Trachte: Ventilation Perfusion Relationships

Question 1

What are the causes of hypoxemia?

Answer 1

1. hypoventilation
2. Diffusion limitation
3. shunt (left-right)
4. Ventilation-perfusion mismatch

Question 2

What are O2 levels in humidified air?

Answer 2

150 mmHg

Question 3

What are O2 levels in the alveoli?

Answer 3

100 mmHg

Question 4

What happens to O2 in the alveoli if it is being perfused normally? What replenishes oxygen?

Answer 4

It’s constantly being REMOVED by blood

Inhalation of fresh air

Question 5

What is the oxygen level in tissues?

Answer 5

1-100 mmHg

Question 6

What is hypoventilation?

Answer 6

If O2 is not replenished fast enough in the alveoli–>

alveolar PO2 DECLINES

Question 7

What causes hypoventilation?

Answer 7

Drugs (opiates)
chest wall damage
paralysis

Question 8

What does hypoventilation do to PCO2?

Answer 8

INCREASES PCO2

Question 9

How do you calculate PCO2?

Answer 9

PCO2 = (VCO2/VA) x K

Question 10

How do you calculate PAO2?

Answer 10

PAO2 = PIO2 – [PACO2/R]

PAO2 is alveolar PO2
PIO2 is inspired air PO2
PACO2 is alveolar PCO2
R is the respiratory quotient (produced CO2/consumed O2)

Question 11

What is R and what is it dependent on?

Answer 11

THe respiratory quotient (produced CO2/consumed O2)

Dependent on SUBSTRATE

Question 12

What is R w/ carbohydrate as substrate? w/ FA as substrate?

Answer 12

R = 1 with carbohydrate as substrate

R = 0.7 with fatty acid as substrate

Question 13

How can impaired diffusion result in hypoxia?

Answer 13

1. Reducing the area available for gas exchange

2. Increasing the diffusion distance

Question 14

Where does a natural shunt occur?

Answer 14

Naturally occurs with mixing of O2 depleted blood from the bronchial circulation and a small amount of blood from the thesbian veins of the heart

Question 15

What else can cause shunts?

Answer 15

1. congenital cardiac abnormalities

2. PDA

Question 16

How do you calculate shunt flow?

Answer 16

Qs/Qt = (CcO2 – CaO2)/CcO2 – CvO2)

Question 17

How do you calculate capillary O2?

Answer 17

From PAO2

Question 18

Can 100% O2 correct the hypoxemia resulting from a shunt?

Answer 18

NO

Question 19

What are two of the influences on Alveolar PO2?

Answer 19

Ventilation and perfusion

Question 20

What is the major cause of hypoxia in lung diseases?

Answer 20

Mismatch of ventilation and perfusion

**also affects alveolar PCO2

Question 21

What happens to PO2 and PCO2 if there is no perfusion of a unit but the alveoli are still ventilated?

Answer 21

PO2 and PCO2 approach that of inspired gas (150 and 0 mmHg)

Question 22

What happens to PO2 and PCO2 if there IS perfusion but NO ventilation?

Answer 22

PO2 and PCO2 approach that of venous blood

40 and 46 mmHg

Question 23

Why do gradations of ventilation-perfusion exist?

Answer 23

Because the lung is perfused and ventilated differently in the APEX and BASE

Question 24

What is the ventilation-perfusion ratio in the APEX? What are PO2 and PCO2?

Answer 24

HIGH

PO2= 132 mmHg

PCO2= 28 mmHg

Question 25

What is the ventilation perfusion ratio in the base of the lung?

Answer 25

LOW ``` PO2= 89 mmHg PCO2= 42 mmHg ```

Question 26

Does pH change from apex to base of the lung? Why?

Answer 26

YES pH is influenced by CO2

Question 27

What affect do ventilation-perfusion mismatches have on PO2 and PCO2? How do lung diseases affect mismatches?

Answer 27

Slight lowering of arterial PO2 <4 mmHg Slight elevation of PCO2 MUCH MORE DRAMATIC EFFECTS

Question 28

Can hypoxia or hypercapnia be rectified by more rapid breathing?

Answer 28

Hypercapnia

Question 29

What is a function of the rate of breathing?

Answer 29

CO2 elimination

Question 30

What determines the differences in O2 and CO2 responses to more rapid breathing?

Answer 30

Different shapes of their dissociation curves

Question 31

What is the dissociation curve like for CO2?

Answer 31

Linear in the physiological range

Question 32

What is the dissociation curve like for O2?

Answer 32

sigmoidal and almost flat at the top of the range

Question 33

Why is PCO2 close to normal even in lung disease?

Answer 33

B/c alterations in arterial PCO2 will result in more rapid breathing--> this increases the work of breathing

Question 34

How do you assess ventilation perfusion abnormalities?

Answer 34

Use the alveolar gas equation to calculate what arterial PO2 should be and compare it w/ what is observed