Elina_Physisology_Four Causes Of Hypoxemia

Front 1

Hypoventilation is

Back 1

Hypoventilation is characterized as an equal decrease in P02 in all three compartments.
• There is no increase in the A-a oxygen gradient.
• Supplemental oxygen can relieve the hypoxemia.
• End-tidal air still reflects the systemic arterial compartment.
• The problem is not within the lung itself.

Front 2

Diffusion impairment means a

Back 2

Diffusion impairment means a structural problem in the lung. This can be produced by a
decreased surface area and/or increased thickness of lung membranes.

Front 3

In marked diffusion impairment, pulmonary end capillary POl alveolar
POl' will be ....

Back 3

In marked diffusion impairment, pulmonary end capillary POl will be less than alveolar
POl' and a difference will exist between the alveolar and systemic arterial blood (A-a). Thus,
end-tidal P02 will not be a good index of systemic arterial POl' An A-a gradient greater than
10 mm Hg on room air tends to indicate a diffusion problem.

Front 4

In summary Diffusion impairment

Back 4

In summary
• There is an increase in A-a oxygen gradient.
• Supplemental oxygen can relieve the hypoxemia.
• End-tidal air does not reflect the arterial values.
• It is characterized by a decrease in DLCO.

Front 5

PULMONARY SHUNT-что это

Back 5

A pulmonary shunt is also known as a right-to-left shunt. Bydefinition, systemic venous blood
is delivered to the left side of the heart without exchanging oxygen and carbon dioxide with the
alveoli.A good example is blood passing through a region of atelectasis. A right-to-left shunt
alwaysleads to hypoxemia.

Front 6

In summary PULMONARY SHUNT

Back 6

In summary
Increase in A-a oxygen gradient
Supplemental oxygen ineffective at returning arterial POl to normal
• End-tidal air does not reflect the arterial values

Front 7

VENTILATION-PERFUSION DIFFERENCES IN THE LUNG
the mean value for intrapleural pressure is

Back 7

At the end of a normal expiration, the mean value for intrapleural pressure is -5 em H20 .However,
there arc regional differences, and the reason for these differences is gravity. In an upright individual,
there is a column of fluid and tissue in the chest cavity. Toward the lung apex (against gravity),
intrapleural pressure decreases (becomes more negative);more toward the lung base, pressure
increases (becomes more positive)

Front 8

VENTILATION-PERFUSION DIFFERENCES IN THE LUNG
At the apex, intrapleural pressure is

Back 8

At the apex, intrapleural pressure is -10 em H20 , which represents a low pressure but
a large force expanding the alveoli. Therefore, at the beginning of inspiration, alveoli
at the apex are large and stiff and contain a large volume of air

Front 9

VENTILATION-PERFUSION DIFFERENCES IN THE LUNG
At the base,

Back 9

At the base, intrapleural pressure is -2.5 em H20, which represents a higher pressure
but a smaller force expanding the alveoli. Therefore, at the beginning of inspiration,
alveoli at the base are small and very compliant and contain a small volume of air.

Front 10

Inspiration at the apex

Back 10

Inspiration atthe apex
Intrapleural pressure decreases from -10 to ~13 em H20. But because the alveoli were almost
completely inflated (low compliance) before inspiration begins, they change little during inspiration.
Thus, very little room air flows into these alveoli. The amount of room air entering an
alveolus during inspiration represents alveolar ventilation (VA). Consequently, under resting
conditions, alveoli at the apex are large but receive a low level of alveolar ventilation.

Front 11

Inspiration at the base

Back 11

Inspiration atthe base
Intrapleural pressure decreases from -2.5 to -5.5 em H20. The base alveoli at the beginning of
inspiration arc small, but they are on the steep part of the pressure-volume curve (very compliant).
Because of this, during inflation there is a large change in size and volume. Consequently,
under resting conditions, alveoli at the base are always smaller than those at the apex but receive
a high level of alveolar ventilation.

Front 12

Regional Differences in Blood Flow

Back 12

In the
upright individual, as blood moves against gravity (toward the lung apex), pressure decreases,
and as blood moves toward the base of the lung, pressure increases.

Front 13

Regional Differences in Blood FlowToward the apex

Back 13

Toward the apex
Pulmonary arterial pressure decreases.
Vessels are less distended and thus represent a higher resistance system.
Therefore, lower perfusing pressures and higher resistance mean less blood flow to the apex.

Front 14

Regional Differences in Blood FlowToward the base

Back 14

Toward the base
Pulmonary arterial pressure increases.
Vessels are more distended, thus a lower resistance system.
Therefore, no loss in perfusing pressure and a lower resistance pathway mean more blood flow
to the base.

Front 15

Ventilation-Perfusion Relationships

Back 15

Apex: Least ventilation (mUmin) and blood flow (mLlmin)
Base: Greatest ventilation (mUmin) and blood flow (mUmin)
Blood flow and ventilation are greatest at the base and decrease toward the apex. Although the
relationship between the two is similar, quantitative differences exist.
In the normal individual, there is an ideal relationship between blood flow and ventilation. The
ideal relationship in a normal individual under resting conditions is close to 0.8.

Front 16

the base can be considered an underventilated region. почему?

Back 16

Even though the base receives the greatest ventilation, it is not high enough for the very high
blood flow.

Front 17

the apex can be considered an overventilated region. почему?

Back 17

Even though the apex receives the least ventilation, it is still too high for the very low blood flow.

Front 18

When the ratio is less than 0.8 under resting conditions, the lung unit is

Back 18

underventilated,

Front 19

When the ratio is greater than 0.8, under resting conditions, the lung unit is

Back 19

it is overventilated.

Front 20

Hypoxic Vasoconstriction

Back 20

Whenever there is a decrease in alveolar P02' a local vasoconstriction of pulmonary blood vessels
is produced. The result is a lowering of blood flow through that lung unit.

Front 21

Consequences of VAIO Mismatches VAlQ<0.8:

Back 21

If we take the situation to the extreme, where there is blood :flowbut minimal ventilation,
this lung unit would resemble a pulmonary shunt. Thus, as the ratio goes below 0.8,
the lung unit begins to act like a shunt. The lower the ratio, the more that lung unit acts like a
pure shunt.

Front 22

Consequences of VAIO Mismatches VAlQ>0.8:

Back 22

VAlQ>0.8: If we take this situation to the extreme, where there is ventilation but minimal blood
flow, the lung unit would act as dead space. The greater the ratio, the more that lung unit acts
like dead space

Front 23

Consequences of VAIO Mismatches Exercise

Back 23

In exercise, there is increased ventilation and pulmonary blood flow. The ideal VA/Q is no
longer 0.8; it is greater than 0.8. Thus, during exercise, ventilation increases more than cardiac
output. Also, the base-apex flows are more uniform.
Insummary
• There is an increase in the A-a oxygen gradient.
Supplemental oxygen can relieve the hypoxemia but is less effectiveas V/Q decreases
to low values.
• End-tidal air does not reflect the systemic arterial values.
• Hypoxemia is often associated with normal or below-normal arterial PC02•

Front 24

SHUNTING OF BLOOD IN THE HEART

Back 24

The consequences are quite different from a pulmonary shunt because pressures are usually
higher on the left side of the heart (atria and ventricles), and thus flow is normally left to right.
A major characteristic is that hypoxemia never develops in a left-to-right shunt. The principal
example is an atrial or ventricular septal defect

Front 25

Atrial septal defect: P02

Back 25

Atrial septal defect: P02 increase first appears in the right atrium.

.

Front 26

Ventricular septal defect

Back 26

Ventricular septal defect: P02 increase first appears in the right ventricle.

Front 27

Patent ductus:

Back 27

Patent ductus: P02 increase appears in pulmonary artery