my 17y/o son has brief episodes of tachycardia about once a week. these episodes last about 20 seconds and only cause palpitations, no other symptoms. these episodes never occur with exercise. He is a cross country runner and never has palpitations when he runs. He does not consume caffeine or other stimulants. He has never had dizziness, chest pain, SOB OR OTHER SYMPTOMS. His resting heart rate is around 50. RECENT NORMAL LABWORK INCLUDING THYROID, CBC, ELECTROLYTES AND EKG. ALSO, HE HAD NORMAL 24 HOUR HOLTER. SHOULD I BE CONCERNED?