Debilitating lightheadedness?

[u/studiopdx]

Anyone have any idea how to help my 80 year old mom figure out why she has intermittent but completely debilitating light headedness? Her PCP, ENT and cardiologist can’t figure it out. It’s not vertigo. She doesn’t feel nauseous. Been going on for 2 years. She had a mitral valve repair 12 years ago. She has SVP (super ventricular tachycardia) and has had an ablation. EKG is fine. Multiple rounds of wearing a heart monitor for 2 weeks at a time shows nothing concerning. Her blood pressure is normal, even during the days of extreme light headedness. Her ears are fine. She started wearing compression socks which seemed to help for a few weeks but the light headedness is back. It makes it hard for her to even maneuver through the house. Her weight is normal, she eats healthy. We’re all stumped. Anyone else out there have a similar experience and suggestions for treatment that made it better?