Gravity and pulmonary perfusion

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