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Arrythmia (without rhythm) refers to irregular heartbeat. There are various types of irregularity possible, resulting from disturbances of the natural pacemaker system of the heart muscle.
Rhythm disturbances can be "regularly irregular" (for example, every fifth beat is skipped) or "irregularly irregular" (random). They may be supraventricular - above the ventricle, or caused by rhythm disturbance in the atria. Or they may be ventricular, caused by problems in the pacemaking center of the heart (the atrioventricular node) or by irritability of the heart muscle itself.
Probably the most commonly encountered significant arrhythmias in pediatric medicine are supraventricular arrythmias (atrial fibrillation) in very young babies. These children often become ill very suddenly, with symptoms of heart failure (rapid breathing, poor feeding, lethargy) that may mimic severe infection. The heart is beating so rapidly that it cannot fill with blood properly for each pumping stroke, and heart pumping failure results. They typically have very high heart rates in the 280 to 300 beats per minute range. They are treated with drugs and sometimes electroshock to restore normal heart rhythm (cardioversion); this immediately restores proper circulation and the symptoms of heart failure quickly disappear. Recurrences happen but are usually avoided by drug therapy, often until about a year of age when the tendency seems to disappear.
A small but significant percentage of these children have a specific conduction defect or "wiring problem" in the pacemaker system of the heart. This syndrome is called Wolff-Parkinson-White syndrome. There is an abnormal conduction pathway that allows the electrical signal from the atria to occasionally bypass the normal pacemaker system and initiate rapid atrial rhythm. This condition is determined by examination of the EKG (electocardiogram) after normal heart rhythm is restored. "WPW" is treated with drugs and sometimes surgical destruction ("ablation") of the abnormal "wiring" pathway.
The most common arrhythmia of all is of course the premature ventricular contraction (PVC), which I suppose just about everybody has to some degree at some time. A few PVC's per minute can be quite normal for anybody, and certainly for children. Because the heart pacemaker maintains its normal rhythm despite a premature contraction, there is a "compensatory pause" in the rhythm of the heart while it waits to resume or pick up the beat (much as a singer pauses to come in correctly on the beat). This is what happens when the heart "skips a beat." PVC's can be caused by caffeine intake - a word to the wise for cola drinkers.