Ventilation perfusion ratio

Question 1

What machine determines oxygen saturation?

Answer 1

pulse oximeter

Question 2

What is the calculation to work out how much oxygen is carried by haemoglobin in blood?

Answer 2

1.34ml xHb (g/L) x haemoglobin saturation of O2

1.34ml = amount of oxygen haemoglobin would carry if it was 100% saturated

Question 3

What % of oxygen carriage is dissolved in the blood? How is this measured?

Answer 3

1.5%
measured in a blood gas - PaO2

Question 4

Define hypoxemia

Answer 4

reduction in PaO2

Question 5

Causes of hypoxemia

Answer 5

anatomical shunt (perfusion that bypasses lung)
intrapulmonary shunt (absent ventilation to areas being perfused eg. atelectasis)
VQ mismatching (low ventilation to areas being perfused)
hypoventilation (underventilation of lung units eg. resp muscle weakness, causes hypercapnia and hypoxemia)
diffusion (fibrosis)
reduced barometric pressure

Question 6

Define hypoxia

Answer 6

local or systemic failure of oxygen delivery

Question 7

What can cause hypoxia?

Answer 7

poor ventilation
poor circulation
histiotoxic (poisoned mitochondria so oxygen can’t be utilised)
severe anaemia or dyshaemoglobinaemias (not enough Hb/Hb that don’t bind well to oxygen)

Question 8

Define alveolar ventilation

Answer 8

rate at which new air reaches gas exchange areas

Question 9

Define dead space air

Answer 9

some air that is breathed never reaches gas exchange areas but fills respiratory passages (eg. nose, pharynx, trachea)
about 150ml

Question 10

Alveolar ventilation rate equation

Answer 10

VA = Frequency x (VT-VD)

VA = volume of alveolar ventilation/min

Frequency = resp rate

VT = tidal volume

VD = dead space volume

Question 11

Alveolar air equation

Answer 11

relationship between CO2 and O2 in the alveoli

PAO2 = (PB-PH2O) x FIO2 - (PACO2/R)

PB = barometric pressure
PH2o = water vapour pressure
FIO2 = fraction of O2 in inspired air (0.21)
R = respiratory quotient (0.8)

if perfect transport and no venous admixture, PAO2 = PaO2

Question 12

What is Aa gradient?

Answer 12

difference between ideal PAO2 and PaO2
(normally less than 15mmHg)

Question 13

What equation estimates Aa gradient in mmHg?

Answer 13

(Age/4)+4

Question 14

Why is A-a gradient useful?

Answer 14

can help see what pathological process is driving hypoxemia

Question 15

Describe expansion of alveoli throughout the lung

Answer 15

alveoli in top of lung are more expanded than those at bottom (due to gravity)

Question 16

Describe pleural pressure throughout the lung

Answer 16

pleural pressure is less (more negative) at apex than base of lung
with inspiration, pleural pressure decreases further
as inspiration begins, alveoli in lungs are at different lung volumes

Question 17

Describe pulmonary circulation

Answer 17

begins with right atrium
deoxygenated blood pumped via right ventricle into pulmonary artery
pulmonary artery divides into right and left main artery then enters lung tissue
ends in mesh-like network of capillaries where RBC flow single file through alveolus
capillaries drain into pulmonary venules
finally 2 large pulmonary veins emerge from each lung to empty into left atrium (4 pulmonary veins total)

Question 18

Why do alveoli not fill with fluid?

Answer 18

normally pulmonary capillaries and lymphatics maintain a slight negative pressure in interstitial spaces
excess fluid will be sucked back into interstitial space from alveoli

Question 19

What causes diffusion problems in the lung?

Answer 19

fibrosis

Question 20

What happens to the VQ ratio as you go from the bottom of the lung to the top?

Answer 20

increases

Question 21

When are ventilation and perfusion matched?

Answer 21

when pulmonary blood flow is proportionally matched to the pulmonary ventilation
results in greatest efficiency for gas exchange

Question 22

How to calculate VQ ratio for alveolus and a lung

Answer 22

single alveolus = alveolar ventilation/capillary blood flow

lung = total alveolar ventilation/cardiac output

Question 23

Describe interpretation of VQ ratio

Answer 23

VQ ratio >1 = ventilation exceeds perfusion

VQ ratio <1 = perfusion exceeds ventilation

Question 24

What is a normal VQ ratio?

Answer 24

0.8 (4L/min / 5L/min)

Question 25

What happens if areas of low PO2 or high PCO2 detected?

Answer 25

arteries constrict so blood is diverted to better oxygenated areas
mechanism thought to involve inhibition of K channels on smooth muscle cells

Question 26

Agents that affect pulmonary vascular resistance - dilators

Answer 26

increased PAO2
decreased PACO2
high pH
H2 agonists
PGI2 (prostacyclin)
PGE1
beta-adrenergic agonists
bradykinin
theophylline
acetylcholine
nitric oxide

Question 27

Agents that affect pulmonary vascular resistance - constrictors

Answer 27

decreased PAO2
increased PACO2
low pH
H1 agonists
thromboxane A2
PGF2alpha
PGE2
alpha-adrenergic agonists
serotonin
angiotensin 2

Question 28

Describe VQ parameters of a normal lung

Answer 28

VA = normal
Q = normal
VQ = ~1

Question 29

Describe blood gases in outflow of a normal lung

Answer 29

PO2 = normal
PCO2 = normal
O2 content = normal
CO2 content = normal

Question 30

Describe VQ parameters of dead space

Answer 30

VA = normal
Q = 0 (no perfusion)
VQ = infinity

Question 31

Describe VQ parameters of dead space effect

Answer 31

VA = normal
Q = low
VQ = high

Question 32

Describe blood gases in outflow of dead space effect

Answer 32

PO2 = high
PCO2 = low
O2 content = normal
CO2 content = low

Question 33

Describe VQ parameters of shunt effect

Answer 33

VA = low
Q = normal
VQ = Low

Question 34

Describe the blood gases in outflow of shunt effect

Answer 34

PO2 = low
PCO2 = high
O2 content = low
CO2 content = high

Question 35

Describe the VQ parameters of true/anatomical shunt

Answer 35

VA = 0
Q = normal
VQ = 0

Question 36

Describe the blood gases in outflow of true/anatomical shunt

Answer 36

PO2 = low
PCO2 = high
O2 content = low
CO2 content = high