Good question. You should go to the ER if you are vomitting so much you get dehydrated, if you are in an Addisonian crisis, you pass out, or if you are going into shock.
I had to go to the ER three times in the past year from AF. The first time I thought I was going into shock. I had passed out dry heaving over the toilet one night after being nauseas and weak several hours beforehand. I woke up not knowing where I was and not being able to move my body because it was limp and simply wouldn't respond. My heart would go into cycles of racing very fast, then beating extremely slow. My body also swung within minutes from being drenched with sweat to shaking with extreme cold, one after the other while my heartbeat went from very fast to extremely slow. My arms were shaking violently. I was also unable to breathe and kept gasping for breathe. Thankfully, at about 1 AM my boyfriend woke up wondering where I was. He called 911, and an ambulance took me to the ER. Thank God my bf was there because my mind was mush and I wasn't able to move or walk or reach for my cell, let alone know where it was. I also couldn't speak to the paramedics when they asked for information from me. At the ER I was given fluids and a valium. The dr told me I had been dehydrated. I felt better after a couple hours and was sent home.
I had to go to the ER 2 other times. The worse visit was when I was at work, and my boss had to call for an ambulance. I was kept in the hospital 3 days and 2 nights. The doctors wanted to keep me longer but I said no. The doctors said I had an Addisonian crisis/ adrenal crisis. I was given 500 mg HC through an IV drip, also fluids. I wouldn't reccomend staying overnight if you can help it because you simply can not sleep in the hospital. Sleep is interrupted every 4 hours or more for blood Sugar checks and blood pressure checks. I was more miserable in the hospital over this extended stay than the shorter stays.
However my AF went from 3B to 3C during that 3 day hospital visit from all the physical stress involved with being in the hospital. They made me fast to take an ACTH test, and being hypoglycemic, the fasting sent me into a tail spin. The loss of blood for this test (five full vials) made me severely weak, and I also had blood drawn for thyroid and parathyroid and CBC. Too much blood loss. I also went another day without any food because the hospital wasn't authorized to prepare gluten and dairy free meals. Dairy makes me vomit, so I went without food for the better part of a day. I kept pushing my hospital emergency button but the nurses ignored me, and weren't able to get me food for a day. Ugh. Bad memories. Leaving the hospital, I was too weak to walk. I had ripped out my own IV because I just couldn't take it. I spent the next week in bed at my own place, in far worse shape than I initially went in to the ER.
Also, the only thing I got from this visit, other than the fluids, was a referral to see a psychiatrist.
So, it's a gamble going to the ER, in my opinion. They could help with dehydration, and they can give you a valium, but I think the complexities of AF are beyond them. When the dr at the hospital told me in a confiding tone that she personally didn't believe that stress contributed to or made worse Adrenal insufficiency and Addison's, I basically stopped listening to her.