It is a condition in which one or more areas of the lung receive oxygen but no blood flow, or they receive blood flow but no oxygen due to some diseases and disorders. The V/Q ratio of a healthy lung is approximately equal to 0.8, as normal lungs are not perfectly matched.,[1] which means the rate of alveolar ventilation to the rate of pulmonary blood flow is roughly equal. The ventilation perfusion ratio can be measured by measuring the A-a gradient i.e. the alveolar-arterial gradient.