L34: Pulmonary Circulation

Question 1

Distribution of blood flow in the lung

Answer 1

Bloodflow ↓ from bottom to top (Apex lowest)
Due to Gravity:
—> Pa + Pv increases downwards
—> hydrostatic pressure difference within blood vessel

Zone 1: PA > Pa > Pv

• Ventilated but unperfused because capillaries are squashed flat by high alveolar pressure
• Not occur unless Pa is reduced (haemorrhage) / PA is increased (positive pressure ventilation)

Zone 2: Pa > PA > Pv

• Pulmonary arterial pressure increases downwards, alveolar pressure stay the same
• (Pa-PA) pressure difference —> bloodflow

Zone 3: Pa > Pv > PA

• Pulmonary venous pressure > alveolar pressure
• (Pa-Pv) pressure difference —> bloodflow
• Hydrostatic pressure increases downwards —> progressive distension of capillaries

Question 2

Regulation of blood flow

Answer 2

By Alveolar Oxygen Tension (PAO2)
—> ↑PAO2 —> dilate arteries and arterioles
—> ↓PAO2 —> constrict arteries and arterioles

• Local reduction in PAO2 —> regional hypoxic vasoconstriction —> divert blood flow from hypoxic regions
• Global reduction in PAO2 —> general hypoxic vasoconstriction —> large rise in pulmonary arterial pressure —> increase work for right heart —> Cor pulmonale + Right heart hypertrophy

Question 3

Ventilation and blood flow matching (VQ matching)

Answer 3

Alveolar ventilation / Perfusion ratio (VA/Q: 4/5 = 0.8 for normal)

• ↓VA (ventilation obstruction) —> PaO2↓ and PaCO2↑ (no air to carry away venous CO2 and bring in fresh O2)
• ↓Q (blood flow obstruction) —> PaO2↑ and PaCO2↓ (oxygenated pulmonary blood less quickly to be transported away)

Question 4

Regional gas exchange in upright lung

Answer 4

Ventilation decreases (PaO2↓ and PaCO2↑) + blood flow decreases (PaO2↑ and PaCO2↓) upwards
—> Blood flow decreases more rapidly
—> High VA/Q at Apex
—> higher PAO2, lower PACO2 at Apex

Question 5

Causes of VQ mismatch

Answer 5

Affecting blood flow (Q)
1. Pulmonary embolism

Affecting ventilation (V)

1. COPD
2. Asthma
3. Pneumonia
4. Pulmonary oedema

Question 6

***Effects of VA/Q mismatching

Answer 6

Having 2 extremes of VA/Q ratio:

1. Depression of PaO2 in effluent blood
   - high ratio area can only add relatively little O2 to blood (sigmoid shape of oxygen dissociation curve), while low ratio area due to steep curve SaO2 drops dramatically
   - major share of blood leaving the lung comes from low VA/Q areas
2. CO2 retention in effluent blood
   - removal of CO2 by high VA/Q alveoli = CO2 retention in low VA/Q alveoli: not much change in CO2 due to linear dissociation curve
   - However, major share of blood leaving the lung comes from low VA/Q areas —> overall effect: CO2 retention

Overall, much less effective in adding O2 than removing CO2 due to difference in shape of O2 and CO2 dissociation curve —> Hypoxaemia a bigger problem in VQ mismatching

Question 7

Ventilatory compensation in VA/Q mismatch

Answer 7

Ventilation compensation on hypoxaemia:
- alveoli with low VA/Q ratio can add larger O2
- but alveoli with high VA/Q ratio can only add little O2 due to flat slope
Overall effect: Partial compensation only (little drop in PaO2)

Ventilation compensation on CO2 retention:
- all alveoli are effective in removing CO2 due to linear dissociation curve
Overall effect: Complete restoration (normal PaCO2)

If patient unable to have ventilatory compensation: severe hypoxaemia and hypercapnia

Question 8

Assessing VA/Q inequality

Answer 8

1. Difference in ideal PaO2 and ParterialO2 (alveolar gas equation: index for gas exchange function)
2. Physiological dead space (infinity VA/Q: only ventilation, no blood flow) (Bohr’s equation: PexpiredCO2↓ —> Vd↑)
3. Physiological shunt (zero VA/Q: only blood flow, no ventilation/gas exchange) (oxygen test, Qs↑ —> CaO2↓)
4. ↓PaO2 (deceptive: since VQ mismatching, hypoventilation, shunt and diffusion impairment can all cause ↓PaO2)