Supraventricular tachycardia (SVT) is not uncommon in kids. They will experience a sudden, unprovoked increase in heart rate to incredibly rapid, often >250 bpm. It occurs when the heart has electrically active areas other than the usual pacemaker in the SA node above the ventricles, so the heart muscle is getting electrical impulses from more than one pacemaker and it's responding to them all. Of course, if it continues for more than a few seconds, cardiac output is severely compromised because the heart has no time to fill. Sometimes it will revert to a normal sinus rate and rhythm spontaneously, but more often than not will require cardioversion. Vagal manouevres will sometimes work to cardiovert, ie ice to the face for infants and small children, valsalva or blowing forcefully through a straw. These measures tend to be temporary, if they succeed at all. The drug of choice initially is adenosine, which, because of its extremely short half life, must be given rapid IV push as close to centrally as possible (NOT in a saphenous peripheral IV!!). If that fails, electric cardioversion is tried. For longer term treatment until the focus (or foci) can be identified and ablated, amiodarone is started. Laser ablation of the offending tissue, done in the cath lab, may be required for permanent effect. Occasionally there are so many areas of electrically active tissue that the child requires a permanent mechanical pacemaker, but this is quite rare.