Resp13 - Ventilation-Pefusion Relationships

Question 1

What is alveolar ventilation?

What is pulmonary perfusion?

What is the ventilation:perfusion (V:Q) ratio

Answer 1

1. ) Alveolar Ventilation - amount of air reaching the alveoli in a given amount of time
   - V = RR x (TV - anatomical dead space)
   - typically expressed as V ml/min
2. ) Pulmonary Perfusion - flow of blood through the pulmonary capillaries surrounding the alveoli
   - typically expressed as Q ml/min
3. ) Ventilation:Perfusion Ratio - ratio between alveolar ventilation and pulmonary perfusion
   - in normal lungs, the V:Q roughly 1 (0.8-1.2) as this is when gas exchange is optimal

Question 2

3 features of maintaining a V:Q of 1

Answer 2

1. ) Low arterial pO2 in Pulmonary Capillary
   - hypoxic vasoconstriction of pulmonary arterioles occurs to divert blood to better ventilated alveoli
2. ) Low pO2 in Alveoli
   - bronchoconstriction occurs to divert air to better perfused lung
3. ) V:Q Mismatch - poorly ventilated alveoli still have significant perfusion (V:Q < 1)
   - rise in alveolar pCO2 due to less blown off
   - fall in alveolar pO2 due to more entering the blood
   - blood equilibruates to new alveolar pCO2 and pO2

Question 3

6 causes of V:Q mismatch (V:Q < 1)

Answer 3

1. ) Asthma - airway narrowing
   - not uniform throughout lungs

2.) Early COPD - not uniform throughout lungs

3. ) Pneumonia - acute inflammatory exudate in alveoli
   - not uniform thoughout lungs
4. ) RDS in Newborn - some alveoli not expanded
   - not uniform thoughout lungs
5. ) Pulmonary Oedema - fluid in alveoli
   - not uniform thoughout lungs

6.) Pulmonary Embolism - leads to redistribution of pulmonary blood fow

Question 4

7 features of mixing of blood from normal segment and affected segment (V:Q mismatch) of the lung

Initial Gas Concentrations
New pCO2/CO2 content
New pO2/O2 content
Hyperventilation
Effect of Hyperventilation on O2 content
Effect of Hyperventilation on CO2 content
Final Result

Answer 4

1. ) Inital Gas Concentrations
   - unaffected segments: normal alveolar pO2 and pCO2
   - affected segment: low pO2 and high pCO2
2. ) Average pCO2 - new pCO2 value is roughly average
   - roughly linear relationship between pCO2 and CO2 content means pCO2 rises
3. ) Much Lower pO2 - lower than avg would be
   - normal pO2 is 13.3 kPa but this doesnt have much more O2 content than 9 kPa due to Hb saturation
4. ) Hyperventilation - occurs due to stimulation of chemoreceptors by hypoxia and hypercapnia, however:
   - unaffected segements have V:Q >1
5. ) Effect of Hyperventilation on O2 content - negligible
   - rise in pO2 barely increases the O2 content due to haemoglobin saturation
   - this is insufficient to compensate for low pO2 in affected segments so the V:Q is still < 1
6. ) Effect of Hyperventilation on CO2 content - large
   - linear relationship between pCO2 and CO2 content leads to significant reduction in total CO2 content
   - this is sufficient to compenste for high pCO2 in affected segments (V:Q <1)
7. ) Final Result - low pO2 w/ normal pCO2
   - hypoxia triggers more hyperventilation