Pulmonary shunt
Encyclopedia
A pulmonary shunt is a physiological condition which results when the alveoli of the lung
are perfused
with blood as normal, but ventilation (the supply of air) fails to supply the perfused region. In other words, the ventilation/perfusion ratio
(the ratio of air reaching the alveoli to blood perfusing them) is zero. A pulmonary shunt often occurs when the alveoli fill with fluid, causing parts of the lung to be unventilated although they are still perfused. Intrapulmonary shunting is the main cause of hypoxemia
(inadequate blood oxygen) in pulmonary edema
and conditions such as pneumonia
in which the lungs become consolidated. The shunt fraction is the percentage of blood put out by the heart
that is not completely oxygenated. A small degree of shunt is normal and may be described as 'physiological shunt'. In a normal healthy person, the physiological shunt is rarely over 4%; in pathological conditions such as pulmonary contusion
, the shunt fraction is significantly greater and even breathing 100% oxygen does not fully oxygenate the blood.
Pulmonary shunting is minimized by the normal reflex constriction
of pulmonary vasculature to hypoxia. Without this hypoxic pulmonary vasoconstriction, shunt and its hypoxic effects would worsen. For example, when alveoli fill with fluid, they are unable to participate in gas exchange with blood, causing local or regional hypoxia, thus triggering vasoconstriction. Blood is then redirected away from this area which poorly matches ventilation and perfusion, to areas which are being ventilated.
Because shunt represents areas where gas exchange does not occur, 100% inspired oxygen is unable to overcome the hypoxia caused by shunting.
A decrease in perfusion relative to ventilation (as occurs in pulmonary embolism
, for example) is an example of increased dead space
. Dead space is a space at which gas exchange does not take place, such as the trachea. It is ventilation without perfusion.
Pulmonary shunting causes the blood supply leaving a shunted area of the lung to have lower levels of oxygen
and higher levels of carbon dioxide
(i.e., the normal gas exchange
does not occur).
A pulmonary shunt is a right to left shunt. The shunt which means V/Q = 0 for that particular part of the lung field under consideration results in a de-oxygenated blood going to the heart from the lungs via the pulmonary veins.
If giving pure oxygen at 100% for five-ten minutes doesn't raise the arterial pressure of O2 more than it does the alveolar pressure of O2 then the defect in the lung is because of a pulmonary shunt. This is because although the PO2 of alveolar gas has been changed by giving pure supplemental O2, the PAO2 ( Arterial gas pressure ) will not increase that much because the V/Q mismatch still exists and it will still add some de-oxygenated blood to the arterial system via the shunt.
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
are perfused
Perfusion
In physiology, perfusion is the process of nutritive delivery of arterial blood to a capillary bed in the biological tissue. The word is derived from the French verb "perfuser" meaning to "pour over or through."...
with blood as normal, but ventilation (the supply of air) fails to supply the perfused region. In other words, the ventilation/perfusion ratio
Ventilation/perfusion ratio
In respiratory physiology, the ventilation/perfusion ratio is a measurement used to assess the efficiency and adequacy of the matching of two variables: It is defined as: the ratio of the amount of air reaching the alveoli to the amount of blood reaching the alveoli.* "V" – ventilation – the air...
(the ratio of air reaching the alveoli to blood perfusing them) is zero. A pulmonary shunt often occurs when the alveoli fill with fluid, causing parts of the lung to be unventilated although they are still perfused. Intrapulmonary shunting is the main cause of hypoxemia
Hypoxemia
Hypoxemia is generally defined as decreased partial pressure of oxygen in blood, sometimes specifically as less than or causing hemoglobin oxygen saturation of less than 90%.-Distinction from anemia and hypoxia:...
(inadequate blood oxygen) in pulmonary edema
Pulmonary edema
Pulmonary edema , or oedema , is fluid accumulation in the air spaces and parenchyma of the lungs. It leads to impaired gas exchange and may cause respiratory failure...
and conditions such as pneumonia
Pneumonia
Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...
in which the lungs become consolidated. The shunt fraction is the percentage of blood put out by the heart
Cardiac output
Cardiac output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. CO may be measured in many ways, for example dm3/min...
that is not completely oxygenated. A small degree of shunt is normal and may be described as 'physiological shunt'. In a normal healthy person, the physiological shunt is rarely over 4%; in pathological conditions such as pulmonary contusion
Pulmonary contusion
A pulmonary contusion is a contusion of the lung, caused by chest trauma. As a result of damage to capillaries, blood and other fluids accumulate in the lung tissue. The excess fluid interferes with gas exchange, potentially leading to inadequate oxygen levels...
, the shunt fraction is significantly greater and even breathing 100% oxygen does not fully oxygenate the blood.
Pathophysiology
Shunt refers to perfusion without ventilation. More specifically, intrapulmonary shunt refers to areas in the lung where perfusion exceeds ventilation.Pulmonary shunting is minimized by the normal reflex constriction
Vasoconstriction
Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries, small arterioles and veins. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in...
of pulmonary vasculature to hypoxia. Without this hypoxic pulmonary vasoconstriction, shunt and its hypoxic effects would worsen. For example, when alveoli fill with fluid, they are unable to participate in gas exchange with blood, causing local or regional hypoxia, thus triggering vasoconstriction. Blood is then redirected away from this area which poorly matches ventilation and perfusion, to areas which are being ventilated.
Because shunt represents areas where gas exchange does not occur, 100% inspired oxygen is unable to overcome the hypoxia caused by shunting.
A decrease in perfusion relative to ventilation (as occurs in pulmonary embolism
Pulmonary embolism
Pulmonary embolism is a blockage of the main artery of the lung or one of its branches by a substance that has travelled from elsewhere in the body through the bloodstream . Usually this is due to embolism of a thrombus from the deep veins in the legs, a process termed venous thromboembolism...
, for example) is an example of increased dead space
Dead space
In physiology, dead space is air that is inhaled by the body in breathing, but does not take part in gas exchange. Not all the air in each breath is able to be used for the exchange of oxygen and carbon dioxide. About a third of every resting breath has no change in O2 and CO2 levels...
. Dead space is a space at which gas exchange does not take place, such as the trachea. It is ventilation without perfusion.
Pulmonary shunting causes the blood supply leaving a shunted area of the lung to have lower levels of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and higher levels of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
(i.e., the normal gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...
does not occur).
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A pulmonary shunt is a right to left shunt. The shunt which means V/Q = 0 for that particular part of the lung field under consideration results in a de-oxygenated blood going to the heart from the lungs via the pulmonary veins.
If giving pure oxygen at 100% for five-ten minutes doesn't raise the arterial pressure of O2 more than it does the alveolar pressure of O2 then the defect in the lung is because of a pulmonary shunt. This is because although the PO2 of alveolar gas has been changed by giving pure supplemental O2, the PAO2 ( Arterial gas pressure ) will not increase that much because the V/Q mismatch still exists and it will still add some de-oxygenated blood to the arterial system via the shunt.