Needing answers...

Last December Caleb was hospitalized with tachycardia and a very mild case of pneumonia (the pneumonia was extremely mild, barely even visible on the x-ray, so he was hospitalized more because of the tachycardia).

We had hoped the tachycardia would resolve.  Unfortunately, he's still having problems with it.  Here are some recent examples of the problems he is having.  The following images are screenshots from where I used a heart rate app on my phone to check his heart rate (I used these while he was hospitalized to compare them to the hospital's heart rate monitor, and they compared very closely.)

This is an example of a resting heart rate for Caleb.  He had been sitting/laying for a long while doing nothing except watching things on his laptop.

This heart rate test was taken immediately after the one above.  The only difference is that I had him stand up.  Just standing up (not even taking a step) caused an increase of over 40 beats per minute from a resting heart rate that was already too high.

This is another resting heart rate.  I was actually really excited to see his heart rate at a normal level - this rarely happens anymore.

This measurement was taken immediately after the one above.  Once again, I had him stand up to see how that affected his heart rate.  His heart rate literally doubled just by standing up, not even taking a step.

This was a measurement I took after Caleb got out of bed yesterday morning.
  Because he is homeschooled he was able to sleep as late as he wanted and gradually wake up (note the time stamp of 9:46am).  There was no hurry to get out of bed.  He stayed in bed until he felt ready to get up, and I just happened to be there in his room and managed to get his heart rate as soon as he got up.  This is way above normal!

Caleb did receive a full evaluation from a pediatric cardiologist, including an echocardiogram of his heart performed by the cardiologist himself.  There does not appear to be anything structurally wrong with his heart.  It could either be a problem with the SA node of the heart (this is the heart's natural "pacemaker") which wouldn't show on the echo, or more likely it's a problem elsewhere in his body.

He is getting all kinds of blood work done to check the most obvious things like thyroid and adrenal gland issues.  He has been put on salt tablets to see if that will help any.  There has been talk of other medications to slow his heart rate, but first we want to figure out why his heart rate is so high.

Postural orthostatic tachycardia syndrome (POTS) is one possibility that is being considered (particularly considering the fact his heart rate increases so much upon standing), but the doctor wants to rule out other problems before focusing on POTS for several reasons, including the fact that even his resting heart rate is usually higher than it should be.

So we are just in a waiting mode, wondering what on earth is wrong with our son to cause his heart rate to be so elevated.  He has been very inactive lately, basically just sits or lays on the floor with his laptop all day long, and I don't even feel comfortable pushing him to do anything more because of his heart rate.  He is not terribly symptomatic, though he does say he feels dizzy and his head hurts when he gets up (he pointed to the center of his forehead to indicate to me where it hurt).  He also looks shaky to me whenever he gets up, but he denies feeling that way.  However, while he is generally inactive, and he has no desire be active (e.g. he doesn't want to walk anywhere with us, even to the nearby coffee shop to get one of the smoothies he loves), when we had friends visiting he was able to play along with their kids, even running along the greenway with them.  However, because of the number of little kids between our 2 families the bigger kids never ran too fast or too far, and Caleb did tell me when he got home he could really feel his heart pounding hard.

Overall, I just really don't think Caleb looks healthy.  As his mom, I sense something not right about him.  He is paler with dark circles under his eyes, despite getting plenty of sleep.  He is not involved in any extracurricular activities at the moment (in the past he was really active, having played baseball, soccer, and doing gymnastics) and is homeschooled so he doesn't have any stress in his life to cause problems.  He eats a very healthy diet full of fresh fruits and vegetables and little to no preservatives, dyes, or other chemicals.  He drinks water all day long and never drinks soda.

The summer is approaching, and I know the heat will be unbearable to him.  I hope we get some answers soon.