1. Department of Anesthesiology KKR Sapporo Medical Center Sapporo Japan
2. Department of Anesthesiology Hokkaido Medical Center for Child Health and Rehabilitation Sapporo Japan
3. Department of Anesthesiology Sapporo Medical University School of Medicine Sapporo Japan
4. Department of Anesthesiology Kushiro City General Hospital Kushiro Japan
5. Department of Biostatistics, School of Public Health, Graduate School of Medicine The University of Tokyo Tokyo Japan