During an arrhythmia, or abnormal heart rhythm, the heart may not be able to pump enough blood to the body, and lack of blood flow affects the brain, heart and other organs, reports CNN.
The teenager consumed three caffeine-laced drinks -- a cafe latte from McDonald's, large Diet Mountain Dew and an energy drink -- in a span of two hours before collapsing in the classroom at Spring Hill High School on April 26, he said.
He collapsed just before 2.30 pm and was pronounced dead at 3.40 pm.
The autopsy showed no undiagnosed heart conditions and that he was healthy and had no conditions that could have triggered by the caffeine intake. Also, no other drugs or alcohol were found in the teen's system, Mr Watts said.
The American Academy of Pediatrics recommends that adolescents, age 12 to 18, should not consume more than 100 milligrams of caffeine per day, CNN reported.
A 2014 study found an estimated 73 per cent of children across the US consume some kind of caffeine each day.
According to the US Food and Drug Administration, adults can consume 400 milligrams of caffeine per day -- equivalent to four or five cups of coffee -- without experiencing side effects.