lung volumes

Question 10

How is RV measured?

Answer 10

1. Via helium dilution
2. Via nitrogen washout
3. Via body plethysmography

Question 11

Does the residual volume ever leave the chest?

Answer 11

NOPE

Question 12

What is special about helium gas? What does this mean?

Answer 12

It is inert = it cannot pass alveolar capillary membranes

Question 13

Describe the helium dilution technique to calculate RV.

Answer 13

1. Add a certain quantity of helium gas in the upside down bucket (C1 x V1)
2. Let the patient breath until the helium equilibrates throughout the lungs, the tube, and the bucket (aka at FRC)
3. Record C2 in the bucket
4. Calculate VT = V1 + V2 = C1 x V1 / C2
5. VT - V1 = V2 = FRC
6. FRC - ERV = RV

Question 14

Describe the nitrogen washout technique to calculate RV.

Answer 14

1. Patient starts at FRC
2. Measure [N2] expired
3. Use 100% O2 gas to wash all of the N2 out into a collecting bag
4. 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
5. FRC x 0.8 = VB x FN2 (in bag) OR FRC = VB x FN2 / 0.8
6. FRC - ERV = RV

Question 15

What is FN2 at FRC?

Answer 15

0.8

Question 16

What is the caveat of both GAS techniques to indirectly measure RV? Does this apply to body plethysmography?

Answer 16

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

Question 17

What is Boyle’s Law?

Answer 17

P x V will stay constant at the same temperature

Question 18

Describe the body plethysmography technique to calculate RV.

Answer 18

1. Place patient in a box with a pressure monitor and a tube they can breath through
2. As patient inspires, the chest expands and the pressure within the box increases and the volume will decrease
3. 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

Question 19

In which case is the body plethysmography technique not valid?

Answer 19

If a child has an ear tube

Question 20

What is minute ventilation?

Answer 20

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

Question 21

Normal respiratory rate?

Answer 21

15 breaths/min

Question 22

Normal minute ventilation?

Answer 22

500 mL x 15 breaths/min = 7,500 mL/min

Question 23

What is the tidal volume equal to? Describe the proportions.

Answer 23

VT = VD (40%) + VA (60%) = 150 mL + 350 mL = 500 mL

VD = Dead space volume
VA = Alveolar volume

Question 24

What is the ventilation to perfusion ratio?

Answer 24

V/Q ~ 1

Question 25

What is the alveolar ventilation?

Answer 25

350 mL x 15 breaths/min = 5,250 mL/min

Question 26

PvO2 in pulmonary artery?

Answer 26

40 mmHg

Question 27

PvCO2 in pulmonary artery?

Answer 27

45 mmHg

Question 28

PaO2 in pulmonary vein?

Answer 28

100 mmHg

Question 29

PaCO2 in pulmonary vein?

Answer 29

40 mmHg

Question 30

PAO2?

Answer 30

100 mmHg

Question 31

PACO2?

Answer 31

40 mmHg

Question 32

How to measure the volume of the anatomical dead space aka the conducting zone? Describe this technique.

Answer 32

Fowler's method: 1. Ask patient to inhale 100% O2 in one maximal inspiration 2. Measure amount of O2 exhaled: first portion will include 100% O2 3. As more air travels up from the respiratory zone we will also detect CO2/N2 4. 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

Question 33

What is the physiological dead space?

Answer 33

Regions of alveoli that are not capable of exchanging gas + anatomical dead space = volume of gas that does not eliminate CO2

Question 34

How to measure the volume of the physiological dead space? What to note?

Answer 34

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

Question 35

What is an example of a case in which an alveolus would be incapable of exchanging gas?

Answer 35

Pulmonary embolism

Question 36

What is the minute ventilation of CO2 equal to in a steady state?

Answer 36

oVeCO2 = oVA x PaCO2 / 0.8

Question 37

How to relate alveolar minute ventilation to VD and VT?

Answer 37

◦VA = VA x RR = (VT - VD) x RR = ◦Ve x (1 - VD/VT) 1 - VD/VT = % of TV that is alveolar

Question 38

What does VD/VT represent?

Answer 38

% of tidal volume that is dead space

Question 39

What can be said of the CO2 production during stead state?

Answer 39

It is constant

Question 40

When is minute ventilation Ve x PaCO2 constant? What does this mean?

Answer 40

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

Question 41

If the PaCO2 increases but the minute ventilation stayed constant, what happened?

Answer 41

One of 2 things: 1. Either the CO2 production rose 2. Or the volume of physiological dead space increased

Question 42

What is total ventilation equal to?

Answer 42

= VT x RR

Question 43

Are physiologic and anatomical dead space usually the same value?

Answer 43

Yes, but physiologic can be increased in many lung diseases

Question 44

Normal anatomical dead space?

Answer 44

150 mL