Lecture 4: Gas Exchange

Question 1

Ve (minute ventilation) =

Answer 1

Vt (tidal volume) x RR

Question 2

Dead space (definition and two components)

Answer 2

Volume of airways and lungs that do not participate in gas exchange, includes anatomical dead space (respiratory tract –> terminal bronchiole), alveolar dead space (ventilated alveolar that are not perfused)

Question 3

Physiological dead space should be equal to what in healthy people?

Answer 3

Anatomical dead space

Question 4

Dead space assumptions (3)

Answer 4

All CO2 in expired air comes from gas exchanged; no CO2 in inspired air; physiological dead space contributes no CO2

Question 5

Dead space equation

Answer 5

Vd/Vt = (PaCO2 - PeCO2) / PaCO2

Question 6

Alveolar ventilation equation

Answer 6

VCO2 = (VA x PACO2) / K

Question 7

If we ventilate more, what happens to PaCO2?

Answer 7

Decreases

Question 8

Alveolar gas equation

Answer 8

PAO2 = FIO2 (Patm - PH2O) - (PaCO2 / R)

Question 9

Where in the lung is ventilation higher?

Answer 9

Bases: gravity causes resting volume of alveoli in bases to b smaller –> more compliant (small pressure difference = large change in volume) –> more air goes here

Question 10

Where in the lung is perfusion higher?

Answer 10

Gravity causes perfusion to be highest in lung bases

Question 11

Zone 1

Answer 11

PA > Pa > Pv

Question 12

Zone 2

Answer 12

Pa > PA > Pv

Question 13

Zone 3

Answer 13

Pa > Pv > PA

Question 14

What is the normal value of V/Q (lung average)?

Answer 14

0.8

Question 15

Alveolar hypoxia causes…

Answer 15

Pulmonary vasoconstriction

Question 16

Regional hypoxia

Answer 16

Vasoconstriction leads diversion of blood away from poorly ventilated alveoli

Question 17

Generalized hypoxia

Answer 17

When PAO2 is low (because PIO2 low or disease) you can get pulmonary hypertension

Question 18

Where is V/Q highest and lowest?

Answer 18

Highest = apex; Lowest = base

Question 19

V/Q = infinity F

Answer 19

Ventilation, no perfusion (dead space); PAO2 = 150; PACO2 = 0

Question 20

V/Q = 0 F

Answer 20

Perfusion, no ventilation (airway obstruction); PaO2 = 40, PaCO2 = 46

Question 21

Diffusion-limited gas exchange

Answer 21

Total amount of gas transported across the alveolar-capillary barrier is limited by the diffusion process (too slow, pressure gradient continues exists)

Question 22

Perfusion-limited gas exchange

Answer 22

Total amount of gas transported across the alveolar-capillary barrier is limited by blood flow (perfusion; partial pressure gradient NOT maintained)

Question 23

Oxygen and carbon dioxide are __________-limited

Answer 23

Perfusion

Question 24

What does the O2-Hb dissociation curve show us? (4 conditions)

Answer 24

O2 preferentially released to tissues that are more metabolically active (decreased pH, increased CO2, increased temp, increased 2,3-DPG)

Question 25

CO binds heme with _____ times more/less affinity. Besides blocking O2 from binding, what else does CO do?

Answer 25

250; more; prevents O2 release by left shifting curve

Question 26

What is diffusing capacity? How does it work?

Answer 26

Quantitative measure of gas transfer in the lungs; low concentration of CO delivered to lungs; amount CO measured at exhale to determine diffusion

Question 27

DLCO (diffusing capacity) equation

Answer 27

DLCO = VCO / PACO

Question 28

FFF

Answer 28

(OR) = (dCO x A) / T

Question 29

How much O2 is dissolved? How else is it carried in blood?

Answer 29

2%; bound to Hb

Question 30

Arterial oxygen content (CaO2) equation

Answer 30

CaO2 = (1.34 x Hgb x O2 saturation) + (0.003 x PaO2)

Question 31

Oxygen delivery (DO2) equation

Answer 31

DO2 = CaO2 x CO (cardiac output)

Question 32

Hypoxia (def)

Answer 32

Decreased oxygen delivery (or utilization) by tissues

Question 33

Hypoxemia (def)

Answer 33

Low PaO2 (can CAUSE hypoxia)

Question 34

Causes of hypoxia (4)

Answer 34

1. Low CO, 2. Low Hb (anemia), 3. Low O2 saturation (hypoxemia, CO poisoning), 4. Low O2 utilization (O2 delivered but NOT used = sepsis, cyanide)

Question 35

Causes of hypoxemia (5)

Answer 35

1. Low PiO2, 2. Hypoventilation, 3. V/Q mismatch, 4. Shunt, 5. Diffusion impairment

Question 36

What can test for hypoxia/hypoexmia?

Answer 36

A-a gradient and effect of supplemental oygen

Question 37

What is the A-a gradient? What is normal? What does hypoxemia with normal A-a gradient indicate? What does hypoxemia with increased A-a gradient indicate?

Answer 37

Difference between PAO2 and PaO2;

Question 38

When does supplemental oxygen help? (4)

Answer 38

Increases FiO2, will increase PaO2 if: low PiO2, hypoventilation, V/Q mismatch, diffusion impairment

Question 39

When will supplemental oxygen not help?

Answer 39

If hypoxemia is secondary to shunt

Question 40

How does supplemental O2 help in w/ hypoventilation?

Answer 40

Breathing supplemental oxygen will require less ventilation to achieve higher PAO2

Question 41

Why does O2 not help in case of a shunt?

Answer 41

Perfused areas of lung already have Hb saturated with O2, so they cannot “make up” for the not perfused portions with the extra O2

Question 42

Why does O2 help in case of V/Q mismatch?

Answer 42

In creased FiO2 will help the less perfused alveoli increase to 100% saturation

Question 43

V/Q mismatch only causes hypoxemia when…

Answer 43

V/Q units are low

Question 44

Why is CO2 not elevated in shunt?

Answer 44

Because of relationship between PCO2 content and and increased ventilation F

Question 45

Why does O2 help diffusion impairment?

Answer 45

If you have diffusion impairment (fibrosis) O2 becomes diffusion limited exchange, so supplemental O2 will increase PAO2 and O2 pressure gradient, which will help

Question 46

When does diffusion impairment manifest?

Answer 46

Exercise: CO increases, speed of blood increases, less time in alveolar capillary –> not enough time for gas to diffuse