Common causes of hypoxemia

Question 1

Hypoxia

Answer 1

Failure of oxygenation at the tissue and cellular level

Question 2

Hypoxemia

Answer 2

Low partial pressure of oxygen in the blood

Question 3

Measuring hypoxemia

Answer 3

Specifically hypoxemia is determined by measuring partial pressure of oxygen in the arterial blood (PaO2)

Question 4

PaO2 of the arterial blood and alveolar gas in the perfect lung + reality

Answer 4

• PaO2 of arterial blood would be the same as that in alveolar gas in the perfect lung
• in reality always be small differences between PAO2 and PaO2 (approx by 10-20 mmHg)

Question 5

Why small differences in PAO2 and PaO2

Answer 5

1) incomplete diffusion (very small)
2) ventilation and perfusion not perfectly matched (even in healthy -especially in different areas of lungs)
3) small percentage of bronchial arterial blood is collected by the pulmonary veins after it has perfused the bronchi and its O2 has been partly depleted

Question 6

Where is ventilation/perfusion the best matched

Answer 6

At the bases of the lungs

Question 7

A-a gradient

Answer 7

-alveolar-arterial difference in oxygen (A-a DO2)

Question 8

Determining A-a DO2

Answer 8

1) Obtain PaO2 and PaCO2 levels by sampling arterial blood gases
2) Use PaCO2 and the alveolar air equation to calculate PAO2
3) Calculate A-a DO2 = PAO2 - PaO2

Question 9

What do large values for A-a DO2 indicate

Answer 9

Problems with gas exchange

Question 10

Estimating normal range for A-a DO2 by age

Answer 10

Age/4 + 4

Increased age affects A-a gradient

Question 11

Causes of hypoxemia - 2 main categories of mechanisms

Answer 11

1) Those that increase A-a DO2

2) Those where A-a DO2 is preserved

Question 12

Five causes of hypoxemia

Answer 12

1. Low inspired oxygen
2. Alveolar hypoventilation (low alveolar ventilation)
3. Diffusion impairment (diffusion abnormality)
4. Ventilation -perfusion inequality (ventilation - perfusion mismatch) - V/Q mismatch

Question 13

How low inspirated oxygen causes hypoxemia

Answer 13

PiO2  = (PB-PH2O) FiO2
-where PiO2 is decreased
FiO2 = Fraction of inspired oxygen
PiO2 = partial pressure of inspired O2
PB = barometric pressure
PH2O= partial pressure of water vapour in lungs

Question 14

A-a DO2 if hypoxemia caused by low inspired oxygen

Answer 14

-will be normal as PAO2 and PaO2 will be decreasing at the same rate

Question 15

Causes of low inspired oxygen

Answer 15

1) Decrease in barometric pressure (breathing at high altitude)
2) Decrease in FlO2 - accidental (anaethetist does not supply enough O2 r leak in breathing circuit)

Question 16

PaCO2 in situations of low inspired oxygen + mechanism

Answer 16

1) Peripheral chemoreceptors sense low arterial PO2 and increase ventilation through input to respiratory centers
2) Hyperventilation
3) PaCO2 is decreased (hypocapnia)

Question 17

How alveolar hypoventilation (low alveolar ventilation) causes hypoxemia

Answer 17

-breathing less (mechanically)-volume entering and exiting alveoli per unit time is reduced

Question 18

A-a DO2 if hypoxemia is caused by alveolar hypoventilation

Answer 18

A-a DO2 is normal as PAO2 and PaO2 will be decreasing at the same rate

Question 19

PaCO2 if hypoxemia is caused by alveolar hypoventilation

Answer 19

-PaCO2 is elevated (hypercapnia) - because not exhaling as much either and body’s primary means of eliminating CO2 is through the lungs
PaCO2 proportional to VCO2/ VA
(alveolar PACO2 is directly proportional to the amount of CO2 produced by metabolism and delivered to the lungs VCO2 and inversely proportional to the alveolar ventilation VA)
-reduction of alveolar ventilation = increased alveolar PCO2 (PACO2) because gas is not being exchanged at normal rate
-in turn CO2 in blood perfusing the alveoli will begin to accumulate and increase the PaCO2

Question 20

Treatment of hypoventilation

Answer 20

Supplemental O2

-increasing fraction of inspired oxygen can alleviate the hypoxemia and the hypercapnia

Question 21

Potential causes of hypoventilation (9)

Answer 21

1. Depression of CNS by drugs
2. Inflammation, trauma or hemorrhage in the brainstem
3. Abnormal spinal cord pathway
4. Disease of the motor neurons of the brain stem/spinal cord
5. Disease of the nerve supplying the respiratory muscles
6. Disease of the neuro muscular junction
7. Disease of the respiratory muscles
8. Abnormality of the chest wall
9. Upper airway obstruction

Question 22

Diffusion impairment - how it causes hypoxemia

Answer 22

-O2 diffusion from alveoli to arteries is reduced

Question 23

A-a DO2 if hypoxemia is caused by diffusion impairment + why

Answer 23

• A-a DO2 is normal at rest but may be elevated during exercise
• because at rest transit time for RBC in pulmonary capillary > then time for PO2 in mixed venous blood to reach eq with alveolar gas
• during exercise is > blood flow so transit time is decreased - in the healthy individual would still be sufficient time for PO2 in mixed venous blood to reach equilibrium with alveolar gas but if have impaired diffusion time to reach eq will be further increased and cause increase PAO2 and decreased PaO2 (or in elite athelete who achieve very high CO during exercise resulting in large decrement in pulmonary transit time)

Question 24

Why does impaired diffusion occur

Answer 24

-When there is an increase in the thickness of the physical seperation between alveolar gas and pulmonary capillary blood + shortened pulmonary transit times (i.e. performing exercise)

Question 25

Treatment of diffusion impairment

Answer 25

• supplemental oxygen
• increasing fraction of inspired oxygen -increases the partial pressure gradient driving O2 across the altered blood air barrier

Question 26

Ventilation-perfusion inequality - why causes hypoxemia

Answer 26

-for efficient gas exchange to occur air must reach the regions of the lung that are being appropriately perfused

Question 27

A-a DO2 in hypoxemia caused by V/Q mismatch

Answer 27

A-a DO2 is elevated

-because alveoli not sufficiently perfused so PAO2 increases and PaO2 does not

Question 28

Average alveolar ventilation

Answer 28

4L/min

Question 29

Average CO

Answer 29

5L/min

Question 30

Normal range V/Q ratio

Answer 30

0.8- 1

Question 31

Why V/Q mismatch occurs even in healthy lungs

Answer 31

• ventilation and blood are both gravity -dependent
• i.e. both increase as move down the lungs
• blood flow shows about 5-fold difference between top and bottom while ventilation shows about a 2-fold difference
• this causes gravity-dependent regional variations in V/Q ratio - range from 0.7 at the base of the lungs to 3.0 at the apex
• this mismtch accounts for approx 2/3 of the normal A-a DO2 seen in healthy individuals

Question 32

At top of the lungs what is the V/Q ratio + consequence

Answer 32

-at top of lungs V/Q is high and is more ventilation compared to perfusion

Question 33

At base of lungs what is the V/Q ratio + consequences

Answer 33

• V/Q is low

- is more perfusion compared to ventilation

Question 34

Treament of hypoxemia caused by V/Q mismatch

Answer 34

-supplemental oxygen will help correct hypoxemia because will provide additional O2 to low V/Q regions (bases)
(but will have little affect on high V/Q regions - blood in these regions will already be nearly saturated)
-but will improve both saturation and PaO2 in blood coming from regions with low V/Q ratio

Question 35

How shunting causes hypoxemia

Answer 35

Shunt = a condition in which deoxygenated blood from the venous system is directed to the arterial system without receiving oxygen from the lungs
Shunts can occur naturally or be the result of a disease process

Question 36

A-a DO2 in hypoxemia caused by shunting

Answer 36

-will be elevated

Question 37

Supplemental oxygen efficacy for hypoxemia caused by shunting

Answer 37

Will not correct hypoxemia - because shunted blood will never be exposed to supplemental oxygen

Question 38

Common causes of shunting

Answer 38

• congenital abnormalities: tetrology of fallot,

- intrapulmonary fistulas (direct communication between a branch of pulmonary artery and pulmonary vein)