New York, Dec 1 (IANS): Do you love diving while holding your breath for a prolonged period? Read this carefully. Athletes who engage in free diving - descending hundreds of feet below the surface of the ocean while holding their breath - undergo significant cardiovascular changes, reveals a study.

These changes can pose potential dangers particularly to inexperienced or cardiac untrained divers, researchers warn.

Apnea is the temporary suspension of breathing. The ancient practice of free or apnea diving has experienced immense growth worldwide over the past decade.

The sport can be dangerous, because divers must hold their breath for prolonged periods while undergoing massive water pressure and physiological changes.

Recreational divers are at greatest risk because of lack of conditioning, but even elite divers have suffered lasting or fatal effects resulting from free diving.

"We wanted to look at the changes that occur in the heart during apnea in real time," explained study author Jonas Dorner from University Hospital of Cologne in Germany.

Researchers used MRI to study the simulated effects of free diving on the cardiovascular systems of 17 elite free divers from Germany and Austria (age range 23 to 58).

The average apnea was 299 seconds (just under five minutes) and 279 seconds or about four and a half minutes for the first and second MRI exams, respectively.

During apnea, the amount of blood flowing to the brain through the carotid arteries increased and then leveled off.

"At the beginning of the apnea period, the heart pumped more strongly than when the heart was at rest. Over time, the heart dilated and began to struggle," explained Dr Dorner.

By the end of the apnea period, Dr. Dorner said the divers' heart function began to fail.

"At that point, not enough blood is being pumped to the brain," he said. "The heart is unable to pump against the high resistance of the blood vessels."

The divers' heart function recovered within minutes of breathing again.

However, for individuals with less training, free diving may be problematic.

"As a recreational activity, free diving could be harmful for someone who has heart or other medical conditions and is not well trained for the activity," noted Lars Eichhorn, from University Hospital of Bonn.

Most recently, champion diver Natalia Molchanova was reported missing and presumed dead off the coast of Spain during a dive in August 2015.

The study was presented at the annual meeting of the Radiological Society of North America (RSNA) recently.