Definitely tell the pediatrician about this issue. The next time your nephew experiences these symptoms, make sure someone takes an accurate pulse in beats per minute. It is not all that uncommon for children to have SVT. If a five year old has a pulse rate of 200 or more, it might be SVT and the child should be seen by a doctor ASAP - at the very least, the pediatrician should be called and it might warrent a trip to the ER.
In my experience, if a healthy five year old is running around playing very actively, their heart rate might be around 160-180, but if a child has a very fast heart rate when inactive, it should definitely be checked out by a doctor. Keep in mind though, that a healthy five year old's resting pulse can be as high as about 120 which can be perfectly normal.
Fortunately, children tend to tolerate SVT better than adults, that is, they may not have dizzyness, faintness, etc. But, it is very important to have an accurate diagnosis to rule out more worrisome conditions like WPW.
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