When ventilation-perfusion ratio V/Q is infinite, it means
Ventilation-perfusion ratio infinite
When perfusion is zero with preserved ventilation ratio of ventilation to perfusion becomes infinite. With zero perfusion CO2 will not be eliminated in alveolus and oxygen will not be absorbed. Thus alveolar air will be similar to external air. This is nothing but physiological dead space.
Ventilation-perfusion ratio zero
If the ventilation is zero and perfusion is preserved the ratio of ventilation to perfusion becomes zero. The alveolar air will same as for venous blood. This is similar to the right to left shunt.