You have my sympathies, EmAnJ.
I’ve had tachycardia symptoms off and on for 6 years. My symptoms are due to ventricular tachycardia — specifically, right ventricular outflow tract ventricular tachycardia. Before I was properly diagnosed, though, the general assumption was that I had supraventricular tachycardia (SVT), as the symptoms were pretty much the same.
When it started, I was in very good health, and exercised regularly (about 6 days per week.) My initial symptoms were a feeling of near fainting that I sometimes felt, most typically after my workout had ended (though later it started occurring while I was working out, and later at random times.) Basically, it appears that I’m very sensitive to changes in my catecholamine (think adrenaline) levels. As a rough description, the problem is that I have some rogue cells near the outflow tract of my right ventricle. Under normal circumstances, the sinus node of your heart should send out signals that keep all 4 chambers of your heart working together at the same pace. Heart cells have a neat property called automaticity, though, that acts as a failsafe in case they don’t “hear” the signal from the sinus node. Automaticity allows them to keep beating along on their own when they don’t hear the signal. In my case, though, the problem areas near my right ventricle tend to get excited and have a mind of their own at times, so that my ventricle goes off on a pace all of its own. Under normal conditions, blood from the body enters the heart through the right atrium, goes into the right ventricle, and then is passed to the lungs for oxygenation before it is circulated back to the rest of the body. When I’m in v-tach, apparently the ventricle is so busy flapping away that it doesn’t allow the atrium to open and pull in blood to feed to the ventricle to send up the the lungs — and you can see the problem there. When you have so much ventricle action without allowing the atria to get in on the act, the supply of oxygenated blood tends to run out — hence the near fainting feeling. (With a lot of people it goes into full-blown fainting, but I’ve never had that happen yet. When it gets really bad, though, I do tend to seek lower ground. In addition to lowering the chance of cracking my skull open from a fall, sitting or lying down takes some of the load off the heart, so what oxygen there is circulating will go further.) In some of the EKG traces they’ve taken, they’ve noticed the ventricles firing off as many as 13 times before it let the atrium open up.
It sounds like you have some testing coming up, which is good. A Holter monitor is a monitor that you wear around the clock for a period of time, like a week. You then download the info so they can look for unusual rhythms. In my case, since I had problems that showed up only at random times, but which I could clearly identify the problem, they had me use an event monitor. The event monitor is similar to the holter monitor, in that it provides an ekg with just a couple of leads. It monitors your heart all the time in 30 second intervals. If I hit a button telling the monitor to save a reading (which I would do when I had symptoms), it would save the reading of the previous 30 seconds as well as the following 30 seconds. I could save up to 5 of these events, and then call in to the monitoring company and download the results. They would then print out the EKG and fax it to my cardiologist for diagnostic purposes. The folks who answered the phone were always very nice and supportive. Though they never told me what things looked like, I could tell by their followup questions when they saw a bad rhythm. They’d usually ask me to take another reading to follow it up.
If you do have an arrhythmia (which it sounds like you do), I would recommend seeing an electrophysiologist. This is a subspecialty in cardiology that specializes in electrical abnormalities of the heart.
If I remember correctly, supraventricular tachycardia (SVT) is a similar process to what I described above, except that this time the problem children causing all the problems are in an atrium rather than a ventricle. (“Supraventricular” means “above the ventricle”, which is where the atrium is located.) In general, I think SVT is considered more benign that v-tach because tachycarida can sometimes destabilize into fibrilliation (which is when the heart fibers, instead of acting together in some sort of rhythm, just start quivering, or fibrillating.) Atrial fibrillation is generally not a huge deal, but ventricular fibrillation can be fatal.
I have had my RVOT treated by a procedure called a catheter ablation. In this procedure, the electrophysiologist runs a catheter up from the femoral vein in the groin up to the heart. After doing some testing to identify where the problem areas are (which is called doing an electrophysiology study), he/she burns the problem areas using a radiofrequency probe. The damage from the burn knocks out the automaticity of the problem areas. My doctor wasn’t able to entirely burn out my problem areas because some of them were located too close to the aorta for him to risk it. While my symptoms haven’t gone away, they are much improved from what they were during the first year or so of this. I have just had to learn to listen to my body, and realize that some days I just need to take it easy and not work out, even though I had planned to and looked forward to it.
As for what it feels like when I’m having an attack ---- it can vary from time to time. Sometimes I have such strong palpitations that it feels like my heart is going to pound out of my chest. Sometimes, it’s not so much, and I first notice it with the “Oh, wow — I just about passed out there” feeling. Frequently it comes along with a huge feeling of uneasiness, almost to the point of impending doom. While this feeling may be shared with other medical problems such as agoraphobia, I think it is really just your body speaking out and saying “Hey! Something is really wrong here, and you need to do something about it.”
As for the disclaimers — IANAD, and the comments above are based on my layman’s understanding of the problem from having had it, and from studying up to be an informed patient. Some of it could be wrong, and, since I have a different diagnosis from you anyhow, it may not be applicable for your situation.
Other quick notes: My family physician (who I first consulted) has SVT. His preferred vagal maneuver is to stand on his head. As for me — at my cardiologist’s advice, when I feel an attack coming on, I sit down, put my feet up, and have a glass of water. (The heart needs water to conduct signals.)