7. Ventilation–perfusion (V ./Q .) mismatch and shunt Flashcards
What is the ventilation–perfusion ratio?
what about dead space
what about shunted ares
The ventilation–perfusion (V /Q ) ratio
is the ratio between the
amount of air getting to the alveoli
(the alveolar ventilation, VA, in L/min)
and
the amount of blood entering the lungs
(the cardiac output, Q, in L/min).
Calculating the V /Q ratio is easy:
V /Q ratio = alveolar ventilation/cardiac output
If alveolar ventilation is 4L/min
and cardiac output is 5L/min then:
V /Q ratio = 4/5 = 0.8
However, this ratio of 0.8 is only
an overall ratio as in reality ventilation
and
perfusion vary across the lung resulting
in a range of V /Q ratios.
In areas of dead space
(i. e. areas that are ventilated but not perfused,
e. g. pulmonary embolus)
the V /Q ratio is infinity
(because mathematically dividing
by zero produces the answer of infinity).
Dead space V /Q ratio = infinity
In areas of shunt
(i.e. areas that are perfused but not ventilated,
e.g. physiological shunt such as an inhaled
foreign body or anatomical shunt
such as a right-to-left shunt)
the V /Q ratio is zero
(because mathematically
dividing zero by any number
is always zero).
Shunt V /Q ratio = zero
If ventilation and perfusion are not matched,
the consequences for gas exchange
are impairment of both
O2 uptake and CO2 elimination.
How does ventilation vary from the apex to the base
of the lung?
> The lungs are suspended
within the thoracic cavity and
therefore the alveoli are subjected
to the effects of gravity.
> In the upright lung intrapleural pressure
varies from the top to the base
For every centimetre of vertical displacement
from the tip of the lung to the base,
intrapleural pressure increases
by about 0.2 cm H2O.
> For an average healthy male,
the intrapleural pressure at the apex
of the lung is about –8 cm H2O and
at the base is about –1.5 cm H2O.
This means that the alveoli at the apex
are exposed to a greater distending
pressure compared to those at the base.
> Consequently, the alveoli at the lung apex
are relatively larger than those at the bases.
The apical alveoli are thus on a
flatter part of their
pressure–volume (i.e. compliance) curve
than the basal alveoli,
which are on the steep portion
of the compliance curve.
Therefore, being relatively more compliant,
the alveoli at the base fill to a greater extent #for a given change in intrapleural pressure
during inspiration compared to
the alveoli at the apex.
Hence, ventilation is preferentially
distributed to the basal alveoli.
How does perfusion vary from the apex to the base of the lung?
> The pulmonary circulation is a
low-pressure,
low-resistance system
and is subject to alveolar pressures.
> In an upright, healthy individual at rest,
pulmonary blood flow is
distributed unevenly through the lung.
Similar to the distribution of ventilation,
pulmonary blood flow is preferentially
directed to the base of the lungs.
> This distribution is dependent on
three relative pressures:
alveolar pressure (PA),
pulmonary arterial pressure (Pa)
and
pulmonary venous pressure (Pv).
On the basis of these pressure relationships,
three functional zones are described (West zones).
Zone 1 (apex):
Zone 1 (apex): PA > Pa > Pv
Alveolar pressure exceeds vascular pressures
resulting in capillary collapse
and no blood flow.
The alveoli in this zone
do not participate in gas exchange
and
are part of the lung’s alveolar dead space.
In healthy subjects zone I does not exist because arterial pressures are just sufficient to raise blood to the top of the lung and exceed alveolar pressure.
Zone 1 may be present in cases
of severe hypotension
(e.g. following major haemorrhage)
as pulmonary arterial pressure
is reduced
or
if alveolar pressure is raised
(e.g. during positive pressure ventilation).
• Zone II (middle):
• Zone II (middle): Pa > PA > Pv
Driving pressure for blood flow is
now determined by the difference
between
arterial
and
alveolar pressures.
Alveolar pressure remains constant
throughout the lung
whereas arterial pressure increases
from the apex to the
base due to the increase in
blood hydrostatic pressure.
Therefore, blood flow gradually
increases down zone II
as the driving pressure (Pa-PA) gradually increases.
Zone III (base):
Zone III (base): Pa > Pv > PA Now both vascular pressures are greater
than alveolar pressure
and
the driving pressure for blood flow
is simply
pulmonary arterial pressure
minus
pulmonary venous pressure.
The increase in blood flow in
zones II and III reflects
also the recruitment and distention
of pulmonary vessels
with increasing intravascular pressures
down the lung.
What factors can cause shifts within the West zones?
West zones are physiological boundaries in the lung that are based upon the relationship between the pressure in the: alveoli, arteries and veins.
The boundaries between these zones can
shift due to
physiological and
pathophysiological changes.
> In healthy subjects,
zone I does not exist
because arterial pressures are
just sufficient to raise blood
to the top of the lung
and exceed alveolar pressure.
Zone 1 may be present in
cases of severe hypotension
(e.g. following major haemorrhage)
as pulmonary arterial pressure is reduced
or
with a pulmonary embolus
as pulmonary artery perfusion
will be disrupted.
> Conversely, during exercise
pulmonary artery pressure is high eliminating
any existing zone I into zone II and
moving the boundary
between zone III and zone II upward.
> Alveolar pressures are increased
during positive pressure ventilation,
which can result in substantial areas of
lung to fall into zone I.
> Changes in body position alter the
orientation of the zones with respect
to the anatomic locations in the lung but the same relationship with respect to gravity and vascular pressure remains.
Draw a graph to show how
ventilation and perfusion are
distributed across the lung.
pg 22
Discuss the effects
of high and low
V . /Q. on alveolar
O2 and CO2 partial pressures.
> The alveolar partial pressure of
oxygen and carbon dioxide
are determined by the ratio of
ventilation to perfusion,
which varies across the lung.
In the upright position,
the gradient for perfusion
is greater than that for ventilation.
> At the apex of the lung, V /Q
ratio is highest
(about 3.0).
Here PaO2 is highest and PaCO2 is lowest
(this explains why organisms that thrive in
high O2 such as TB flourish in the lung apex).
> At the base of the lung, V /Q ratio is lowest
(about 0.6) and now PaO2 is
lowest and PaCO2 is highest.
With a patient in the lateral position, how is ventilation
distributed in the following situations:
awake patient,
anaesthetise patient breathing spontaneously (GA/SV),
anaesthetised and ventilated patient (GA/IPPV)
and patient with an upper-chest thoracotomy?
Upper lung (V) Lower lung (V)
Awake 40% 60%
GA/SV 55% 45%
GA/IPPV 60% 40%
Thoracotomy 70% 30%
Why are the above changes in ventilation distribution seen?
The explanation centres on the
effect of anaesthesia on
lung volume and hence
the change in compliance of
different areas of the lung.
The lung pressure–volume curve illustrates
the regional variation in lung compliance
and shows how under anaesthesia
the alveoli at the top of the lung
(in the upper lung)
move to a steeper portion
of the compliance curve
as their resting volume falls.
Conversely, the basal alveoli (in the lower lung) move to a flatter, less compliant part of the curve.
Lung pressure–volume curve
Fig. 7.2 Lung pressure–volume curve illustrating the effect of anaesthesia on lung compliance
Note how under anaesthesia
lung volume falls;
the alveoli in the upper lung have
a reduced volume resulting
in increased compliance and hence
improved ventilation.
The alveoli in the lower lung also
undergo volume
reduction under anaesthesia.
However, the reduction in volume
leaves the lower lung alveoli less compliant
and, therefore, ventilation is reduced.
What do you understand by the term ‘shunt’?
Shunt is an extreme form of V. /Q. mismatch,
whereby blood enters the arterial
system without passing through
ventilated areas of the lung.
It may be classified into
intrapulmonary and extrapulmonary causes.
What are the causes of shunt? >
Intrapulmonary:
• Physiological:
Bronchial arterial blood passing
into the pulmonary veins.
Coronary venous blood draining
into the left ventricle.
• Pathological:
Lung collapse or consolidation
with loss of ventilation.
> Extrapulmonary:
• Cyanotic congenital heart disease,
i.e. right-to-left intracardiac shunting,
e.g. Tetralogy of Fallot.
What is the shunt equation?
what does it allow
how does it work?
The shunt equation allows the
amount of shunt caused
by the addition of venous blood
to the arterial circulation to be calculated.
It requires the subject to be breathing 100% oxygen.
Of fundamental importance is the fact
that of all of the causes of hypoxia,
shunt cannot be corrected
by breathing 100% oxygen
because the shunted blood bypasses
ventilated alveoli and thus
is never exposed to
the higher alveolar PO2.
The shunted blood therefore continues
to depress the arterial oxygen content.
Shunt eqn on page 24
formula
Q.S CcO2 –CaO2
= _______________
Q.T CcO2 –CvO2
Q.S = Shunt blood flow
Q.T = Total blood flow
(measured via cardiac output monitors)
CcO2 =
End-capillary oxygen content
(estimated from alveolar gas equation)
CaO2
Arterial oxygen content
ABG then calculate oxygen content)
CvO2
Mixed venous oxygen content
(mixed venous blood sample from a
PAFC then calculate venous oxygen content).
