1. Department of Cardiovascular Surgery, Fukushima Medical University School of Medicine, Fukushima, Japan.
2. Ota Nishinouchi Hospital, Fukushima, Japan.
3. Hoshi General Hospital, Fukushima, Japan.
4. Iwaki Kyoritsu General Hospital, Fukushima, Japan.
5. Ohara Medical Center, Fukushima, Japan.
6. Aizu Chuo Hospital, Fukushima, Japan.