Heart palpitation when lying down

I suggest you see a cardiologist.
I was having uneven heart rhythms and dizzy when I changed positions and stood up.
My diagnosis was POTS but you need to see a professional.

What you describe is possibly/likely to be atrial fibrillation. Many who have AF find that they cannot lie on their left side or their heart will start fibrillating...which is what we feel as 'palpitations'. Palpitations is not a diagnosis or a condition, it's what physicians use to label what their patients say they feel when their chest is thumping.

You don't say why you were asked to take a nuclear stress test (I have had two of them). These are normally given to patients who are suspected of having atherosclerosis to the extent that they have significant ischemia (shortness of oxygen to muscle tissue due to obstructions in the local arteries).

Just so you know, if your stress test came back as 'normal' or 'acceptable', and you have a heart arrhythmia, you should ask for a polysomnography, an overnight sleep lab. It's a weird night away from home, and you'll be wired up with about 20 different leads, in your scalp, on your chest, on your legs....and you'll be encouraged to try your darndest to relax, accept that it's a necessary event, and to try to sleep at least four hours. Sleep apnea is what we're hoping you DON'T have, but if you do, it will be the likely cause of your heart's distress.

Thank you for trying to help.
Cardiologist prescribed that nuclear test and echocardiogram. I went to see him because some service diagnostic imaging facility did for me an echocardiogram ordered by my primary doctor and they wrote that l have a pulmonary hypertension. After an echocardiogram in cardiologist office, he told me that l don’t have a pulmonary hypertension. And after monitoring my heart for 3 weeks he told me that nothing abnormal about my heart, except a palpitations. Prescribed metoprolol for a while but my blood pressure went low.

This is one of the drawbacks to metoprolol. It does a fair job of controlling the high rate when in arrhythmia such as atrial fibrillation or supraventricular tachycardia. But, when the heart is behaving, it can beat down the rate to the point of bradycardia, sometimes to a dangerous rate. Happened to me and at least one other person I know on the www. It is commonly reported.

Unless you were experiencing some kind of tachyarrhythmia, I don't know why he would have placed you on metoprolol. And if it is indeed a tachyarrhythmia, it's not normal....period. Further, these things have a way of progressing as the heart ages or endures stress such as morphological changes like collagen deposition in the myocardium and fibrosis.

It sounds like you need to clarify some things with him, or find another cardiologist for a second opinion. Right now you're not in any great duress or urgency, especially if it self-corrects every time. However, it would be nice to have a plan for a night, say zero dark-thirty on a Saturday, when you awaken to find your heart racing and it goes on for an hour, two hours, three...

I have also read that sleeping on left side the worst position. Your heart is on the left and putting pressure on it? I was told to sleep on my back because I had a fracture on my left arm, and dislocated shoulder on the right. Also learned that if I am propped up with pillows, alleviates sleep apnea instead of having to wear what I call a snorkel machine. I am 76, and live alone, so do what it takes and doesn't bother anyone. My dogs think I am a bit strange though.

Position matters for at least some people, based on my own experience. I have paroxysmal atrial fibrillation, two episodes in the last 8 months (although I realize it could be happening in my sleep). If I feel palpitations/irregular beats when I lay down at night, I "assume the position" - which for me is either R side, or flat on my back. This is often sufficient to calm things down.

In terms of self care, I try to avoid letting myself get over-tired. I don't drink alcohol at home at all any more, and rarely have more than 1 drink in a social situation. I eat smaller meals, have lost weight (hello, clothes that fit again!), and have taken up a walking program.