LOS ANGELES - This is your heart on an energy drink, and it's contracting significantly faster than it was before you opened that can full of liquid stimulant.

So says a team of cardiac radiologists who wanted to figure out why energy drinks like Red Bull, Monster, 5-Hour Energy and Rockstar are sending tens of thousands of people to emergency rooms each year, including nearly 21,000 in the United States alone, according to a 2013 report from the Substance Abuse and Mental Health Services Administration.

A typical energy drink can have up to three times as much caffeine as coffee or soda, according to Jonas Doerner, a resident at the University of Bonn in Germany and member of the research team. High caffeine consumption can cause rapid heart rate, palpitations, a spike in blood pressure and even seizures or death, he said in a statement.

Taurine is also a major ingredient in energy drinks, Doerner said.

To get more detail on exactly how energy drinks caused medical problems, Doerner and his colleagues imaged the hearts of 15 men and three women with a 1.5-Tesla MRI scanner. Then the volunteers drank a beverage containing high amounts of caffeine and taurine and had their hearts scanned again.

One hour after consuming the experimental energy drink, the researchers found that radiologic measurements of heart strain were significantly higher than at baseline.

Specifically, the team measured the peak strain and peak systolic strain rate of the heart's left ventricle, which is responsible for pumping oxygenated blood from the lungs to the aorta and then on to the rest of the body. Both showed changes that were too big to be due to chance.

In addition, the team found a small change in peak diastolic strain rate, but it wasn't large enough to be statistically significant.

(In the language of blood pressure, the systolic measurement quantifies the pressure in the arteries when the heart muscle contracts and diastolic measurement quantifies pressure in the arteries when the heart muscle is relaxed between heartbeats.)

The results were presented Monday at the annual meeting of the Radiological Society of North America.