Dead space ventilation is a term used clinically to describe a pathology of the respiratory system in which there is a decrease in alveolar blood flow in part of the lung, but ventilation is normal, and blood and gas are not exchanged in this part of the alveoli, resulting in an imbalance in the ratio of ventilation to blood flow, which does not allow for effective and normal gas exchange, which is equivalent to an increase in alveolar dead space in the respiratory system, hence the term dead space. This is called dead space ventilation. There are many clinical causes of dead space ventilation, such as diffuse intravascular coagulation, pulmonary arteriovenous fistula, and pulmonary vasoconstriction and spasm due to various causes.