Lecture 5- V/ Q mismatch

Question 1

ventilation and perfusion of the lungs as a whole in a healthy person

Answer 1

• Ventilation
  
  • tidal volume =500ml
  • anatomical dead space (trachea , bronchi etc)= 150ml
  • respiratory rate= 14/min
  • alveolar ventilation (V)= (500-150) x 14/min= 4900ml/min
• Pulmonary blood flow (Q)
  
  • =4900ml/min
• therefore Ventilation/perfusion ratio= 4900/4900= 1

Question 2

will a well ventilated but poorl perfused lung have a V/Q ratio of above or below 1

Answer 2

give the equation your own numbers

2= done fully

1= not done fully

therefore

V= 2 (well ventilated)

Q= 1 (poorly perfused)

2/1= 2 therefore >1

therefore if its above 1= well ventilated, poorly perfused

Question 3

will a well perfused but poorly perfused lung be above or below 1

Answer 3

give the equation your own numbers

2= done fully

1= not done fully

therefore

V= 1 (poorly ventilated)

Q= 2 (well perfused)

1/2= 0.5 therefore <1

therefore if its below 1= well perfused, poorly ventilated

Question 4

blood perfusing an area of lung that has no ventilation will

Answer 4

remain un-oxygenated- no gas exchange possibel –> SHUNT

V/Q= 0

Question 5

Aeea of lung with normal venitlation but no perfusion- gas entering and leaving alveoli without gas exchnage- no o2 extraction, no CO2 added–> no gas exhange beecause there is no blood flow to the area–> this becomes….

Answer 5

dead space

Question 6

when venitlation exceed perfusion

Answer 6

V/Q >1 = wasted air

Question 7

when perfusion exceeds ventilation

Answer 7

V/Q <1 - low V/Q

• this is what is typically meant by V/Q mismatch
• arterial blood will have low Oxygen

Question 8

Ventilation and perfusion has to be

Answer 8

be matched throughout the lungs

Question 9

• Gas exchange optimal when: V/Q ratio of individual alveolar units=

Answer 9

1

Question 10

• 300 million alveoli – may have differing amounts of

Answer 10

ventilation and perfusion

Question 11

• Ideally:
  
  • Alveoli with increased ventilation should have ……….. perfusion
  • Alveoli with …………. ventilation should have decreased perfusion

Answer 11

increased

decreased

Question 12

when alveolar pAO2 is low due to impaired ventilation…..

Answer 12

• hypoxic vasoconstriction of pulmonary arterioles occurs
  
  • This diverts blood to better ventilated alveoli
• However, this process is not complete, so in disease states, poorly ventilated alveoli still have significant perfusion

Question 13

in normal lungs v/Q is around

Answer 13

• 0.8 or 0.9

Question 14

V/Q above 1 at the …………….and below 1 the ……………….

Answer 14

top of the lungs

further down the lungs

Ventilation increases more slowly than blood flow increases

Question 15

Why do we have V/Q mismatch in normal lungs?

Answer 15

• As a reserve- V:Q match approaches 1 when we exercise
• V<q>
  </q><li>V&gt;Q= &gt;1 (high V/Q)</li></q>

Question 16

V/Q is the most common cause of

Answer 16

• of systemic arterial hypoxaemia in people with cardio-pulmonary disease- hypoxaemia= low pp oxygen in arterial blood
• Increases with ages

Question 17

specific diseases associated with systemic arterial hypoxaemia caused by V/Q mismatch

Answer 17

• Asthma- airway narrowing but not uniformly dispersed in lungs
• Early stage COPD- airway narrowing but not uniformly dispersed in lung
• Pneumonia- acute inflammatory exudate in affected alveoli
• Respiratory distress syndrome in Newborn- some alveoli open, others not
• Pulmonary oedema- fluid in alveoli
• Pulmonary embolism

Question 18

example diagram of healhy lung V/Q =1

Answer 18

Question 19

diagram showing consequence of low V/Q: V/Q <1

Answer 19

e. g. due to poor ventilation e.g. asthma
- perfusion all good
- arterial blood O2 will begin to fall
- stimulating hyperventilation
- However nearby alveoli can slightly compensate for poorly
- over-ventilated alveolus cannot fully compensate for e ventilated alveolus oxyge
- therefore pulmonary vein will have lower than normla PpVO2
- If the alveoli didn’t compensate the ppvo2 and Ppvco2 of the pulmonary vein would be lower pO2= 6 and PCO2=10, instead it is pO2= 11 and PCO2=5.4

Question 20

Why cant over- ventilated alveoli fully compensate for under-ventilated alveoli?

Answer 20

• 98% of our oxygen carried on Hb
• Only 2% oxygen dissolved
  
  • PP reflects dissolved oxygen
• Going form a partial pressure of 13kPa to 15 kPa only increases dissolved oxygen by 0.02 mmol/L
• Hb is already carrying all that is can carry- saturated
• So very little additional oxygen for loading onto haemoglobin from poorly perfused alveoli

Question 21

graph showing relationship between pCO2 and total CO2 contents in blood

Answer 21

• Relatively straight line in the physiological range of CO2 i.e. there is a linear relationship between pCO2 and blood total CO2
• This because CO2 is very soluble in blood
• The dissociation curve is also quite steep- meaning that it is easy for CO2 to drop with even small decreases in PP
• These two factors explain why CO2 in blood is directly proportional to alveolar minute ventilation

Question 22

which two factors explain why CO2 in blood is directly proportional to alveolar minute ventilation

Answer 22

1. This because CO2 is very soluble in blood
2. The dissociation curve is also quite steep- meaning that it is easy for CO2 to drop with even small decreases in PP

Question 23

Consequence of NO ventilation but still perfused- SHUNT

Answer 23

• Blockage e.g. tumour or mucus plug
• Blood still perfusing the alveolar- however due to blockage low pp in alveolar therefore pp of pulmonary artery will not change once passing this alveoli
• Pt now hyperventilating to increase oxygen level (which you can see in the alveoli compensating for the shunted alveoli)
  
  • However compensation not enough – still too low in pulmonary vein (can never be compensated fully in a SHUNT situation)

Question 24

example of a V/Q mismatch higher than 1

Answer 24

Pulmonary embolism

Question 25

**Pulmonary embolism and V/Q mismatch**

Answer 25

* In PE there are alveoli that receive no blood, though they are still ventilated * Dead space (like trachea and bronchi that cant exchange gas) * V/Q= 1/0 (absolutley no perfusion) = infinity * But the way V/Q mismatch occurs is because the diverted blood perfuses other alveoli * If ventilation to those other alveoli cant increase to commensurate with the increased blood flow then we have caused a V/Q mistatch \<1 in the perfused other alveoli