The short answer is: Whenever you have an acute onset of something serious, such as sudden, severe bleeding, shortness of breath, palpitations (irregular heartbeat) or severe weakness, fatigue or pain.

You should also head right to the ER if you have sudden changes in vision, severe or persistent vomiting or diarrhea, if you are coughing or vomiting blood or if you have suicidal or homicidal feelings, according to new guidelines released in September by the American College of Emergency Physicians.

“I would rather see you for what turns out to be a stable situation than have you misdiagnose yourself and not come in. That heartburn could be a heart attack, that pulled muscle in your back could be an aneurysm [a potentially dangerous weakening of a blood vessel wall],” said Dr. Richard O’Brien, a spokesman for the emergency physicians’ group and an emergency physician at the Moses Taylor Hospital in Scranton, PA.

If you have time, call your own doctor on your way to the hospital, said Dr. Alasdair Conn, chief of the emergency service at Massachusetts General Hospital. He or she “can call ahead and give us a lot of information,” he said, and once you’re stabilized in the emergency department, you can be sent home safely if emergency physicians know your own doctor will see you the next day.

To speed up care in the emergency room, write out a little card with key information on it and keep it in your wallet. The card should include your medications, allergies to food and medications and whether you have a pacemaker or have had a joint replacement. It also helps to bring an adult family member or friend with you to be your advocate.

A final bit of advice: Try not to get acutely ill on a Thursday. “That’s the worst day for hospitals,” said Conn. Although it’s the lightest day in terms of the numbers of patients flocking to the ER (the heaviest is Monday), Thursday is when in-patient bed occupancy is highest, which creates a bottleneck for patients in the ER who need to be admitted.