Ventilation - Perfusion mismatching, factors affecting blood gases

Question 1

What are the functions of the Pulmonary circulations?

Answer 1

• Primary function is: to facilitate the exchange of systemic blood gases with the environment.

NON-respiratory functions;

• Blood reservoir
• Filtration of emboli (fat globules, air, blood clots)
• Metabolism of vasoactive hormones.

Question 2

Name 3 differences between Pulmonary and Systemic circulation.

Answer 2

• Driving pressure and mean capillary pressure are much lower in pulmonary circulation.
• Much lower resistance to flow in pulmonary circulation. (because of the bullet point below)
• Pulmonary vessels are very compliant.
• As a result of points 2&3 there is an inverse relationship between pulmonary artery pressure and Pulmonary vascular resistance.
  Thus able to handle large increases in cardiac output without large increases in pulmonary artery pressure.
  (this is a passive mechanism)

Question 3

At what lung volume is Pulmonary vascular resistance lowest?

Answer 3

Around FRC

Question 4

When assessing V/Q what are the 2 measurements used?

And what are the normal figures for these?
V=?
Q=?
V/Q=?

Answer 4

Alveolar ventilation = V
Pulmonary blood flow = Q

V= 5250ml/min
Q= 5000ml/min

so V/Q= around 0.8

Question 5

Why is it important that V/Q must be matched at the alveolar capillary level?

Answer 5

This is because that’s where gas exchange occurs. Overall V/Q may be misleading.

See LC 15:00 if confused.

Question 6

What does dead space mean?

and what would the V/Q be?

Answer 6

This is where there is normal ventilation but no perfusion (no blood flow in the capillary over the alveoli) = wasted ventilation.

V/Q = ∞ (bc anything divided by 0 is infinity)

Question 7

What occurs in a shunt?

And what would the V/Q be?

Answer 7

No ventilation (e.g. a ligature over the top of the lung) but normal perfusion.

Mixed venous blood flows past and equilibrates but with O₂ levels too low and CO₂ levels slightly increases.

V/Q= 0

Question 8

What is a right to left shunt and give the 2 examples of normal right to left shunts.

Answer 8

A (R) to (L) shunt is where blood moves from the right side of the heart to the left without taking part in gas exchange.

Normally 98% of venous blood (which is on the right side) takes part in gas exchange.
The 2% that doesn’t are the examples of R to L shunt:

• 1% occurs from the bronchiole circulation as they empty directly to the pulmonary artery.
• The other 1% occurs in the walls of the left ventricle (Thebesian veins).

Question 9

What is an abnormal R to L shunt and give 1 E.g.

Answer 9

When circulation is fine but there’s no ventilation, e.g. Pneumonia an area of the lung filled with pus, and fluid etc. No air can get to the alveoli, so no gas exchange, there’s just wasted ventilation. = Right to left shunt. So deoxygenated blood from the right is added to the oxygenated blood on the left.

Can also occur in:

• Collapsed lung
• Consolidation (pneumonia)
• Congenital heart diseases. (e.g. Fallot’s tetralogy)

Question 10

What are the 2 cardiac examples of a R to L shunt?

Answer 10

These aka “Holes in the heart”

• Atrial septal defect (ASD)
• Ventricular septal defect (occur in 0.2% of live births) (VSD)

ASD’s and VSD’s initially cause a L to Right shunt due to higher pa on the left side of the heart.

However, for VSDs the consequent high pa in the pulmonary circulation can lead to pulmonary vascular remodelling and high resistance and so, R to L shunt.

Question 11

How do you calculate the effect of __% shunt on arterial O₂ and CO₂ contents?

use 20% as an example.
The shunted blood O₂ content= 150ml.1¯¹
and CO₂ content = 520ml.l¯¹

and unshunted blood, O₂ content = 200ml.l¯¹ and CO₂ content = 480ml.l¯¹

Answer 11

The effect of an 20% shunt on arterial O₂ and CO₂ contents can be calculated from a weighted average of contents in the shunted and unshunted blood.

SO; if in the shunted blood O₂ content= 150ml.1¯¹
and CO₂ content = 520ml.l¯¹

and unshunted blood, O₂ content = 200ml.l¯¹ and CO₂ content = 480ml.l¯¹ you do the following calculation.

O₂ content= (80÷100) x 200 + (20÷100) x 150 =
190ml/L

So due to the R to L shunt 10ml of O₂ content is lost.

Question 12

What’s the effect of hypoxaemia on the ventilatory drive?

And the effect of increased ventilation on a R to L shunt.

Does giving the patient 100% O₂ help?

Answer 12

Low arterial PO₂ and high arterial PCO₂ =

• Stimulate chemoreceptors
• increase ventilation
• Ventilated areas loose more CO₂ but gain little extra O₂ (Hb already saturated) Shunt blood unaffected as there’s no ventilation anyway.

final blood gases: Low PaO₂, normal or low PaCO₂

Giving the patient 100% O₂ wont increase arterial PO₂ much as it wont reach shunted blood, and the unshunted blood is already near 100% saturation.

(see LC at 38:00 if confused)

Question 13

True or False & fill in the blank:
In some diseases rather than having a region with no gas exchange, there is a variety of V/Q ratio’s.

E.g. some regions are under perfused in relation to ventilation so behave like they are ____ ___ ___.

Some regions are under-ventilated in relation to perfusion, qualitatively they behave like ___ __ ____ _____.

And there may be some regions where V/Q are matched

Answer 13

True

Alveolar dead space

Right to left shunt.

Question 14

What would you describe the following as and what might be causing these results? :
High Va
Low Q
High Va/Q

Answer 14

Pulmonary embolus or clot in the pulmonary circulation.

Dead space

Question 15

What would you describe the following as and what might be causing these results?:
Low Va
High Q
Low Va/Q

Answer 15

Asthma

Shunt effect

Question 16

If these were the reading in the blood after taking part in gas exchange when it triggers Peripheral & central chemoreceptors what would the result be?

O₂ content = Low
CO₂ content = High
PO₂ = Low
PCO₂ = High

Answer 16

The chemoreceptors would increase ventilation due to being hypoxic (low O₂) and hypercapnic so overall the final effect would be:

O₂ content = Low
CO₂ content = normal/low (decreases bc you breath out more CO₂ than you can produce)
PO₂ = Low
PCO₂ = normal/low
(Would be hypoxic)

Question 17

When there are regions which have a variety of V/Q’s some regions with high V/Q and other regions have low V/Q, the areas with high V/Q don’t compensate for the low V/Q areas in their effects on arterial blood gases, especially for O₂, why is this?

Answer 17

• More blood tends to come from the Low V/Q areas (as the high V/Q areas are mostly caused by low perfusion (= blood flow))
• Also high V/Q areas don’t have high O₂ content, alveolar PO₂ is high in these regions but once the blood is saturated raising local PO₂ further does little to increase O₂ content

(If confused see LC at 45:00)

Question 18

How would the patients below respond to being given O₂ enriched air?

Patient A: “Pure” R to L shunt. (i.e. V/Q = 0 e.g. due to consolidated/collapsed lung lobe)

Patient B: V/Q mis match

Answer 18

A: Any improvement to PO₂ would be small
(as O₂ wont be able to reach the collapsed/consolidated lobe)

B: Improvement often marked due to PO₂ in under-ventilated lung areas is improved.

Question 19

Why is there V/Q mis match when you’re sat/stood upright?

Answer 19

This is due to the effects of gravity:

The bottom of the lung is more squashed and the alveoli at the top of your lung will be held more open at rest.

So when you breath in there is a larger relative increase in size of alveoli during inspiration at the bottom of the lung as the ones at the top are already held quite opens so can’t be opened much more.

So ventilation is greater at the bottom than at the top of the lung, the same applies to perfusion.

But the effect gravity has on perfusion is larger than its effect on ventilation, this is what leads to a V/Q mismatch, so V/Q is higher at the top than at the bottom.

(If confused watch LC at 49:00)

Question 20

What are the effects of V/Q ratios on regional gas exchange?

Which areas of the lung would you expect to have the highest alveolar PO₂ and why?

Answer 20

The higher the V/Q ratio / closer it is to 1, the better the gas exchange so the higher the alveolar PO₂

Alveolar PO₂ is highest at the top of the lung than at the bottom bc there’s a higher V/Q ratio at the top than at the bottom. (e.g. V/Q top of lung MIGHT = 0.9 and at bottom of lung V/Q MIGHT = 0.6
(JUST AN EXAMPLE NOT ACCURATE))

Question 21

Reminder

Answer 21

go to LC at 50:44 try understand the red text in the box and what that actually means

Question 22

What is the pulmonary blood vessels response to hypoxia?

And why is it beneficial?

Answer 22

The pulmonary blood vessels vasoconstrict in response to hypoxia this is called “Hypoxic pulmonary vasoconstriction” = HPV.

This is beneficial as it diverts blood flow from poorly ventilate areas to well ventilated areas, so improves V/Q matching and arterial oxygenation.

Question 23

When is HPV not helpful?

Answer 23

Not helpful in the presence of global hypoxia (e.g. hypoxic lung disease or in response to high altitude) (when all the alveoli are hypoxic) so doesn’t divert blood as all pulmonary blood vessels vasoconstrict which increases pressure on the right side of the heart and causes pulmonary hypertension.

If this occurs in a hypoxic patient leads to right ventricular failure secondary to a hypoxic lung disease.

Question 24

How can you treat pulmonary hypertension?

Answer 24

Drugs = vasodilators.

Best when inhaled as its delivered to areas of the lung with good ventilation where as if its delivered via blood then it’ll vasodilate all areas including where there was poor ventilation which may make things worse rather than better.

Question 25

How can you asses V/Q mismatch

Answer 25

• Isotope Ventilation scan
• Isotope Perfusion scans
• Could use modified Bohr and shunt equations
• Use “A-a” PO₂ gradient.

Question 26

What are the 5 causes of arterial hypoxia

(low PaO₂)?

Answer 26

1. Low inspired PO₂ (e.g. hypobaric hypoxia)
2. Hypoventilation
3. Diffusion impairment (may be due to fibrosis or heart failure)
4. R-L shunt (either pulmonary e.g. pneumonia with consolidation) or cardiac)
5. Va/Q Mismatch either high or low.

(3,4,5 will all increase the A-a PO₂ gradient)

Question 27

What are the 4 causes of Tissue Hypoxia (insufficient O₂ at a cellular level)?

Answer 27

1. Hypoxic hypoxia
2. Anaemic hypoxia
3. Circulatory hypoxia
4. Histotoxic hypoxia

Question 28

What is Hypoxic hypoxia?

Answer 28

Low PaO₂ accompanied by inadequate Hb saturation (slide 30 on LC)

Question 29

What is Anaemic hypoxia?

Answer 29

Reduced O₂ carrying capacity

Question 30

What is Circulatory hypoxia?

Answer 30

Supply of blood inadequate for demands e.g. heart failure, peripheral vascular disease, emboli, capillary microthrombosis

Question 31

What is Histotoxic hypoxia?

Answer 31

Normal O₂ delivery but cells unable to se O₂ available to them e.g. Cyanide poisoning.