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PCRS > Ventilation perfusion relationships > Flashcards

Ventilation perfusion relationships Flashcards

1
Q

What is the formula for calculating minute ventilation?

A

Minute ventilation = Tidal volume × Respiratory rate.

Example: 500 mL × 15/min = 7500 mL/min.

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2
Q

How is alveolar ventilation calculated?

A

Alveolar ventilation = (Tidal volume - Anatomical dead space) × Respiratory rate.

Example: (500 mL - 150 mL) × 15 = 5250 mL/min.

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3
Q
A
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3
Q

What is the approximate pulmonary blood flow at rest?

A

Pulmonary blood flow is approximately 5000 mL/min.

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4
Q

What is the ventilation-perfusion (V/Q) ratio for the whole lung?

A

The V/Q ratio is approximately 1, indicating balanced ventilation and perfusion.

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5
Q

What is the normal ventilation-perfusion (V/Q) ratio in the lungs?

A

The normal V/Q ratio is approximately 0.8, indicating balanced ventilation and perfusion.

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6
Q

What happens when ventilation (𝑉𝐴 ) is zero but perfusion(𝑄) is normal?

A

There is no gas exchange, resulting in a V/Q ratio of 0 (e.g., shunt)

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7
Q

What happens when perfusion (𝑄) is zero but ventilation (𝑉𝐴) is normal?

A

There is no gas exchange, and the V/Q ratio becomes infinity (e.g., dead space).

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8
Q
A
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8
Q

Why can the overall V/Q ratio of 0.8 be misleading?

A

Ventilation and perfusion must be matched at the 
alveolar capillary level, because that is where gas 
exchange occurs. Overall VA/Q may be misleading

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9
Q

What happens to alveolar PO₂ and PCO₂ in a shunt?

A

PAO2 =40mmHg
PACO2 =46mmHg

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10
Q

What is the V/Q ratio in a shunt, and why?

A

The V/Q ratio is 0 because there is no ventilation despite normal perfusion.

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11
Q

What are the normal alveolar gas values during effective gas exchange?

A

PAO2 =100mmHg
PACO2 =40mmHg

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12
Q

What happens to alveolar PO₂ and PCO₂ in dead space?

A

PAO2 =150mmHg
PACO2 =0mmHg

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13
Q

What is the V/Q ratio in dead space, and why?

A

The V/Q ratio is infinity because there is ventilation but no perfusion.

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14
Q

What percentage of venous blood normally passes through the lungs for gas exchange?

A

Over 98% of venous blood.

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15
Q

What veins contribute to the normal right-to-left shunt?

A

Bronchial veins (draining the lungs).

Thebesian veins (small veins draining the walls of the left ventricle).

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16
Q

What is an abnormal right-to-left shunt?

A

It is when blood bypasses the lungs without gas exchange, often due to specific pathological conditions.

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17
Q

What 3 conditions can cause an abnormal right-to-left shunt?

A
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18
Q

Describe abnormal right to left shunts.

A
  • No ventilation in part of a lung but perfusion is still occurring.
  • That would be a right to left shunt.
  • The blood would be O2 poor and CO2 rich and just gets added to the oxygenated blood.
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18
Q
A
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19
Q

What are the blood gas values for the patient with a consolidated lung?

A
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20
Q

What is an Atrial Septal Defect (ASD)?

A

A defect in the atrial septum allowing blood to flow from the left atrium (LA) to the right atrium (RA), typically causing a left-to-right shunt.

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21
Q

What is a Ventricular Septal Defect (VSD)

A

A defect in the ventricular septum allowing blood to flow from the left ventricle (LV) to the right ventricle (RV), also causing a left-to-right shunt initially.

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22
Q

How do ASDs and VSDs affect arterial oxygen content initially?

A

They usually do not lower arterial oxygen content or 𝑃𝑂2 because they are left-to-right shunts, meaning oxygenated blood mixes with deoxygenated blood in the right heart and lungs.

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23
Q

What can happen with prolonged left-to-right shunting in VSDs?

A

High pressure in the pulmonary circulation can cause pulmonary vascular remodeling, leading to increased resistance and a reversal to a right-to-left shunt.

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24
Q

What are the four components of Fallot’s tetralogy?

A
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25
Q

What is the effect of a 20% shunt on arterial oxygen and carbon dioxide contents?

A

The arterial oxygen and carbon dioxide contents are calculated as weighted averages of shunted and unshunted blood.

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26
Q

How is the oxygen content in the blood affected by a 20% shunt?

A
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27
Q

How is the carbon dioxide content in the blood affected by a 20% shunt?

A
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28
Q

Can pressures (PO2 and PCO2) be directly averaged in a shunt calculation?

A

No, pressures cannot be averaged; contents must be calculated using dissociation curves.

29
Q

How does a moderate rise in CO2 content affect PCO2?

A

A moderate rise in CO2 content causes a very small rise in PCO2
​

30
Q

How does a moderate fall in O2 content affect PO2?

A

A moderate fall in O2 content causes a large fall in PO2

31
Q

What is the effect of low arterial 𝑃𝑂2 and high arterial 𝑃𝐶𝑂2 in a right-to-left shunt?

A
32
Q

What are the final blood gas levels in a right-to-left shunt with increased ventilation?

A

Low 𝑃𝑂2, normal or low 𝑃𝐶𝑂2

33
Q

Why does breathing 100% oxygen have only a modest effect on arterial 𝑃𝑂2 in a right-to-left shunt?

A

The oxygen does not reach shunted blood, and blood in ventilated regions is already near full saturation.

34
Q

What does the low 𝑃𝐶𝑂2 in ventilation-perfusion mismatching depend on?

A

It depends on the ventilatory response.

35
Q

What happens if alveolar hypoventilation occurs?

A

The ventilatory response becomes poorer, and 𝑃𝐶𝑂2 rises, such as in exhaustion due to acute severe asthma.

36
Q

What are the blood gas levels in acute respiratory acidosis due to severe asthma?

A
37
Q
A
37
Q

What are the blood gas levels in acute respiratory acidosis due to severe asthma?

A

It is an ominous clinical finding.

38
Q

How does chronic hypoventilation affect pH?

A

Renal compensation normalizes pH, with 𝐻𝐶𝑂3− levels high and 𝑃𝐶𝑂2 remaining high

39
Q

What is ventilation-perfusion mismatching?

A

It refers to the presence of regions with varying ventilation-perfusion ratios (𝑉𝐴/ /Q) in diseases, rather than regions with no gas exchange.

40
Q

How do under-perfused regions behave?

A

They behave qualitatively like alveolar dead space.

41
Q

How do under-ventilated regions behave?

A

They behave qualitatively like a right-to-left shunt.

42
Q

What happens in areas with high 𝑉𝐴/Q?

A
43
Q

What happens in areas with low 𝑉𝐴/Q?

A
44
Q

Can the effects of high 𝑉𝐴/Q areas balance the effects of low 𝑉𝐴/Q areas?

A

Look at black box

45
Q

What is the overall effect on ventilation due to 𝑉𝐴/Q mismatch?

A

Peripheral and central chemoreceptors are stimulated, leading to increased ventilation.

46
Q
A
47
Q
A
48
Q

What is the effect of oxygen-enriched inspired air on pure right-to-left shunts?

A

Any improvement in arterial 𝑃O2 is small because 𝑉𝐴/Q=0 (e.g., completely collapsed lung lobe).

49
Q

How does ventilation-perfusion mismatching respond to oxygen-enriched inspired air?

A

Improvement is often marked because 𝑃𝑂2 in under-ventilated lung areas is improved.

50
Q

Where is the 𝑉𝐴 /Q ratio higher in the lungs?

A

𝑉𝐴 /Q is higher at the top of the lungs compared to the bottom.

51
Q

How does gravity affect lung perfusion and ventilation?

A

Gravity increases both perfusion and ventilation, with a greater effect on perfusion, especially at the bottom of the lungs.

52
Q

Why is V(A)/Q higher at the top of the lung rather than the bottom?

A
53
Q
A
54
Q
A
55
Q
A
56
Q

Why are regional V(A)/Q differences important?

A

Regional VA/Q differences localise some diseases to top or bottom of the lung e.g. Tuberculosis at apex (high P(A)O2)

57
Q

What is hypoxic pulmonary vasoconstriction (HPV)?

A

It is the constriction of pulmonary blood vessels in response to hypoxia, diverting blood flow from poorly ventilated areas to well-ventilated areas to improve 𝑉𝐴/Q matching.

58
Q

How does HPV improve oxygenation?

A

By redirecting blood flow to well-ventilated areas, enhancing 𝑉𝐴/Q matching and arterial oxygenation.

59
Q

Why is HPV not beneficial during global hypoxia?

A

Global hypoxia, such as in respiratory failure or high altitude, increases pulmonary vascular resistance, causing pulmonary hypertension and strain on the right heart.

60
Q

which is more detrimental, systemic or topical vasodilators?

A

systemic

61
Q

What are the three methods used to assess ventilation-perfusion mismatching?

A
62
Q
A

Around 5–10 mmHg (0.6–1.2 kPa), with a normal upper limit of ~15 mmHg (2 kPa).

63
Q
A
64
Q

What does a ventilation-perfusion scan reveal in pulmonary embolism?

A

Ventilation remains normal (E), but there is no perfusion to the affected area (F), indicating an occlusion in the pulmonary artery.

65
Q

What finding on an X-ray suggests a pulmonary embolism?

A

Occlusion in the right pulmonary artery (D).

66
Q

What are the mechanisms leading to arterial hypoxia (low PaO2)?

A
  • Low inspired PO2 - the problem is not in the lung but with the air or the alevolar gas has a reduced PO2, will make you hypoxaemic.
  • Hypoventilation - air is not moving in or out of the lung, resulting in high alveolar PCO2 and low PO2
  • Diffusion impairment - problems with surface area or thickness of the gas exchange interface.
  • Right to left shunt - blood isn’t in the right place so you get shunting of the blood without it taking part in gas exchange.
  • Ventilation-perfusion mismatch - mix of high V(A)/Q areas (dead space) and low V(A)/Q right to left shunts.
  • Only hypoventilation results in high PaCO2 (in hypoxia).
  • Diffusion impairment, right to left shunt and ventilation-perfusion mismatch increases the A-a PO2 gradient (alveolar and arterial).
67
Q

Where must ventilation and perfusion be matched in the lungs?

A

At the alveolar/capillary level, not the whole lung.

68
Q

What are the two extremes of 𝑉𝐴 /Q mismatching?

A
69
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70
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71
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72
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A

PCRS (11 decks)

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