In a normal heart rhythm, the sinus node generates an electrical impulse which travels through the right and left atrial muscles producing electrical changes which is represented on the electrocardiogram (ECG) by the p-wave.  The electrical impulse then continue to travel through specialized tissue known as the atrioventricular node, which conducts electricity at a slower pace.  This will create a pause (PR interval) before the ventricles are stimulated.  This pause is helpful since it allows blood to be emptied into the ventricles from the atria prior to ventricular contraction to propel blood out into the body.  The ventricular contraction is represented electrically on the ECG by the QRS complex of waves.  This is followed by the T-wave which represents the electrical changes in the ventricles as they are relaxing.  The cardiac cycle after a short pause repeats itself, and so on.

An irregular heartbeat is an arrhythmia. The most common irregularity occurs during breathing. When a child breathes in, the heart rate normally speeds up for a few beats. When the child breathes out, it slows down again. This variation with breathing is called sinus arrhythmia. It's completely normal.

Arrhythmias (also called dysrhythmias) may occur at any age. Many times they have no symptoms. Often parents and children never suspect an arrhythmia and are surprised when a doctor finds one during a routine physical exam. Rhythm abnormalities are usually evaluated much like other health problems. Your child's history — or what you and your child report about the problem — is very important. You may be asked questions like:

• Is your child aware of unusual heartbeats?
• Does anything bring on the arrhythmia? Is there anything your child or the family can do to make it stop?
• If it's a fast rate, how fast?
• Does your child feel weak, lightheaded or dizzy?
• Has your child ever fainted?

Some medicines may make arrhythmias worse. Be sure to tell your doctor about all the prescribed and over-the-counter medications that your child takes. If your child has an arrhythmia, discuss this with the doctor and ask what to look for.

• Premature atrial contraction (PAC) and premature ventricular contraction (PVC)
  Premature beats or extra beats most often cause irregular heart rhythms. Those that start in the upper chambers (atrium) are called premature atrial contractions or PACs. Premature ventricular contractions or PVCs start in the ventricles. If you've ever felt your heart "skip a beat," it was probably from this type of arrhythmia. The heart really doesn't skip a beat. Instead, an extra beat comes sooner than normal. Then there's a pause that causes the next beat to be more forceful. You felt this more-forceful beat. Premature beats are very common in normal children and teenagers — most people have them at some time. Usually no cause can be found and no special treatment is needed. The premature beats may disappear later. Even if they continue, your child will stay well and won't need any restrictions. Occasionally premature beats may be caused by disease or injury to the heart. Your child's doctor may recommend more tests to make sure your child's heart is OK.
• Tachycardia
  A fast heart rate is called tachycardia. The definition of "too fast" usually depends upon the person's age and physical activity. A newborn has tachycardia if the resting rate is more than 160 beats a minute. A teenager has it if the resting heart rate is more than 100 beats a minute. An exercising teenager may have a normal heart rate of up to 200 beats a minute. Sinus tachycardia is a normal increase in the heart rate. It occurs with fever, excitement and exercise. No treatment is needed. Rarely, disease, such as anemia (low blood counts) or increased thyroid activity can cause this fast heart rate. In these cases, when the disease is treated, the tachycardia goes away. Supraventricular tachycardia (SVT)
  The most common abnormal tachycardia in children is supraventricular tachycardia (SVT). It's also called paroxysmal atrial tachycardia (PAT) or paroxysmal supraventricular tachycardia (PSVT). The fast heart rate involves both the heart's upper and lower chambers. This isn't a life-threatening problem for most children and adolescents. Treatment is only considered if episodes are prolonged or frequent. For many infants, SVT is a time-limited problem. Treatment with medications often stops after six to twelve months.
• Wolff-Parkinson-White syndrome
  If an abnormal conduction pathway runs between the atria and ventricles, the electrical signal may arrive at the ventricles sooner than normal. This condition is called Wolff-Parkinson-White syndrome (WPW syndrome). It's named after the three people who first described it. WPW syndrome is recognized by certain changes on the ECG. Many people with WPW syndrome don't have symptoms or episodes of tachycardia.