MY Lung Doctor Cannot (as Yet) Determine What's Happening That

mY lung doctor cannot (as yet) determine what's happening that my heart rate goes way up and o2 levels way down. Also very low blood o2 when sleeping. Heart guy says my heart is fine. Lung tests say my lungs are fine. Will be going in for another nucular test - - he (my doc) mentioned there may be a shunt in the blood going to my lungs. Ever heard of anything like that? It's a puzzle. Thanks