1. Department of Cardiology, National Hospital Organization Kasumigaura Medical Center , 2-7-14 Shimotakatsu, Tsuchiura, Ibaraki 300-8585 , Japan
2. Department of Cardiology, Faculty of Medicine, University of Tsukuba , 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577 , Japan
3. Faculty of Health Science, Tsukuba University of Technology , 4-12-7 Kasuga, Tsukuba, Ibaraki 305-8521 , Japan