I seem to have an irregular heartbeat. Could that be dangerous? —D.S., Lady Lake, Fla. 

It could be. Minor, short-lived variations in heart rate or an occasional "skipped" beat are relatively common and usually completely harmless. But more frequent or sustained irregularities— like a recurrent racing heartbeat, or erratic rhythms that last for more than a few seconds—could indicate several serious problems, including heart disease, an overactive thyroid, or a reaction to certain drugs or dietary supplements. And the irregular rhythms themselves can cause dizziness, fainting, stroke, heart attack, or even sudden death in susceptible people. You should see your doctor right away to determine the cause of the perceived irregularity. If an electrocardiogram doesn’t turn up anything, your doctor will likely have you wear a continuous-recording monitor for at least 24 hours to provide a more complete picture.