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According to Shukan Gendai, N. Korea maintained an affiliation with former E. Germany's clinic for privileged party elites. N. Koreans regularly sent doctors and medical staffs to receive training there and this relationship is continuing even after German unification.

There were several doctors and nurses in Pyongyang who took the training at this E. German clinic, but they were unable to deal with Kim's problem.

It seems almost certain that those who were operated upon are not ordinary folks, but very important figures, despite denial from heart institute. After all, 6 doctors were flown in to do a surgery. If not Kim Jong-il, they must be Kim's most trusted confidante or his immediate family member. However, I won't rule out that it was actually Kim Jong-il himself.