What Causes Sudden Episodes Of Dizziness, Nausea And A Sensation Of Fainting When Diagnosed With Hypertension And Stage 3 CKD?

My Dad is 84 with PMH of aortic stenosis( AVR 2017), HTN, CKD stage 3, a fib. About every 2 months he gets really dizzy, feels nauseated, feels like passing out but doesn t, and gets very diaphoretic . episode lasts about 10 minutes then he feels great. BP and heart rate after these episodes are normal.