The man may have suffered a heart attack, but it sure looks he was practically killed by TEPCO and the Fukushima prefectural government.
This bit of information from Mainichi Shinbun (5/14/2011):
原発からの救急搬送を巡り、県災害対策本部と消防、東電の３者は原発事故後、約２０キロ南の拠点施設ナショナルトレーニングセンター「Ｊヴィレッ ジ」（楢葉町）で受け渡すよう取り決めている。このため同７時３５分ごろ業務用車両に男性を乗せ原発を出発、８時１０分ごろ着いたＪヴィレッジで、常駐医 師が心臓マッサージなどをしたが回復せず、同３５分ごろ救急車に乗せられた。
As to medical emergency transfer from the plant, the Fukushima prefectural government headquarters that deal with the plant accident, the Fukushima fire department, and TEPCO decided after the accident that the handover [of sick and injured people] must be done at "J-Village", 20 kilometers south of the plant. Accordingly, the man was put in a TEPCO's vehicle at 7:35AM, and arrived at "J-Village" at 8:10AM. There, the doctors did the CPR but couldn't resuscitate the man. He was then put on an ambulance at 8:35AM.
The ambulance arrived at a hospital in Iwaki City at 9:07AM, and the man was pronounced dead at 9:33AM.
And why the handover at "J-Village"? It's for the safety of emergency medical crew.
And I have this sinking feeling that the TEPCO's vehicle carrying the man observed the speed limit and all the traffic lights along the way, since it was not an emergency vehicle. (The police might have issued a ticket for traffic violation.) It took them 35 minutes to cover 20 kilometers, so they did less than 40 km/hour, or 25 miles/hour.
The ambulance didn't seem to do any better. It took 30 minutes to cover the distance between "J-Village" and the hospital. Depending on which hospital - no one is saying - in Iwaki City, but the distance between "J-Village" and Iwaki City is roughly 28 kilometers. So the speed was 56 km/hour, or 35 miles/hour.
In Japan, ambulances announce to the cars and trucks on the road, "We are turning, we are turning..." Very polite, and very reserved in speed. But I'm sure there was a medical reason for going slower than the speed limit.
Even with 56 km/hour though, if the ambulance had come to the plant and took the worker to the hospital directly to Iwaki City, it would have arrived at the hospital in less than an hour, instead of 2 hours, and with a medical treatment provided in the ambulance.
The Self Defense Force also has this thing called "helicopter", but I guess that is reserved for VIPs like Prime Minister Kan and Nuclear Safety Commission chief Madarame, and we have to be worried for the safety of the SDF helicopter pilot.