ventilation/perfusion relationships

Question 1

how does concentration of oxygen, carbon dioxide and nitrogen, PO2, PCO2 change from inspiration to expiration

Answer 1

O2 - decreases
CO2 - increases
N2 - remains the same
PO2 - decreases
PCO2 - increases

Question 2

how is the partial pressure of gas calculated

Answer 2

air is saturated in water vapour
water in water vapour exerts water vapour pressure of 47 mmHg
Pb - 47 %
or Pb - %gas

Question 3

how are arterial blood gas partial pressures measured

Answer 3

using arterial blood samples and a blood gas analyser

Question 4

how are alveolar PCO2 values measured

Answer 4

by measuring end-tidal values

Question 5

what is the thickness of alveolar capillary membrane

Answer 5

very thin 0.5 µm (microns)

Question 6

what is the purpose of the alveolar capillary membrane being very thin

Answer 6

there is rapid, complete equilibration of O2 and CO2 between the alveolar gas and the blood (perfusion rather than diffusion limited)

Question 7

diffusion is 1.___ proportional to 2.____ and 3.___ and 4.___ proportional to 5.____

Answer 7

1. directly
2. pressure difference
3. surface area
4. inversely
5. distance

Question 8

gas exchange in emphysema is reduced by

1. pressure difference
2. surface area
3. increased distance
4. resistance

Answer 8

2.reduced surface area

Question 9

gas exchange in fibrosis a is reduced by

1. pressure difference
2. surface area
3. increased distance
4. resistance

Answer 9

3.increased distance

Question 10

diffusing capacity or transfer factor definition

Answer 10

the extent to which oxygen passes from the air sacs of the lungs into the blood

Question 11

diffusing capacity or transfer factor calculation

Answer 11

Rate of transfer of gas from lung to blood/ Partial pressure difference = Rate of trans/PACO - PaCO

Question 12

why carbon monoxide used for diffusing capacity calculation

Answer 12

because the binding to haemoglobin is so strong and the PCO in the blood is zero so the partial pressure difference is the alveolar PCO

Question 13

how is the diffusing capacity calculated in practice

Answer 13

subject inhales a CO mixture, holds their breath for 10 s, exhales and the alveolar air analysed
CO consumption and alveolar PCO are measured and diffusing capacity is calculated

Question 14

diffusing capacity and transfer factor calculation units

Answer 14

DLCO = ml/min/kPa

TLCO - mmol/min/kPa

Question 15

diffusing capacity is depends on what factors

Answer 15

haemoglobin, age, sex

Question 16

reduced diffusing capacity is because of

Answer 16

lung fibrosis, pneumonia, oedema, emphysema

Question 17

what has a greater oxygen pressure arterial Po2 or alveolar Po2 and why

Answer 17

arterial (a) blood PO2 is slightly less (95 mmHg) than alveolar (A) PO2 (A-a PO2 gradient) because of venous admixture which is caused by - anatomical shunt, ventilation/perfusion mismatch

Question 18

if the alveolar (A) is greater than arterial (a) PO2 what does suggest

Answer 18

gradient suggests a problem with gas exchange, i.e., anatomical shunting or with (ventilation/perfusion)V/Q mismatch

Question 19

respiratory exchange ratio

Answer 19

CO2 production / O2 consumption

Question 20

why is ventilation/perfusion matching important

Answer 20

for normal gas exchange in the lungs

Question 21

what conditions of the alveoli and pulmonary capillaries must be in

Answer 21

alveoli must be in close proximity to pulmonary capillaries

Question 22

average V/Q ratio in lung

Answer 22

0.8

Question 23

is there a slight mismatch of ventilation to perfusion yes or no

Answer 23

yes

no consistent throughout the lung

Question 24

ventilation and perfusion at the base of lungs

Answer 24

high ventilation - not stretched, high compliance
higher perfusion -
lower PAO2 and V/Q ratio

Question 25

ventilation and perfusion at apex of lungs

Answer 25

low ventilation - not stretched, high compliance
lower perfusion - less blood flow
higher PAO2 and V/Q ratio

Question 26

what would the V/Q ratio look like in respiratory disease

Answer 26

increased V/Q ratio - overventilation and underperfusion
or
decreased V/Q ratio.- underventilation and overperfusion

Question 27

respiratory diseases that cause high V/Q ratio

Answer 27

increased V/Q ratio - overventilation and underperfusion

embolus and emphysema

Question 28

how does increased V/Q ratio mean to alveolar Vd

Answer 28

wasted ventilation

Question 29

how does increased V/Q ratio mean to alveoli

Answer 29

“shunting” where deoxygenated venous blood bypasses the exchange area and enters the left heart causing arterial hypoxaemia

Question 30

respiratory diseases that cause low V/Q ratio

Answer 30

obstruction - COPD , asthma, bronchitis

small V / Large V

Question 31

dead space

Answer 31

ventilation and no perfusion V/Q = infinity

Question 32

V/Q = infinity is indicative of what respiratory disease

Answer 32

pulmonary embolism

Question 33

true shunt

Answer 33

where blood flows through a region with zero ventilation

Question 34

examples of shunts

Answer 34

abnormal right-left shunts in the heart, atelectasis, consolidation

Question 35

what will oxygen therapy improve

Answer 35

oxygen therapy will improve PaO2 with a low V/Q ratio but not with “true shunt

Question 36

how does overventilation/underperfusion affect alveolar PO2 PCO2

Answer 36

increases alveolar PO2 and decreases PCO2

Question 37

how does underventilation/overperfusion affect alveolar PO2 PCO2

Answer 37

decreases alveolar PO2 and increases PCO2

Question 38

how does a decrease in PAO2 and increase in PACO2 affect smooth muscles in airway and pulmonary arterioles

Answer 38

causes relaxation of airway smooth muscle but contraction of pulmonary arterioles

Question 39

why does expired air have a higher percentage of oxygen than alveolar air

Answer 39

mixing with dead space air prior to exhalation

Question 40

what is reduced in emphysema and fibrosis

Answer 40

exchange