Respiratory Physiology - Pulmonary Gas Exchange

Question 1

Hypothetical oxygen cascade in a hypothetical perfect lung

Answer 1

Question 2

What causes the drop in PO2 in alveoli from atmosphere

Answer 2

Both ventilation (addition of O2 to alveoli) and pulmonary blood flow (removal of O2 from alveoli) can be thought of as continuous
(there is an element of pulsatility but overall negligible)

Taking both into account, overall result is a lower PO2

Question 3

Effect of hypoventilation on oxygen cascade

Answer 3

Reduced ventilation so less O2 added
Therefore lower PO2

Question 4

Alveolar gas equation

Answer 4

PAO2 = alveolar PO2
PiO2 = inspired PO2
PACO2 = alveolar PCO2
R = respiratory exchange ratio

F is generally neglected as represents very minimal effect when breathing air

Question 5

Respiratory exchange ratio

Answer 5

Ratio of CO2 production to O2 uptake

Also known as respiratory quotient when referring to tissue level

Question 6

Normal respiratory exchange ratio at rest

Answer 6

~0.8

Question 7

Equation for PiO2 (inspired PO2)

Answer 7

PiO2 = FiO2 x 713 mmHg

Question 8

Causes of hypoxaemia

Answer 8

Hypoventilation
Diffusion limitation (eg pulmonary fibrosis)
Shunt
Ventilation perfusion mismatch - most common

Question 9

Causes of hypoventilation

Answer 9

1, 2 = central respiratory centres
3 = interference with nerve tracts from respiratory centres
4 = anterior horn disease
5 = nerve disease
6 = neuromuscular junction
7 = muscular disease
8 = cage wall abnormality
9 = upper airway obstruction

Question 10

Examples of central respiratory centre abnormalities causing hypoventilation

Answer 10

Encephalitis
Drugs eg opioids

Question 11

Examples of interference with nerve tracts causing hypoventilation

Answer 11

C spine dislocation causing spinal cord compression

Question 12

Example of anterior horn disease causing hypoventilation

Answer 12

Polio myelitis

Question 13

Example of nerve disease causing hypoventilation

Answer 13

Neuritis eg Guillan-Barre Syndrome

Question 14

Example of alterations at neuromuscular junction causing hypoventilation

Answer 14

Neuromuscular blockade drugs in anaesthesia

Question 15

Example of muscular disease causing hypoventilation

Answer 15

Muscular dystrophy

Question 16

Example of thoracic cage wall abnormality causing hypoventilation

Answer 16

Rib fractures
Flail chest

Question 17

Examples of upper airway obstruction causing hypoventilation

Answer 17

Cancer
Enlarged lymph nodes

Question 18

Oxygen cascade in physiological lung - not a perfect lung

Answer 18

Drop in PO2 with diffusion and shunt

Question 19

Shunt definition

Answer 19

Blood reaching the arterial system without passing through ventilated areas of lung

Exists in small amount even in healthy lung - bronchial artery circulation and small amount from cardiac thebesian vein into left ventricle

Question 20

Drainage of bronchial arterial system

Answer 20

Most enters right atrium via azygous vein and bronchial venous system

Small amount drains downstream of pulmonary capillaries contributing to shunt

Question 21

Shunt equation for calculating degree of shunt

Answer 21

Question 22

Method to measure mixed venous concentration of oxygen

Answer 22

CvO2

Pulmonary artery catheter required

Clinically sometimes a CVC near the right atrium is used as an estimate

Question 23

Why does increase FiO2 to 1.0 not increase PaO2 in shunt (to the expected level)

Answer 23

Increasing FiO2 only impacts blood passing by ventilated lung, but this is not the cause of low PaO2 with shunt

Unoxygenated blood still mixes with end capillary blood

Question 24

What is normal V/Q ratio

Answer 24

1.0 (in all regions of healthy lung it clusters around this value despite regional lung differences)

Ventilation and perfusion should match in healthy situations

Therefore can assume alveolar PO2 is the same as end pulmonary capillary PO2

Question 25

Effects of changing ventilation perfusion ratio

Answer 25

A = normal
B = reducing ventilation (shunt)
C = reducing perfusion (dead space)

Can think about it as the scale at the bottom (Ventilation Perfusion line)

Question 26

Effect of changing V/Q ratio on alveolar PO2 and PCO2

Answer 26

Question 27

Changes in ventilation, blood flow and V/Q ratio in the upright lung from bottom to top

Answer 27

Question 28

Why is pulmonary tuberculosis typically at the apices of the lung

Answer 28

Alveolar PO2 at top of lung is higher than at bottom (as V/Q ratio higher) so more favourable for pulmonary TB

Question 29

Cause for difference between alveolar and arterial PO2

Answer 29

Difference in blood flow between top and bottom of lung is greater than ventilation difference

Therefore relative more blood flow gets O2 from base of lung (with lower PO2) than from the top, but end expired gas is more mixed from top and bottom (as difference in ventilation is lower)

Question 30

Cause for arterial PCO2 being higher than alveolar PCO2

Answer 30

Similar concept to O2 differences with relative differences in ventilation and blood flow, but now base of lung has higher PCO2 and top has lower PCO2 due to V/Q ratio differences

Therefore mixed arterial blood has higher PCO2 than mixed expired alveolar gas

Question 31

Why does reduced V/Q ratio cause hypoxaemia

Answer 31

Shunted blood has lower PO2 therefore reducing overall mixed arterial PO2

Question 32

Effect of emphysema on V/Q ratio

Answer 32

Emphysema results in alveolar wall breakdown including loss of capillaries

Therefore some areas of lung have maintained V/Q around 1.0, but other areas of damage have higher V/Q (still ventilated but loss of blood flow)

Tends to have higher effect on PCO2 as no areas of shunt

Question 33

Effect of chronic bronchitis on V/Q ratio

Answer 33

Some areas of lung have maintained V/Q around 1.0, but other areas have lower V/Q (maintained blood flow but lower ventilation due to airway inflammation / mucus to lung units)

Question 34

Stages of impairment of gas exchange when ventilation perfusion mismatch enforced

Answer 34

Moves down stages as physiology aims to maintain PO2 supply for tissue demand and to maintain normal PCO2 by eventually increasing ventilation

Question 35

How to calculate the alveolar-arterial PO2 difference

Answer 35

Use alveolar equation to calculate alveolar PO2 minus the arterial PO2 from this

For alveolar PCO2, use arterial PCO2 as this is almost equivalent to the “ideal alveolar PCO2”

Question 36

Normal alveolar-arterial PO2 difference

Answer 36

< 10 mmHg

Question 37

Significance of raised Alveolar-arterial PO2 difference

Answer 37

Suggests ventilation perfusion mismatch