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RESP > Ventilation/Perfusion relationships > Flashcards

Ventilation/Perfusion relationships Flashcards

1
Q

How is partial pressure for dry gas in air calculated?

A
  • (PB- 47) x %gas
  • example for O2: (760-47) x 0.21 = 150mmHg
  • usually O2 is 160 mmHg

water exerts a saturated water vapour pressure of 47

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2
Q

How is arterial blood gas pressure measured?

A
  • arterial blood sample + blood gas analyser
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3
Q

How is arterial PCO2 measured?

A
  • measuring end-tidal volumes
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4
Q

What is Capnography?

A
  • the continous recording of CO2 concentration in respiratory gas
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5
Q

How thick is the alveolocapillary membrane?

A
  • 0.5µm

alveolar lining fluid, alveolar epithelium and capillary endothelium

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6
Q

What advantage does the thin alveolar-capillary membrane give?

A
  • rapid and complete equilibration of O2 and CO2
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7
Q

What is Fick’s law of diffusion?

A
  • gas exchange by simple diffusion
  • proportional to pressure difference and surface area
  • inversely proportional to distance
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8
Q

What happens to gas exchange in emphysema and lung fibrosis?

A
  • REDUCED
  • Reduced SA in emphysema
  • Increased diffusion distance in lung fibrosis
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9
Q

What is diffusing capacity/transfer factor?

A
  • extent to which a gas passes from air sacs of the lung into the blood
  • DL = rate of transfer of gas from lung to blood/partial pressure difference

mmol/min/kPa

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10
Q

How is Diffusion capacity measured?

A
  • small, non-lethal amounts of Carbon monoxide
  • pCO in blood is zero, so partial pressure difference is alveolar pCO
  • units = ml/min/kPa
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11
Q

What factors are dependent on DL?

A
  • haemoglobin
  • age
  • sex
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12
Q

What is a normal DLCO?

A
  • > 75%
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13
Q

Why is arterial PO2 slightly less than alveolar PO2?

A

Venous admixture:
- Anatomical shunt (bronchial and thesbian veins)
- V/Q mismatch

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14
Q

What is anatomical shunt?

A
  • Blood bypasses the alveoli and enters systemic circulation without being oxygenated
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15
Q

Why is a greater than normal Arterial to alveolar PO2 gradient bad?

A
  • suggests problems with gas exchange
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16
Q

Why happens if there is a rise in CO2 in the blood?

A
  • can displace oxygen molecules
  • hypoxemia
17
Q

What is the RER?

A
  • Respiratory exchange ratio
  • CO2 production/O2 consumption
18
Q

What is V/Q ratio?

A
  • Ventilation/Perfusion matching
  • normal is 0.8 for average of entire lung (ventilation is 80% of perfusion)
19
Q

Is V/Q ratio varied in different parts of the lungs?

A
  • Yes
  • Near the apex = high VQ
  • Near the base = low VQ
20
Q

Why is V/Q higher at the apex?

A
  • low ventilation
  • even lower perfusion
  • more air goes to base during inspiration
  • TB more likely at apex since higher PO2
21
Q

What does an increased V/Q mean?

A
  • alveolar dead space increased
  • wasted ventilation
22
Q

What does a decreased VQ mean?

A
  • shunting: deoxygenated venous blood bypasses alveoli, enters arteries
  • hypoxaemia
23
Q

What diseases could cause an increased VQ?

A
  • Embolus, emphysema
  • blockage in vessel
24
Q

What diseases could cause a decreased VQ?

A
  • COPD
  • asthma

obstructive airway diseases

25
Q

What is a “true shunt”?

A
  • Blood flows through a region with zero ventilation
  • abnormal right-left shunts in heart, atelectasis, consolidation
26
Q

How does an increased VQ affect PO2 and PCO2?

A
  • increased alveolar PO2
  • decreased alveolar PCO2
  • lung try to constrict airway + dilate vessel to combat
27
Q

How does a decreased VQ affect PO2 and PCO2

A
  • decreased alveolar PO2
  • increased alveolar PCO2
  • lung try to relax airway and constrict vessel
  • HYPOXIC PULMONARY VASOCONSTRICTION
28
Q

Why is Hypoxic vasoconstriction good in the lungs?

A
  • Diverts blood flow away from unventilated areas, where blood flow will be wasted since gas exchange cannot occur, to ventilated areas

RESP (20 decks)

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