It happened to my boss on a NW flight from NRT->MSP a couple of year's ago. The guy in the row (coach) behind him had heart problems midway over the Pacific. So, they found an on-board doctor, upgraded him to biz class and about 30min later he had a heart attack and died. They covered the poor guy with blankets and reclined the seat for the rest of the flight. Then, they kept on flying for another 6+ hrs until arriving in MSP with a "slight Customs problem"!