Conflict of interest

This study was supported by the grants-in-aid for scientific research from Japanese Society of Anesthesiologists (Grant Number 17B0010).

Ethical approval and consent to participate

Okayama University Hospital Institutional Review Board waived the need for obtaining informed consent and approved this study and its submission for publication (No 1801-005). If you have any question, please contact following address. Mailing address: 2-5-1 Shikatachou, Kitaku, Okayama, Okayama 700-8525, Japan. Phone: + 81-86-235-6503. Fax: + 81-86-235-7552. Email: mae6605@adm.okayama-u.ac.jp.

Consent for publication

Not applicable.