ventilation perfusion ratio

Question 1

what levels of the bronchi does gas exchange occur in

Answer 1

17-23

Question 2

why is pAO2 (alveolar) greater thsn paO2 (arterial)

Answer 2

diffusion is imperfect and so 100% of the O2 in the alveoli does not get transported into the blood

Question 3

what does pO2 in an ABG measure and what is this a proxy for

Answer 3

partial pressure of O2 in the artery; proxy for blood leaving the lung

Question 4

equation for the oxygen-carrying capacity of the blood

Answer 4

1.34mL x [Hb] (g/L) x Hb saturation of oxygen

Each gram of hemoglobin can carry about 1.34 ml of oxygen when fully saturated

Question 5

what device is used to determine oxygen saturation

Answer 5

pulse oximeter

Question 6

hypoxemia vs hypoxia

Answer 6

hypoxemia - reduciton in PaO2
hypoxia -local or systemic failure of oxygen delivery

Question 7

what can cause hypoxemia (5)

Answer 7

reduciton in altitude; hypoventilation; diffusion; shunts; VQ mismatch

Question 8

what can cause hypoxia (4)

Answer 8

ventilation (paO2 affected); circulation ( ↓CO = ↓O2 to tissue);
histiotoxic (Mt. not working);
sever anaemia/dyhaemoglobinaemias (O2 doesnt bind well)

Question 9

what is alveolar ventilation

Answer 9

the rate at which new air reavched gas exchange areas

Question 10

alveolar ventilation rate equation

Answer 10

Va = RR x (Vt-Vd)
Vt = tidal volume
Vd = dead space volume

Question 11

what is the alveolar air equation

Answer 11

pAO2 = (pB-pH2O) x FIO2 -(pACO2/R)

pB - barometric pressure (usually 150mmHg)
pH20 - water vapour pressure (usually 147mmHg)
FIO2 - fraction of O2 in inspired air (0.21)
R - respiratory quotient (0.8)

Question 12

what does the alveolar air equation describe

Answer 12

the ideal case of what pAO2 should be if there is perfect transport and no venous admixture => ideally pAO2 = paO2 but paO2 is affected by disease

Question 13

what is the Aa gradient

Answer 13

the difference between ideal pAO2 and paO2 (should be <15mmHg)

Question 14

what does an increased Aa gradient indicate

Answer 14

hypoxaemia

Question 15

how to estimate an Aa gradient of a pt

Answer 15

Age/4 +4

Question 16

what are 4 types (causes) high Aa gradient

Answer 16

intrapulmonary shunt (e.g. due to atelectesis, pneumonia, ARDS); intracardiac R ->L shunt (e.g. due to PFO, PDA); V/Q mismatch (e.g. due to acute PE, ILD, pneumonia); diffusion limitation (e.g. due to heavy exercise, severe ILD with exercise)

Question 17

where in the lungs do alveoli receive more tidal volume and why

Answer 17

the base of lungs as these alveoli are underinflated and so more compliant

Question 18

why are the alveoli smaller at the base

Answer 18

gravity causes increased pleural pressure at the base which causes reduced alveolar volume

Question 19

why are the alveoli smaller at the base

Answer 19

gravity causes increased pleural pressure at the base which causes reduced alveolar volume

Question 20

what is diffusion inversely proportional to

Answer 20

membrane thickness

Question 21

what is diffusion inversely proportional to

Answer 21

membrane thickness

Question 22

V/Q definition for lung

Answer 22

total alveolar ventilation/cardiac output

Question 23

if V or Q exceeds the other how does this affect V/Q ratio

Answer 23

if ventilation exceeds perfusion: V/Q >1
if perfusion exceeds ventilaition: V/Q <1

Question 24

what is a normal V/Q ratio (number)

Answer 24

around 0.8 (i.e. perfusion is slightly higher than ventilation)

Question 25

how does the graph of V/Q change as rib number decreases (towards apex)

Answer 25

exponentially (y=b^x) i.e. ventilation exceeds perfusion as it goes towards the apex

Question 26

paO2 values for hypoxemia vs hypoxia

Answer 26

arterial hypoxemia: paO2 <80mmHg (adult at sea level)
hypoxia: paO2 <60mmHg

Question 27

6 major causes of hypoxemia

Answer 27

1.anatomical shunt (perfusion that bypasses lung)
2. intrapulmonary shunt (absent ventilation to areas being perfused)
3. V/Q mismatching (low ventilation to areas being perfused)
4. hypoventilation (underventilation of lung units)
5. diffusion
6. reduced barometric pressure

Question 28

V/Q ratio at apex vs base

Answer 28

apex - 3.3
base - 0.8

Question 29

what is the most common cause of pathological shunts

Answer 29

atelectesis - usually due to obstrution by mucous plug, airway oedema, foreign body, tumour etc.

Question 30

how do most respiratory diseases cause hypoxemia

Answer 30

V/Q mismatch - individual airways will have varying degrees of abnormal ventilation but perfusion will be normally distributed -> alveolar and end capillary gases will vary according to degree of obstruction

Question 31

what is the effect of the mixing of high and low V/Q regions on ABGs

Answer 31

combine to give: low O2, high CO2, low pO2, slightly high pCO2 –> peripheral and central chemoreceptors detect this –> increased ventilation –> result in: low O2 content, normal/low CO2 content, low pO2, normal/low pCO2

Question 32

what mainly regulates pulmonary blood flow

Answer 32

pO2 and pCO2 - not autonomic regulation

Question 33

how is pulmonary blood flow regulated

Answer 33

arteries constrict so blood is diverted to better oxygenated areas (though to involve K+ ch inhibition on smooth muscle cells)

Question 34

examples of pulmonary vascular dilators

Answer 34

↓pACO2, ↑pAO2, ↑pH, H2 agonists, bradykinin, Ach, NO, b-adrenergic agonists, PCI2,PGE1

Question 35

examples of pulmonary vascular constrictors

Answer 35

↓pAO2, ↑pACO2, ↓pH, H1 agonists, PGE2, a-adrenergic agonists, angiotensin II, serotonin

Question 36

how does hypoventilation affect V/Q ratio and its consequence

Answer 36

V/Q ration ↓ -> underventilation brings less fresh gas to avleoli -> O2 levels in alveoli will decrease and CO2 levels increase