Funding for this research was provided by:
Beijing Science and Technology Planning Project (Z191100004419003)
Text and Data Mining valid from 2020-03-27
Received: 25 February 2020
Accepted: 6 March 2020
First Online: 27 March 2020
Ethics approval and consent to participate
: The study was approved by the Ethics Committee of Beijing Emergency Medical Center (no.2020-01), and the written informed consent was waived because the study is retrospective in nature and belongs to the emergency medical service.
: Not applicable.
: The authors declare that they have no competing interests.