Sick without a diagnosis - Palpitations

[Meldinoor]

I started feeling sick about 6 months ago, with unexplained lightheadedness and fatigue. When I went to the ER in February, the doctor did an EKG, then told me I had suffered a panic attack and was otherwise healthy. But in the months that followed I just kept getting sicker, until I was only able to work a few hours at a time without feeling out of breath and exhausted (I was a waiter at a restaurant).
Then, last month, I started having palpitations and bouts of sudden tachycardia. When I started having chest-pain as well, I went to the ER. But each time I went, they found nothing. I ended up getting several chest x-rays, a CT-scan of my chest, blood tests, a stress echo-cardiogram, and EKGs, none of which showed anything aside from some mild sinus arrythmia.
My pulse sometimes drops just below 50 when I'm calm and at rest, and sometimes shoots up to 120 when I stand up, yet no doctor I have spoken to yet has been worried.
I am fatigued, light-headed, sore, sometimes short of breath, and have frequent palpitations, yet every test I've taken so far has shown nothing unusual? Please help! I don't know what's wrong with me!

[regula22]

sounds like bradycardia  caused by  sick sinus syndrome

[Meldinoor]

Thanks for responding, Jerry.
The echocardiogram came out just fine. The cardiologist told me he couldn't see anything unusual or worrisome on it. So I assume my ejection fraction's ok, even though I don't know exactly what my numbers were.
I also have a portable monitor now. Unfortunately, it's not a Holter. It measures my heart's electrical activity for a few seconds when I place my thumbs on it, which seems strange to me, but apparently it works. The upside is that it sends in all my readings to the doctor immediately. The downside is that I never seem to catch any palpitations with it. Usually they're sudden and don't last long enough for me to switch it on and get a measurement. The doctor hasn't called me about anything unusual showing up on my readings, so I guess that's good news.

Nobody's checked my lung capacity yet, but they did several x-rays and a CT-scan that included my lungs. My oxygen levels have been good ( around 98-99%) whenever they've been tested. I don't know what my oxygen levels are like on exertion.

[Jerry_NJ]

Symptoms sure sound like heart/lung related.  I assume your checks have included your lungs: capacity and whatever else they can test.  I know it is easy for the doctors (nurse) to check oxygen level in the blood (just an electronic device that clips on the finger as I recall) - were those levels normal (I think they should be a high number, like 90%+).  

You didn't mention an echocardiogram.  Has your ejection fraction been measured, and the operation of your heart valves (a couple of things the echo is used for)?  The echo measures all physical/operational aspects of the heart (not the electrical - that's the EKG).

I think you should start by thinking back and try to identify any changes in your "life style" (eat, drink, sleep, mental...) that could possibly have brought about the onset of symptoms.  The doctors must have already gone over this path of thinking with you, but sometimes it is best to sit down in the quiet of one's own mind and think about such questions, then discuss them with your doctor.