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A pulmonary shunt is a pathological condition which results when the alveoli of the lungs 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 an 'anatomical shunt'. Anatomical shunting occurs when too much of the blood supplying the lung tissues via the bronchial arteries is returned via the pulmonary veins, bypassing the gas exchange. In addition, some of the smallest cardiac veins drain directly into the left ventricle of the human heart. This drainage of deoxygenated blood straight into the systemic circulation is why the arterial PO2 is normally slightly lower than the alveolar PO2, known as the alveolar–arterial gradient, a useful clinical sign in determining cause of Hypoxia.
Shunt refers to the passage of deoxygenated blood from the right side of the heart to the left, without participation in gas exchange in the pulmonary capillaries. While shunt is an absolute (ventilation is zero), lung units with a V/Q ratio of less than 0.005 are indistinguishable from shunt from a gas exchange perspective.
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.
A pulmonary shunt occurs as a result of blood flowing right-to-left through cardiac openings or in pulmonary arteriovenous malformations. The shunt which means V/Q = 0 for that particular part of the lung field under consideration results in 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.