Ventilation perfusion relationships

Question 1

What is the formula for calculating minute ventilation?

Answer 1

Minute ventilation = Tidal volume × Respiratory rate.

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

Question 2

How is alveolar ventilation calculated?

Answer 2

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

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

Question 3

What is the approximate pulmonary blood flow at rest?

Answer 3

Pulmonary blood flow is approximately 5000 mL/min.

Question 4

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

Answer 4

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

Question 5

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

Answer 5

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

Question 6

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

Answer 6

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

Question 7

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

Answer 7

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

Question 8

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

Answer 8

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

Question 9

What happens to alveolar PO₂ and PCO₂ in a shunt?

Answer 9

PAO2 =40mmHg
PACO2 =46mmHg

Question 10

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

Answer 10

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

Question 11

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

Answer 11

PAO2 =100mmHg
PACO2 =40mmHg

Question 12

What happens to alveolar PO₂ and PCO₂ in dead space?

Answer 12

PAO2 =150mmHg
PACO2 =0mmHg

Question 13

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

Answer 13

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

Question 14

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

Answer 14

Over 98% of venous blood.

Question 15

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

Answer 15

Bronchial veins (draining the lungs).

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

Question 16

What is an abnormal right-to-left shunt?

Answer 16

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

Question 17

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

Answer 17

Question 18

Describe abnormal right to left shunts.

Answer 18

• 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.

Question 19

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

Answer 19

Question 20

What is an Atrial Septal Defect (ASD)?

Answer 20

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.

Question 21

What is a Ventricular Septal Defect (VSD)

Answer 21

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.

Question 22

How do ASDs and VSDs affect arterial oxygen content initially?

Answer 22

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.

Question 23

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

Answer 23

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

Question 24

What are the four components of Fallot's tetralogy?

Answer 24

Question 25

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

Answer 25

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

Question 26

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

Answer 26

Question 27

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

Answer 27

Question 28

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

Answer 28

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

Question 29

How does a moderate rise in CO2 content affect PCO2?

Answer 29

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

Question 30

How does a moderate fall in O2 content affect PO2?

Answer 30

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

Question 31

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

Answer 31

Question 32

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

Answer 32

Low 𝑃𝑂2, normal or low 𝑃𝐶𝑂2

Question 33

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

Answer 33

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

Question 34

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

Answer 34

It depends on the ventilatory response.

Question 35

What happens if alveolar hypoventilation occurs?

Answer 35

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

Question 36

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

Answer 36

Question 37

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

Answer 37

It is an ominous clinical finding.

Question 38

How does chronic hypoventilation affect pH?

Answer 38

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

Question 39

What is ventilation-perfusion mismatching?

Answer 39

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

Question 40

How do under-perfused regions behave?

Answer 40

They behave qualitatively like alveolar dead space.

Question 41

How do under-ventilated regions behave?

Answer 41

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

Question 42

What happens in areas with high 𝑉𝐴/Q?

Answer 42

Question 43

What happens in areas with low 𝑉𝐴/Q?

Answer 43

Question 44

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

Answer 44

Look at black box

Question 45

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

Answer 45

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

Question 46

Answer 46

Question 47

Answer 47

Question 48

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

Answer 48

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

Question 49

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

Answer 49

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

Question 50

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

Answer 50

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

Question 51

How does gravity affect lung perfusion and ventilation?

Answer 51

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

Question 52

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

Answer 52

Question 53

Answer 53

Question 54

Answer 54

Question 55

Answer 55

Question 56

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

Answer 56

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

Question 57

What is hypoxic pulmonary vasoconstriction (HPV)?

Answer 57

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.

Question 58

How does HPV improve oxygenation?

Answer 58

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

Question 59

Why is HPV not beneficial during global hypoxia?

Answer 59

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

Question 60

which is more detrimental, systemic or topical vasodilators?

Answer 60

systemic

Question 61

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

Answer 61

Question 62

Answer 62

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

Question 63

Answer 63

Question 64

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

Answer 64

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

Question 65

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

Answer 65

Occlusion in the right pulmonary artery (D).

Question 66

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

Answer 66

- 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).

Question 67

Where must ventilation and perfusion be matched in the lungs?

Answer 67

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

Question 68

What are the two extremes of 𝑉𝐴 /Q mismatching?

Answer 68

Question 69

Answer 69

Question 70

Answer 70

Question 71

Answer 71

Question 72

What is the inevitable result of hypoventilation?

Answer 72