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
- Zone 1 - PA>Pa>Pv
- Zone 2 - Pa>PA>Pv
- Zone 3 - Pa>Pv>PA
- 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
