Re: Pounding Heart!
A 'clear' ECG is generally a good sign. It's very unlikely there's anything structurally wrong with your heart is your ECG is unremarkable.
Having said that, have you asked about a holter monitor? This way, your heart's rhythm can be tracked over a period of time. Some of the more modern monitors allow you to press a button on the device when you have symptoms. Older monitors require you to write down the time (and what you were doing) when you had the symptoms.
I'm wondering if you're experiencing sinus arrhythmia. THIS IS BENIGN but I had the same thing last year (caught on a holter monitor). I felt like my heart was racing and having skipped beats. This happened without any trigger and was a horrible feeling. But the monitor confirmed this benign phenomenon.
Anxiety can definitely cause what you're experiencing as well. Us lot with anxiety are known to have sudden surges of adrenaline for no real reason.
However, PLEASE follow up with your doctor as (I'm sure you'll appreciate) you cannot get diagnosed online.
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Healthcare Professional with Health Anxiety
Be kind, for everyone you meet is fighting a hard battle