Carrie Fisher Heart Failure a Real Problem on Commercial Flights

Carrie Fisher's heart failure aboard a United jet is the latest fatal incident in what has become a real problem on commercial airliners.

Several EMTs who work LAX tell us ... they frequently encounter passengers with heart problems, and a number of the fatalities are preventable.

Passengers on long flights who experience chest pain, shortness of breath and other signs of distress often tell themselves they can wait it out until the plane lands, losing precious time. As one EMT put it, "They think, 'only one hour to go' and they can just bite the bullet."

And the EMTs say passengers often assume no one on board is capable of dealing with heart issues, but they add flight attendants have training and there are often passengers on board with medical training.

Another problem ... people frequently don't take critical meds during long flights ... they just lose any sense of time.

We're told male passengers tend to be more stubborn than females ... ignoring signs of trouble.

As for Carrie, we did some checking and there's no obvious evidence she was in distress before her heart stopped. Family sources tell us they received no texts from her during the flight complaining of ill health.