Even IST is a dysautonomia diagnosis, but this sounds more like POTS. As angelawood said, he needs a table test.
Your son needs to be on beta blockers. If you let your heart beat fast for a long period of time it will enlarge and loose the ability to pump. The tilt table test is an excellent idea. He should have some of his test repeated. Event monitors usally find the most heart problems because you wear them for 30 days. The more your son worries the more anxiety he will have and the faster his heart will beat. have him get on this forum so he can see how mwny of us have the fast heart rates the pvc's pac's all of the anoying problems with our hearts. This will help ease his anxiety to see so many that have lived so long with these kinds of problems. I do agree with most post he needs to get out of bed before he develops blood clots, NOT GOOd :( I wish you and your son all the best.
He needs a table test. This is when you put him flat on a table that turns back and forth like a seesaw. It checks his ability to maintain normal BP and HR with suddeen changes in equilibrium. It sounds like he possibly has some form of SVT, such as IST as mentioned by another person. By the way, do take 120 heart rate seriously. If ignored it will later becomne 140 heart rate and then possibly higher. He is not playing you. Youy cannot fake constant increased heart rates unless you are doing drugs.
I once was in this state...the longer you stay in bed the worse it gets. My anxiety over it made it worse. He needs to get out of bed, even if to sit in a chair (and not a recliner either) with his feet on the floor. I had too...if your son has IST inactivity can make it worse and staying in bed causes anxiety to be higher... exercise reduces adrenalin and in time will lower the heart rate but it will not happen overnight. One month in bed will take 3 months recovery...I have been there done that...I was 3 months in bed. With the help of meds possibly Beta Blockers and anti-anxiety meds. with therapy and your son will recover. My pulse was a constant 110-120... that was in January. I have been up and exercising since then...I have psychiactric therapy and physical therapy and I am in close monitoring by a cardiologist. My pulse still goes high, but now it is mostly in the 70's. But I do have more problems than IST though that I am also working through. My daughter who is 14 also suffers like me...she now runs in track.... Don't give up... if all the work ups say he is okay... help him to believe that he is okay...encourage him out of bed...a little bit to a time, a little more each day...add some really light hand weights where he can flex his muscles and have him do leg lifts while sitting...add exercise a little to a time....as for the dizziness...I got that from not used to standing...BP takes a bit to come back to normal from no exercise...de-conditioning is a nasty thing..so is anxiety. I suggest that since they found nothing out of the ordinary with a medical and cardiac workup that you have him also mentally evaluated...If he is suffering from GAD (Generalized anxiety disorder) or Panic attacks...some meds. to help calm may be the extra temporary boost that he needs. Oh... no caffine and no sugar...this has helped me and and my daughter too. Best of luck to you and your son. Don't give up, lots of love...but don't cater to him so much...Get a commode next to his bed... with a pulse of 120 he can get up and use it...each day move it farther from his bed...and get him a cane for his unsteadiness...in a week the dizziness should get less. Let us know how it is going...
I'm really sorry to say, but I think your son is 'playing you'. A heart rate of 120 is no where near high enough to be causing ANYONE except perhaps someone with severe heart disease, to have the kind of problems your son is describing. A pulse rate to 140+ yes, not 120. He needs to get out of bed. Remember that when a person is used to laying down all the time, when that person stands up, he is going to get dizzy. That happens to EVERYONE. Any person who has been in a hospital can tell you that happens. It is ridiculous that your family needs to stay home to supervise your son for something like this. If he is drinking things that have caffine in it, especially Mountain Dew, he should stop drinking those things. Maybe he is going through a depression, you might want to consider that as a possibility.
You say the HR goes up to 120 when standing, what is the HR when fully at rest?
I take medication to keep my HR lower, but it usually runs high anyway, right now it is running at 96 even though I have been sitting at the computer for at least 30 minutes. I would stay my HR is usually lower, more like 80 at rest, but for some reason is higher now, but I note I did have a big Easter dinner and that may causing my HR to be higher to digest a larger than normal meal. When normal my HR would easily be 100 when walking around slowly and 120 with an serious walking, even on level ground. I don't have much of a problem with dizziness, however. may be due to something other than the heart, and HR. What other symptoms does he suffer from: e.g., stomach or digestive disorder, or headache or other pain? I can't say knowing answers to the question I ask would help me suggest anything helpful, but maybe it will help others.
I do hope you find an answer an a cure.
Not sure what I just said helps, I'm simply making the point that while a HR of 120 is definitely too high for a rest HR, it isn't as much out-of-line for someone moving around. The weakness and dizziness
Have any of the doctors mentioned IST, or have you heard of it? IST is Inappropriate Sinus Tachycardia, which is when your heart rate escalates inappropriately in response to exertion where it shouldn't. You also have a higher than average resting heart rate (usually greater than 95bpm average in 24hrs). The hr is normal, just fast. usually with a high hr, there's shortness of breath and palpitations. i have this, but am on medication for it.
But, it also sounds like POTS, Postural Orthostatic Tachycardia Syndrome, where hr and blood pressure reacts funnily upon standing. The dizziness you mention fits this more.
I'm not a dr, and don't know the full story, but you should look up both of these conditions and see if they sound like what your son has. If so, I would mention it to the drs. Keep in mind though, that many drs don't know about IST. It's a newly thought about condition, and often it is unexplainable.
good luck to you (and your son). Let us know how everything goes!