1 Division of Respiratory Medicine, Mito Medical Center, University of Tsukuba, Miya-machi 3-2-7, Mito, Ibaraki, 310-0015, Japan.
2 Division of Cardiology, Mito Medical Center, University of Tsukuba, Ibaraki, Japan.
3 Division of Respiratory Medicine, Mito Medical Center, University of Tsukuba, Miya-machi 3-2-7, Mito, Ibaraki, 310-0015, Japan. hirosato@md.tsukuba.ac.jp.