When I was first diagnosed with moderate to severe aortic stenosis about 18 years ago, they warned me that AS patients may experience light-headedness or "near-syncope" episodes, especially when standing from a crouch or getting up from a chair. That was because when sitting or crouched, the vascular system is "compressed" and does not require the full normal amount of blood to reach its "target" blood pressure, but then the heart cannot quickly increase its pumping volume to fill the uncompressed system when you stand. That would explain the "upon standing" incidents but not the "while seated" situation. I would look into hydration level, BP and/or heart rate to try to explain near-syncope while seated. I've had this happen a couple of times since valve replacement, but never had it go any farther than light-headedness.
I also remember an incident at the gym before my valve replacement. At the time I had undiagnosed bradycardia (transient low heart rate). I always wore a heart rate monitor at the gym, and one day, before I started my workout I noted that my heart rate was low. As I stood there it went from 60 BPM down to 52, then 50, then 48. . . and at that point I decided that I had better get moving to "rev it up" or else maybe it would stop. Once I got onto the treadmill, all was well. Who knows what was happening? I just know that immediately after valve replacement I ended up with a pacemaker, and now if my heart rate goes below 60 BPM it means I need to get the pacer looked at. (That happened last summer and I ended up having one of the leads and the pacer itself replaced.)
