2. Outline
• Distribution of ventilation
• Perfusion of the lung
• Ventilation : perfusion ratio
• V/Q relationship
3. Distribution of ventilation
• When inspiration occurs the fresh gas entering the lung is not
evenly distributed
• In a normal patient breathing spontaneously,
– The resting position of the lung is such that the apices are already
reasonably expanded,
– Whilst the bases are more squashed (though not collapsed).
• The resulting positions on the compliance diagram mean that
– The bases and mid-zones are on the steep part of the curve ,and
– Receive more ventilation.
5. Pulmonary circulation
• The pulmonary circulation is a low-pressure, low resistance
system
• Blood passes through pulmonary capillaries in about 0.5–1.0
seconds,
– Depending on the cardiac output,
– During which time it is oxygenated and excess CO2 is removed
• The pulmonary artery is a thin-walled structure which arises
from the RV
– Feed the lung up to the level of the terminal bronchioles
• Bronchial arteries from the thoracic aorta supply:
– Oxygenated blood to the supporting tissue of the lung
6. Pulmonary vascular resistance
• PVR = 80 x (MPAP - PCWP ) / CO
• Pulmonary vascular resistance (PVR) is influenced by the
following factors:
– Autonomic innervation
– Nitric oxide (NO)
– Prostacyclin (prostaglandin I2)
– Endothelins – potent vasoconstrictor peptides
– Vascular transmural pressure
– Lung volume
– Lung disease
– Hypoxic vasoconstriction
7. Hypoxic pulmonary vasoconstriction
• HPV is a powerful physiological reflex which diverts blood
from poorly ventilated areas to better ventilated areas
• The site of HPV lies in the:
– small pulmonary arteries - predominate site
– The remaining resistance capillary bed and venous system
• The exact mechanism of HPV is unknown
• It is potentiated by acidosis and modified by various drugs
9. Distribution of perfusion
• In addition to PVR & HPV, gravity also plays a large part in
directing blood flow
• Blood flow is not evenly distributed throughout each lung
• Gravity plays a large part in directing blood flow
– by setting up a hydrostatic pressure gradient which is higher at the
base
– reduces the perfusion pressure by 1 cm H2O for every cm in height
above the level of the heart
• Blood is preferentially directed to the lung bases
11. Cont..
Functional zones of the lung
• As well as the affect of gravity on Q & V, the transmular
pressure also affects
• Transmular pressure is the pressure d/ce across the wall of
pulmonary vessels
– Capillary , arteries and postcapilary venous pressure
• In the lung,
– intravascular hydrostatic pressure gradient that increases from top to
base
– extravascular pressure is effectively equal to PA, which is equal to Amp
12.
13. Cont..
• Zone 3
• Arteries, capillaries and veins are patent and pulmonary blood
flow is continuous.
• There is some increase in perfusion moving down the zone,
– as pulmonary arterial pressure increases due to the gravitational
pressure gradient.
• This increase in blood flow is achieved by:
– Recruitment of closed pulmonary vessels
– The perfusion of open but not perfused vessels
– Dilatation of vessels already perfused
14. Ventilation: perfusion ratio
• For efficient gas exchange in the lung ventilation (V˙ ) must
match perfusion (Q˙ )
• At rest total alveolar ventilation is about 5250 ml min−1 and
total pulmonary blood flow about 5000 ml min−1
• Thus, it is normally assumed that the optimum V/Q ratio for
any unit of lung tissue is also 1.
• In an area of lung tissue:
– If ˙ V/˙Q < 1, it increases the ‘physiological shunt’
– If ˙ V/˙Q > 1, it increases the ‘alveolar dead space’
15. Distribution of V / Q ratios in the lung
• The V/Q ratios are not uniform and vary from the lung
apices to the bases.
• At the top V/Q ratios about 3.3.
• At the bottom, where the V/Q ratios are about 0.6.
• In diseased lungs V/Q ratios vary over a wider range
The effect of increased V /Q mismatch:-
– An increase in shunt fraction
– An increase in the alveolar–arterial PO2 difference
– An increase in alveolar dead space
– A decrease in PaO2 or an increase in PaCO2
16. Extreme V/Q Mismatching
• The extremes of V/Q mismatching occur when an area of lung
has:
– A total absence of ventilation
– A total absence of perfusion
• No ventilation (V/Q = 0)
– This area of the lung is classed as shunt.
17. Extreme V/Q Mismatching
• No perfusion (V/Q = ∞)
– Perfusion may cease because of occlusion by embolus (thrombus or
air) or fall in CO
– This area of the lung is classed as alveolar dead-space.
18. Less Extreme V/Q Mismatching
• General anaesthesia almost inevitably produces some degree
of V/Q mismatching.
This model of a collapsible tube with variable flow resistance
due to downstream occlusion by external pressure is
sometimes referred to as the Starling resistor or ‘waterfall’
effect.
