PARIS (Reuters) - French artificial heart maker Carmat will not carry out any more human transplants until it has determined the cause of the death of the first patient fitted with its device, one of the company's co-founders said on Sunday.

Carmat's first patient, a 76-year-old man, died on March 2 in Paris, two and a half months after his operation.

Before he was fitted with the device, the man was suffering from terminal heart failure, when the sick heart can no longer pump enough blood to sustain the body, and was said to have only a few weeks, or even days, to live.

"Patients are still being chosen, but of course we will wait to hear a little more on the causes of the death of the first patient before transplanting another artificial heart," Philippe Pouletty, director general of Truffle Capital, one of the main shareholders in Carmat, told i>Tele television.

The device's inventor, French surgeon Dr. Alain Carpentier, told the weekly Journal du Dimanche on Sunday that the heart had stopped after a short circuit, although the exact reasons behind the death were still unknown.

Carmat's device is designed to replace the real heart for as many as five years, mimicking nature's work using biological materials and sensors. It aims to extend life for terminally ill patients who cannot hope for a real organ because they are too old and donors too scarce.

"We are trying to understand where this electronic problem came from and why," Carpentier said. "Our engineers are working night and day to understand and they will find (the reason)."

Three more patients in France with terminal heart failure and no other therapeutic option are due to be fitted with Carmat's device.

Pouletty, who said his firm had last week bought back shares in the company to show he backed the device, said the decision on the next transplant would be made with an independent committee once the analysis was complete in a "few weeks."

"I just want to remind you that for the first tests the survival expectation was 30 days and the first patient survived two and a half months after his transplant," he said.

Carmat has previously said that if it passed this first safety test, it would fit the device into about 20 more with less severe heart failure later this year, with an aim to request the right to market its device in Europe by 2015.