Pulmonary shunt

Pulmonary shunt

A pulmonary shunt is a physiological result when the alveoli of the lung is 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.cite book |author=Garay S, Kamelar D|chapter=Pathophysiology of trauma-associated respiratory failure |editor= Hood RM, Boyd AD, Culliford AT |title=Thoracic Trauma |publisher=Saunders |location=Philadelphia |year=1989 |pages=328–332 |isbn=0-7216-2353-0 |oclc= |doi= |accessdate=] A pulmonary shunt often occurs when the alveoli fill with fluid, causing parts of the lung to be unventilated although they are still perfused.cite book |author=Fraser, Robert |title=Diagnosis of Diseases of the Chest |publisher=Saunders |location=Philadelphia |year=1988 |pages=139 |isbn=0-7216-3870-8 |oclc= |doi= |accessdate=] 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 anatomic shunt is rarely over 4%; in pathological conditions such as pulmonary contusion the shunt is larger and even breathing 100% oxygen does not fully oxygenate the blood.

Pathophysiology

Pulmonary shunting is the result of the fact that the blood vessels of the lungs constrict when they are in a hypoxic environment. When the alveoli fill with fluid, they would not be able to oxygenate the blood; thus blood is shunted away from them to areas which are being ventilated. In the absence of ventilation, hypoxia ensues; the areas of the lung that are not ventilated have a lowered oxygen level. The hypoxia causes blood vessels to constrict; as a result blood is directed to other areas of the lung. The blood that is shunted away does not come in contact with alveoli, so even if a person is breathing 100% oxygen, the blood cannot be fully oxygenated.

An increase in perfusion relative to ventilation (as occurs in pulmonary embolism, for example) is known as an increased physiologic dead space.cite journal |author=Prentice D, Ahrens T |title=Pulmonary complications of trauma |journal=Critical Care Nursing Quarterly |volume=17 |issue=2 |pages=24–33 |year=1994 |month=August |pmid=8055358 |doi= |url=] Dead space is a part of the lung at which gas exchange does not take place, such as the trachea.

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).

ee also

* Shunt equation
* Cardiac shunt
* Systemic-to-pulmonary shunt
* Shunt (medical)

References

External links

*
* [http://www.medstudents.com.br/calculat/shunt.htm calculator at medstudents.com.br]


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