What does ventilation-perfusion coupling refer to in regard to the lungs?

V/Q mismatch= PE? Pulmonary zones of good ventilation with good perfusion result in appropriate oxygen transport into the bloodstream. Mismatch of ventilation and perfusion results in poorly oxygenated blood (example pulmonary emboli where vascular clots prevent good perfusion to regions of optimal ventilation ---ie oxygen cannot be transported into bloodstream.
Match. Sounds like you are referring to matching perfusion and ventilation, which is the normal situation for healthy people. V/q scans look for pulmonary embolisms primarily by seeing if there is a mismatch between ventilation and perfusion.