According to WJLA TV, John Warner was hospitalized briefly today for "atrial fibrillation," and a "routine procedure is scheduled for Wednesday." According to the Mayo Clinic:
Atrial fibrillation is a common heart rhythm problem. More than 2 million Americans have atrial fibrillation, which can cause palpitations, shortness of breath, fatigue and stroke.[...]
Although atrial fibrillation usually isn't life-threatening, it can lead to complications. Treatments for atrial fibrillation may include medications and other interventions to try to alter the heart's electrical system.
Atrial fibrillation is not a heart attack; in fact, Senator Warner returned to work after going to the hospital this morning.
[UPDATE: The latest news is that "John Warner was treated Wednesday for an irregular heartbeat but planned to return to work in the Senate next week."]