While in a<unk> , the ventilation/perfusion ratio is zero, lung units with a V/Q (where V = ventilation, and Q = perfusion) ratio of less than 0.005 are indistinguishable from shunt from a gas exchange perspective.<unk> ing is minimized by the normal reflex constriction of<unk>  vasculature to hypoxia. Without this hypoxic<unk>  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. This vasoconstriction is triggered by a smooth muscle reflex, as a consequence of the low oxygen concentration itself. 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. For instance, if there is a certain alveolus that is not being ventilated, blood will still flow through the capillary which irrigates it instead of going elsewhere, as the problem does not reside in the perfusion. The rest of capillaries will be working as normal, being saturated of oxygen at the 100% of their capacity. Therefore, there is no use in providing 100% inspired oxygen to the patient, as the blood that is not being oxygenated will not still be able to catch this oxygen, and the other capillaries cannot get it either because they are already 100% saturated. A decrease in perfusion relative to ventilation (as occurs in<unk>  embolism, for example) is an example of increased dead space. Dead space is a space where gas exchange does not take place, such as the trachea; it is ventilation without perfusion. A pathological example of dead zone would be a capillary blocked by an embolus. Although ventilation at that area is unaffected, blood will not be able to flow through that capillary; therefore, at that zone there will be no gas exchange. Dead zones may be corrected by supplying 100% inspired oxygen; when a capillary is blocked, the blood inside of it goes backwards and distributes between other capillaries that are exchanging gases without problem. The resulting blood that flows through them will not be 100% saturated, as it contains some unoxygenated blood (the one that came from the blocked capillary). For this reason, blood will actually be able to obtain the extra oxygen we supply to the patient.<unk> ing 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<unk>  occurs as a result of blood flowing right-to-left through cardiac openings or in<unk>  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<unk>  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<unk> . 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.
Pulmonary shunt
Cardiac shunt
Atriocaval shunt
Pulmonary-to-systemic shunt
Pulmonary artery banding
Portacaval shunt
Blalock–Taussig shunt
Aortopulmonary window