Holly Jackson is so vibrant and joyful that a stranger would never guess she’s had two heart attacks and is a candidate for a pacemaker.

The 19-year-old Ark resident has spent the past six months coping with repeated episodes of supraventricular tachycardia (SVT), or a heartbeat that’s so fast it causes dizziness and fainting. Repeated visits to a cardiologist for EKGs, Holter monitor testing, a tilt table test, the administration of a drug that stops the heart momentarily, and cardiac catheterization have all failed to determine why Jackson’s teenaged heart suddenly starts beating at speeds of up to 220 beats per minute.

The condition has doctors puzzled. Jackson doesn’t fit the mold of the heart patient who smokes, drinks alcohol, and lives a sedentary lifestyle. Active all her life in team sports such as softball, basketball, field hockey and cheerleading, she said the problem never occurred until last November.

After enduring several bouts with SVT, ...