The heart’s regular rhythm or beat is maintained by a small electrical circuit that runs through nerves in the walls of the heart. When the circuit is working properly, the heartbeat is quite regular; but when there’s a problem in the circuit, an irregular heartbeat, or arrhythmia, can occur. Some children are born with abnormalities in this heart circuitry, but arrhythmias also can be caused by infections or chemical imbalances in the blood. Even in healthy children, there can be other variations in the rhythm of the heartbeat, including changes that occur just as a result of breathing. Such a fluctuation is called sinus arrhythmia, and requires no special evaluation or treatment because it is normal. If your paediatrician says that your child has a true arrhythmia, it could mean that her heart beats faster than normal (tachycardia), very fast (flutter), fast and with no regularity (fibrillation), slower than normal (bradycardia), or that it has isolated early beats (premature beats). While true arrhythmias are not very common, when they do occur they can be serious. On rare occasions they can cause fainting or even heart failure. Fortunately, they can be treated successfully so it’s important to detect arrhythmias as early as possible.