Im not a cardiologist so I can't answer this for you, but I can give my observations and experiences in my 11 yr history with panic:
Firstly, and foremost, they let you leave the hospital. If there were anything dire that required help and treatment in a quick time frame they cannot release you. That being said, it must not be serious.
Second, although some do not agree in this, it has been noted by many in the throes of a panic attack that an EKG will read abnormal when in fact the heart is fine. This can be because of skips and flutters that we feel as well as things we don't. But it's all no big deal. If palps are a concern for you know that it has been found that palpitations of skips, stops, flutters, thuds, thumps, double beats, etc are not indicative, predictive, or diagnostic of nearly anything.
In the end, I would do 2 things. Ask the ER for a copy of your EKG, while you got them on the phone ask them if it be possible that the ER doc explain the EKG readings. Then take it to your PCP and discuss with him/her.