No its not about congenital abnormalities.
Ive got a PBL case about respiratory failure, and mechanical ventilation. The patient is in the ICU and requires positive end expiratory pressure (PEEP) ventilation. And it says:
In the extreme case when the V/Q ratio equals 0, pulmonary blood flow does not participate in gas exchange because the perfused lung unit receives no ventilation. This condition is intrapulmonary shunting and is calculated by comparing the oxygen contents in arterial blood, mixed venous blood, and pulmonary capillary blood (see Other Tests). In healthy people, the percentage of intrapulmonary shunt is less than 10%. When the intrapulmonary shunt is greater than 30%, resultant hypoxemia does not improve with supplemental oxygenation because the shunted blood does not come in contact with the high oxygen content in the alveoli. Instead, treatment consists of recruiting and maximizing lung volume with positive pressure. PaO2 continues to fall proportionately as the shunt increases.
Where V/Q is the ratio between ventilation and perfusion in the lungs.