Respiratory Physiology: Ventilation/Perfusion Abnormalities

Question 1

Explain regional differneces in ventilation in the lungs

Answer 1

• lower, dependent portion of the lung is better ventilated than the upper, non-dependent zone
• intrapleural pressure is less negative at the bases compared with the apices (balance weight of base)
• lung bases are compressed -> lower resting volume
• higher compliance at bases so better ventilated
• apices stiff due to higher resting volume

Question 2

Zone 1

Answer 2

• when standing blood flow increased from apex to base
• At the top of the lung, the arterial pressure (P_a) may fall below alveolar pressure (P_A)
• capillaries in this area (zone 1) will be compressed and there is no flow
• normally pulmonary arterial pressure is sufficient unless:
  • arterial pressure falls (haemorrhage)
  • IPPV
• If ventilated and not perfused = alveolar deadspace

Question 3

Zone 2

Answer 3

• pulmonary artery pressure increases and exceeds alveolar pressure
• Venous pressure (P_v) is very low -> perfusion determined by difference between arterial and alveolar pressure
• Venous pressure only has an effect if it is significantly raised

Question 4

Zone 3

Answer 4

• near the base venous pressure>arterial pressure
• perfusion determined by atriovenous gradient
• low lung volumes -> extra alveolar vessels compressed
• reduction in regional blood flow

Question 5

Alveolar PO2 (P_AO₂) is determined by:

Answer 5

1. The rate of removal of oxygen from the blood (metabolic rate)
2. The rate of addition of oxygen to the blood (alveolar ventilation)

Question 6

Alveolar gas equation

Answer 6

Question 7

Shunt

Normal shunt causes

Answer 7

• Shunt refers to blood that enters the arterial circulation without passing through ventilated lung
• Doesn’t cause increased PCO2 (rasied PCO2 detected by chemoreceptor and increase RR)
• Normal shunt
  
  • Bronchial arteries draining directly into pulmonary veins
  • Coronary venous blood draining into the left ventricle via Thebesian veins

Question 8

The shunt equation

Answer 8

Question 9

How does Ventilation/Perfusion Ratio change within the lung zones?

Answer 9

• Base
  
  • V/Q <1
  • perfusion > ventilation
  • shunt
• Apex
  
  • V/Q >1
  • ventilation > perfusion
  • dead space

Question 10

V/Q <1

Answer 10

• decrease ventilation
  
  • eg obstruction
  • P_AO₂ falls
  • ​P_ACO₂ rises
• Complete obstruction
  
  • no oxygenation
  • V/Q =0
  • PO₂ and PCO₂ equal
    
    • alveoli
    • end capillary
    • mixed venous

Question 11

V/Q >1

Answer 11

• ventilation unchanged
• decreased perfusion
• increase P_AO₂
• decrease P_ACO₂
• no perfusion
  
  • no gas transfer taking place
  • PO₂ PCO_{2 =} inspired air
  • V/Q = infinity

Question 12

A-a O2 gradient

Explain

Normal value

Answer 12

• Change in ventilation between apex and base less than change in perfusion
• most blood from lung base
• less PO_2, higher PCO₂
• lowers P_aO₂ and increases P_aCO₂ compared to alveolar gas
• A-a gradient = P_AO₂ - P_aO₂
• Measure of V/Q inequality
• Normal value = 4mmHg

Question 13

Causes of increased A-a O2 gradient

Answer 13

• highe or lower V/Q ratio
• biggest effect with low V/Q ratio
• Low V/Q ratio (shunt)
  
  • decreased ventilation
  • eg obstruction
  • decreased O2 to lungs
  • decrease P_AO₂
  • greater fall P_aO₂
  • increase A-a gradient
• High V/Q ratio (dead space)
  
  • decreased perfusion to lung
  • eg hypotension/PE
  • P_AO₂ same
  • decreased P_aO₂
  • increased A-a gradient