This may be totally off track, but I was speaking to a nurse yesterday about hiatal hernia's. She said she cannot lay down flat and has to use lots of pillows because she gets palpitations otherwise. I have read that these hernia's are quite common, and the most common is the sliding hernia. When you swallow, the esophagus contracts, pulling the stomach through the hiatus slightly and this can affect the vagus nerve, press on the lungs or other issues. When upright the stomach drops back through quite easily, I assume through gravity. When laying down I would have thought it less likely. An X-ray doesn't reveal these hernia's unless very large so a barium solution and CT are required.
Just a thought...
Mine get worse when I lay on my right side, most people will report they are worse when lying on their left side whole others say laying on their backs will trigger them. My doctor says PVCs can be positional, especially bending forward or lying on your sides. I don't think they are happening more often, I just think I feel them more based on my physiology, maybe everyone is different, who knows?
All I know is they were real annoying at one time, now I just ignore them when they flare up.
I asked my cardiologist about "positional palpitations" but he said that he never heard of them. There must be some truth to the idea that lying down causes more palps because it happens to me when I try to sleep sometimes.
It is real to me, so you are not alone.