Gravity and pulmonary perfusion

Describe the gravity dependent processes which affect pulmonary blood flow. What changes take place when the pressure increases in the pulmonary vessels? (97A2)

Hydrostatic pressure

Pulmonary blood pressures (both arterial and venous) are low

=> mean pulmonary blood pressure = 15mmHg

In an erect person, there is about 30cmH20 (23mmHg) difference between apex and base due to hydrostatic pressure.

=> both pulmonary arterial and venous pressure increases from apex to base

=> blood flows at different levels change as per West's zone

West's zone

1. Zone 1 - PA>Pa>Pv
2. Zone 2 - Pa>PA>Pv
3. Zone 3 - Pa>Pv>PA
4. Zone 4 - very low volume

PA - pressure in alveoli

Pa - pressure in pulmonary artery

Pv - pressure in pulmonary

Zone 1 - PA>Pa>Pv

Pressure in alveoli is > than pulmonary arterial pressure

=> capillary is squashed flat

=> no perfusion

=> "alveolar dead space"

Zone 1 doesn't happen in normal person but happens when

• PA increased - e.g. positive pressure ventilation
• Pa decreased - e.g. massive haemorrhage

Zone 2 - Pa>PA>Pv

• Moving down from zone 1, hydrostatic pressure raises both Pa and Pv so that PA is less than Pa but greater than Pv
• Blood flow is present but dependent on the pressure difference between Pa and PA
• Recruitment effect dominates here.
• Pv has no effect on blood flow
• Capillaries colllapse at downstream end and pressure at the point of collapse (PA) limits flow
  => "Starling resistor" or "waterfall effect"

Zone 3 - Pa>Pv>PA

• Moving down from zone 2, hydrostatic pressure increases both Pa and Pv further and now PA is less than Pv.
• Blood flow is dependent on the pressure difference between Pa and Pv.
• Distension effect dominates here.

Zone 4 - very low volume

At very low volume

=> reduction in radial traction

=> extra-alveolar vessels narrow

=> pulmonary vascular resistance increase

=> decrease in blood flow

NB: PVR is lowest at FRC

Recruitment, distension, and transudation

(Also see PVR)

When pulmonary blood pressure increases (e.g. due to hydrostatic pressure), PVR would decrease because:

• recruitment - some capillaries, which were closed or open but with no blood flow, begins to conduct blood
• distension - capillaries change from near flattened to more circular

Both mechanisms contribute to increased perfusion, but:

• at low pulmonary arterial pressure (e.g. zone 2)
  => recruitment dominate
• at high pulmonary arterial pressure (e.g. zone 3)
  => distension dominate

When Pa is much higher than PA and the difference exceed oncotic pressure

=> transudation (movement of plasma from capillaries into alveoli) occurs

 

Exercise and postural changes

With exercise

cardiac output increase and blood pressure increase

=> Zone 1 and 2 reduces in size, and zone 3 expands

=> greater blood flow

=> difference between apex and base due to gravity not as great

 

With postural changes

the dependent part of the lung will have higher Pa and Pv due to hydrostatic pressure

=> lower PVR and better blood flow

(as long as there is no significant alveolar collapse (zone 4))

Examiner's comment

• West's 4 zones, and changes to the zones when BP change
• Need to discuss recruitment and distension, and transudation
• Perfusion changes due to postural changes and exercise
• ???? (extra) long term changes with pulmonary hypertension